Merge pull request #2838 from rtfeldman/abilities-typechecking

Inference and checking for abilities

Cargo.lock

@@ -3351,6 +3351,7 @@ dependencies = [
  "roc_parse",
  "roc_problem",
  "roc_region",
+ "roc_reporting",
  "roc_target",
  "roc_types",
  "roc_unify",
@@ -3519,6 +3520,7 @@ dependencies = [
  "roc_module",
  "roc_parse",
  "roc_region",
+ "roc_reporting",
  "roc_target",
  "roc_types",
  "snafu",
@@ -3703,6 +3705,7 @@ dependencies = [
  "roc_constrain",
  "roc_load_internal",
  "roc_module",
+ "roc_reporting",
  "roc_target",
  "roc_types",
 ]
@@ -3734,7 +3737,6 @@ dependencies = [
  "roc_target",
  "roc_types",
  "roc_unify",
- "strip-ansi-escapes",
  "tempfile",
  "ven_pretty",
 ]
@@ -3893,6 +3895,7 @@ dependencies = [
  "roc_collections",
  "roc_constrain",
  "roc_exhaustive",
+ "roc_load",
  "roc_module",
  "roc_mono",
  "roc_parse",
@@ -3902,6 +3905,7 @@ dependencies = [
  "roc_target",
  "roc_test_utils",
  "roc_types",
+ "tempfile",
  "ven_pretty",
 ]
 
@@ -3916,11 +3920,13 @@ dependencies = [
  "roc_builtins",
  "roc_can",
  "roc_collections",
+ "roc_error_macros",
  "roc_load",
  "roc_module",
  "roc_parse",
  "roc_problem",
  "roc_region",
+ "roc_reporting",
  "roc_solve",
  "roc_target",
  "roc_types",
@@ -3968,6 +3974,7 @@ version = "0.1.0"
 dependencies = [
  "bitflags",
  "roc_collections",
+ "roc_error_macros",
  "roc_module",
  "roc_types",
 ]
@@ -4518,6 +4525,7 @@ dependencies = [
  "roc_load",
  "roc_module",
  "roc_mono",
+ "roc_reporting",
  "roc_target",
  "test_mono_macros",
 ]

ast/Cargo.toml

@@ -19,6 +19,7 @@ roc_unify = { path = "../compiler/unify"}
 roc_load = { path = "../compiler/load" }
 roc_target = { path = "../compiler/roc_target" }
 roc_error_macros = { path = "../error_macros" }
+roc_reporting = { path = "../reporting" }
 arrayvec = "0.7.2"
 bumpalo = { version = "3.8.0", features = ["collections"] }
 page_size = "0.4.2"

ast/src/module.rs

@@ -19,6 +19,7 @@ pub fn load_module(src_file: &Path) -> LoadedModule {
         }),
         subs_by_module,
         TargetInfo::default_x86_64(),
+        roc_reporting::report::RenderTarget::ColorTerminal,
     );
 
     match loaded {

ast/src/solve_type.rs

@@ -227,12 +227,16 @@ fn solve<'a>(
             );
 
             match unify(subs, actual, expected, Mode::EQ) {
-                Success(vars) => {
+                Success {
+                    vars,
+                    must_implement_ability: _,
+                } => {
+                    // TODO(abilities) record deferred ability checks
                     introduce(subs, rank, pools, &vars);
 
                     state
                 }
-                Failure(vars, actual_type, expected_type) => {
+                Failure(vars, actual_type, expected_type, _bad_impl) => {
                     introduce(subs, rank, pools, &vars);
 
                     let problem = TypeError::BadExpr(
@@ -267,7 +271,7 @@ fn solve<'a>(
         //
         //                    state
         //                }
-        //                Failure(vars, _actual_type, _expected_type) => {
+        //                Failure(vars, _actual_type, _expected_type, _bad_impl) => {
         //                    introduce(subs, rank, pools, &vars);
         //
         //                    // ERROR NOT REPORTED
@@ -320,13 +324,17 @@ fn solve<'a>(
                     );
 
                     match unify(subs, actual, expected, Mode::EQ) {
-                        Success(vars) => {
+                        Success {
+                            vars,
+                            must_implement_ability: _,
+                        } => {
+                            // TODO(abilities) record deferred ability checks
                             introduce(subs, rank, pools, &vars);
 
                             state
                         }
 
-                        Failure(vars, actual_type, expected_type) => {
+                        Failure(vars, actual_type, expected_type, _bad_impl) => {
                             introduce(subs, rank, pools, &vars);
 
                             let problem = TypeError::BadExpr(
@@ -391,12 +399,16 @@ fn solve<'a>(
 
             // TODO(ayazhafiz): presence constraints for Expr2/Type2
             match unify(subs, actual, expected, Mode::EQ) {
-                Success(vars) => {
+                Success {
+                    vars,
+                    must_implement_ability: _,
+                } => {
+                    // TODO(abilities) record deferred ability checks
                     introduce(subs, rank, pools, &vars);
 
                     state
                 }
-                Failure(vars, actual_type, expected_type) => {
+                Failure(vars, actual_type, expected_type, _bad_impl) => {
                     introduce(subs, rank, pools, &vars);
 
                     let problem = TypeError::BadPattern(
@@ -699,12 +711,16 @@ fn solve<'a>(
             let includes = type_to_var(arena, mempool, subs, rank, pools, cached_aliases, &tag_ty);
 
             match unify(subs, actual, includes, Mode::PRESENT) {
-                Success(vars) => {
+                Success {
+                    vars,
+                    must_implement_ability: _,
+                } => {
+                    // TODO(abilities) record deferred ability checks
                     introduce(subs, rank, pools, &vars);
 
                     state
                 }
-                Failure(vars, actual_type, expected_type) => {
+                Failure(vars, actual_type, expected_type, _bad_impl) => {
                     introduce(subs, rank, pools, &vars);
 
                     // TODO: do we need a better error type here?
@@ -1281,7 +1297,7 @@ fn adjust_rank_content(
     use roc_types::subs::FlatType::*;
 
     match content {
-        FlexVar(_) | RigidVar(_) | Error => group_rank,
+        FlexVar(_) | RigidVar(_) | FlexAbleVar(..) | RigidAbleVar(..) | Error => group_rank,
 
         RecursionVar { .. } => group_rank,
 
@@ -1536,7 +1552,7 @@ fn instantiate_rigids_help(
             };
         }
 
-        FlexVar(_) | Error => {}
+        FlexVar(_) | FlexAbleVar(_, _) | Error => {}
 
         RecursionVar { structure, .. } => {
             instantiate_rigids_help(subs, max_rank, pools, structure);
@@ -1547,6 +1563,11 @@ fn instantiate_rigids_help(
             subs.set(copy, make_descriptor(FlexVar(Some(name))));
         }
 
+        RigidAbleVar(name, ability) => {
+            // what it's all about: convert the rigid var into a flex var
+            subs.set(copy, make_descriptor(FlexAbleVar(Some(name), ability)));
+        }
+
         Alias(_, args, real_type_var, _) => {
             for var_index in args.all_variables() {
                 let var = subs[var_index];
@@ -1772,7 +1793,7 @@ fn deep_copy_var_help(
             copy
         }
 
-        FlexVar(_) | Error => copy,
+        FlexVar(_) | FlexAbleVar(_, _) | Error => copy,
 
         RecursionVar {
             opt_name,
@@ -1797,6 +1818,12 @@ fn deep_copy_var_help(
             copy
         }
 
+        RigidAbleVar(name, ability) => {
+            subs.set(copy, make_descriptor(FlexAbleVar(Some(name), ability)));
+
+            copy
+        }
+
         Alias(symbol, mut args, real_type_var, kind) => {
             let mut new_args = Vec::with_capacity(args.all_variables().len());
 

cli/src/build.rs

@@ -6,6 +6,7 @@ use roc_build::{
 use roc_builtins::bitcode;
 use roc_load::LoadingProblem;
 use roc_mono::ir::OptLevel;
+use roc_reporting::report::RenderTarget;
 use roc_target::TargetInfo;
 use std::path::PathBuf;
 use std::time::{Duration, SystemTime};
@@ -68,6 +69,8 @@ pub fn build_file<'a>(
         src_dir.as_path(),
         subs_by_module,
         target_info,
+        // TODO: expose this from CLI?
+        RenderTarget::ColorTerminal,
     )?;
 
     use target_lexicon::Architecture;
@@ -374,6 +377,8 @@ pub fn check_file(
         src_dir.as_path(),
         subs_by_module,
         target_info,
+        // TODO: expose this from CLI?
+        RenderTarget::ColorTerminal,
     )?;
 
     let buf = &mut String::with_capacity(1024);

compiler/can/src/abilities.rs

@@ -1,55 +1,63 @@
-use roc_collections::all::{MutMap, MutSet};
+use roc_collections::all::MutMap;
 use roc_module::symbol::Symbol;
+use roc_region::all::Region;
 use roc_types::types::Type;
 
-use crate::annotation::HasClause;
-
 /// Stores information about an ability member definition, including the parent ability, the
 /// defining type, and what type variables need to be instantiated with instances of the ability.
-#[derive(Debug)]
+#[derive(Debug, Clone, PartialEq, Eq)]
-struct AbilityMemberData {
+pub struct AbilityMemberData {
-    #[allow(unused)]
+    pub parent_ability: Symbol,
-    parent_ability: Symbol,
+    pub signature: Type,
-    #[allow(unused)]
+    pub region: Region,
-    signature: Type,
+}
-    #[allow(unused)]
+
-    bound_has_clauses: Vec<HasClause>,
+/// A particular specialization of an ability member.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct MemberSpecialization {
+    pub symbol: Symbol,
+    pub region: Region,
 }
 
 /// Stores information about what abilities exist in a scope, what it means to implement an
 /// ability, and what types implement them.
 // TODO(abilities): this should probably go on the Scope, I don't put it there for now because we
 // are only dealing with inter-module abilities for now.
-#[derive(Default, Debug)]
+#[derive(Default, Debug, Clone, PartialEq, Eq)]
 pub struct AbilitiesStore {
     /// Maps an ability to the members defining it.
-    #[allow(unused)]
     members_of_ability: MutMap<Symbol, Vec<Symbol>>,
 
     /// Information about all members composing abilities.
     ability_members: MutMap<Symbol, AbilityMemberData>,
 
-    /// Tuples of (type, member) specifying that `type` declares an implementation of an ability
+    /// Map of symbols that specialize an ability member to the root ability symbol name.
-    /// member `member`.
+    /// For example, for the program
-    #[allow(unused)]
+    ///   Hash has hash : a -> U64 | a has Hash
-    declared_implementations: MutSet<(Symbol, Symbol)>,
+    ///            ^^^^ gets the symbol "#hash"
+    ///   hash = \@Id n -> n
+    ///   ^^^^ gets the symbol "#hash1"
+    ///
+    /// We keep the mapping #hash1->#hash
+    specialization_to_root: MutMap<Symbol, Symbol>,
+
+    /// Maps a tuple (member, type) specifying that `type` declares an implementation of an ability
+    /// member `member`, to the exact symbol that implements the ability.
+    declared_specializations: MutMap<(Symbol, Symbol), MemberSpecialization>,
 }
 
 impl AbilitiesStore {
-    pub fn register_ability(
+    /// Records the definition of an ability, including its members.
-        &mut self,
+    pub fn register_ability(&mut self, ability: Symbol, members: Vec<(Symbol, Region, Type)>) {
-        ability: Symbol,
-        members: Vec<(Symbol, Type, Vec<HasClause>)>,
-    ) {
         let mut members_vec = Vec::with_capacity(members.len());
-        for (member, signature, bound_has_clauses) in members.into_iter() {
+        for (member, region, signature) in members.into_iter() {
             members_vec.push(member);
             let old_member = self.ability_members.insert(
                 member,
                 AbilityMemberData {
                     parent_ability: ability,
                     signature,
-                    bound_has_clauses,
+                    region,
                 },
             );
             debug_assert!(old_member.is_none(), "Replacing existing member definition");
@@ -61,14 +69,83 @@ impl AbilitiesStore {
         );
     }
 
-    pub fn register_implementation(&mut self, implementing_type: Symbol, ability_member: Symbol) {
+    /// Records a specialization of `ability_member` with specialized type `implementing_type`.
-        let old_impl = self
+    /// Entries via this function are considered a source of truth. It must be ensured that a
-            .declared_implementations
+    /// specialization is validated before being registered here.
-            .insert((implementing_type, ability_member));
+    pub fn register_specialization_for_type(
-        debug_assert!(!old_impl, "Replacing existing implementation");
+        &mut self,
+        ability_member: Symbol,
+        implementing_type: Symbol,
+        specialization: MemberSpecialization,
+    ) {
+        let old_spec = self
+            .declared_specializations
+            .insert((ability_member, implementing_type), specialization);
+        debug_assert!(old_spec.is_none(), "Replacing existing specialization");
     }
 
+    /// Checks if `name` is a root ability member symbol name.
+    /// Note that this will return `false` for specializations of an ability member, which have
+    /// different symbols from the root.
     pub fn is_ability_member_name(&self, name: Symbol) -> bool {
         self.ability_members.contains_key(&name)
     }
+
+    /// Returns information about all known ability members and their root symbols.
+    pub fn root_ability_members(&self) -> &MutMap<Symbol, AbilityMemberData> {
+        &self.ability_members
+    }
+
+    /// Records that the symbol `specializing_symbol` claims to specialize `ability_member`; for
+    /// example the symbol of `hash : Id -> U64` specializing `hash : a -> U64 | a has Hash`.
+    pub fn register_specializing_symbol(
+        &mut self,
+        specializing_symbol: Symbol,
+        ability_member: Symbol,
+    ) {
+        self.specialization_to_root
+            .insert(specializing_symbol, ability_member);
+    }
+
+    /// Returns whether a symbol is declared to specialize an ability member.
+    pub fn is_specialization_name(&self, symbol: Symbol) -> bool {
+        self.specialization_to_root.contains_key(&symbol)
+    }
+
+    /// Finds the symbol name and ability member definition for a symbol specializing the ability
+    /// member, if it specializes any.
+    /// For example, suppose `hash : Id -> U64` has symbol #hash1 and specializes
+    /// `hash : a -> U64 | a has Hash` with symbol #hash. Calling this with #hash1 would retrieve
+    /// the ability member data for #hash.
+    pub fn root_name_and_def(
+        &self,
+        specializing_symbol: Symbol,
+    ) -> Option<(Symbol, &AbilityMemberData)> {
+        let root_symbol = self.specialization_to_root.get(&specializing_symbol)?;
+        debug_assert!(self.ability_members.contains_key(root_symbol));
+        let root_data = self.ability_members.get(root_symbol).unwrap();
+        Some((*root_symbol, root_data))
+    }
+
+    /// Finds the ability member definition for a member name.
+    pub fn member_def(&self, member: Symbol) -> Option<&AbilityMemberData> {
+        self.ability_members.get(&member)
+    }
+
+    /// Returns an iterator over pairs (ability member, type) specifying that
+    /// "ability member" has a specialization with type "type".
+    pub fn get_known_specializations(&self) -> impl Iterator<Item = (Symbol, Symbol)> + '_ {
+        self.declared_specializations.keys().copied()
+    }
+
+    /// Retrieves the specialization of `member` for `typ`, if it exists.
+    pub fn get_specialization(&self, member: Symbol, typ: Symbol) -> Option<MemberSpecialization> {
+        self.declared_specializations.get(&(member, typ)).copied()
+    }
+
+    /// Returns pairs of (type, ability member) specifying that "ability member" has a
+    /// specialization with type "type".
+    pub fn members_of_ability(&self, ability: Symbol) -> Option<&[Symbol]> {
+        self.members_of_ability.get(&ability).map(|v| v.as_ref())
+    }
 }

compiler/can/src/annotation.rs

@@ -19,6 +19,22 @@ pub struct Annotation {
     pub aliases: SendMap<Symbol, Alias>,
 }
 
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum NamedOrAbleVariable<'a> {
+    Named(&'a NamedVariable),
+    Able(&'a AbleVariable),
+}
+
+impl<'a> NamedOrAbleVariable<'a> {
+    pub fn first_seen(&self) -> Region {
+        match self {
+            NamedOrAbleVariable::Named(nv) => nv.first_seen,
+            NamedOrAbleVariable::Able(av) => av.first_seen,
+        }
+    }
+}
+
+/// A named type variable, not bound to an ability.
 #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct NamedVariable {
     pub variable: Variable,
@@ -27,21 +43,40 @@ pub struct NamedVariable {
     pub first_seen: Region,
 }
 
+/// A type variable bound to an ability, like "a has Hash".
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct AbleVariable {
+    pub variable: Variable,
+    pub name: Lowercase,
+    pub ability: Symbol,
+    // NB: there may be multiple occurrences of a variable
+    pub first_seen: Region,
+}
+
 #[derive(Clone, Debug, PartialEq, Default)]
 pub struct IntroducedVariables {
     pub wildcards: Vec<Loc<Variable>>,
     pub lambda_sets: Vec<Variable>,
     pub inferred: Vec<Loc<Variable>>,
     pub named: Vec<NamedVariable>,
+    pub able: Vec<AbleVariable>,
     pub host_exposed_aliases: MutMap<Symbol, Variable>,
 }
 
 impl IntroducedVariables {
+    #[inline(always)]
+    fn debug_assert_not_already_present(&self, var: Variable) {
+        debug_assert!((self.wildcards.iter().map(|v| &v.value))
+            .chain(self.lambda_sets.iter())
+            .chain(self.inferred.iter().map(|v| &v.value))
+            .chain(self.named.iter().map(|nv| &nv.variable))
+            .chain(self.able.iter().map(|av| &av.variable))
+            .chain(self.host_exposed_aliases.values())
+            .all(|&v| v != var));
+    }
+
     pub fn insert_named(&mut self, name: Lowercase, var: Loc<Variable>) {
-        debug_assert!(!self
+        self.debug_assert_not_already_present(var.value);
-            .named
-            .iter()
-            .any(|nv| nv.name == name || nv.variable == var.value));
 
         let named_variable = NamedVariable {
             name,
@@ -52,19 +87,36 @@ impl IntroducedVariables {
         self.named.push(named_variable);
     }
 
+    pub fn insert_able(&mut self, name: Lowercase, var: Loc<Variable>, ability: Symbol) {
+        self.debug_assert_not_already_present(var.value);
+
+        let able_variable = AbleVariable {
+            name,
+            ability,
+            variable: var.value,
+            first_seen: var.region,
+        };
+
+        self.able.push(able_variable);
+    }
+
     pub fn insert_wildcard(&mut self, var: Loc<Variable>) {
+        self.debug_assert_not_already_present(var.value);
         self.wildcards.push(var);
     }
 
     pub fn insert_inferred(&mut self, var: Loc<Variable>) {
+        self.debug_assert_not_already_present(var.value);
         self.inferred.push(var);
     }
 
     fn insert_lambda_set(&mut self, var: Variable) {
+        self.debug_assert_not_already_present(var);
         self.lambda_sets.push(var);
     }
 
     pub fn insert_host_exposed_alias(&mut self, symbol: Symbol, var: Variable) {
+        self.debug_assert_not_already_present(var);
         self.host_exposed_aliases.insert(symbol, var);
     }
 
@@ -78,6 +130,10 @@ impl IntroducedVariables {
         self.named.extend(other.named.iter().cloned());
         self.named.sort();
         self.named.dedup();
+
+        self.able.extend(other.able.iter().cloned());
+        self.able.sort();
+        self.able.dedup();
     }
 
     pub fn union_owned(&mut self, other: Self) {
@@ -91,22 +147,26 @@ impl IntroducedVariables {
         self.named.dedup();
     }
 
-    pub fn var_by_name(&self, name: &Lowercase) -> Option<&Variable> {
+    pub fn var_by_name(&self, name: &Lowercase) -> Option<Variable> {
-        self.named
+        (self.named.iter().map(|nv| (&nv.name, nv.variable)))
+            .chain(self.able.iter().map(|av| (&av.name, av.variable)))
+            .find(|(cand, _)| cand == &name)
+            .map(|(_, var)| var)
+    }
+
+    pub fn named_var_by_name(&self, name: &Lowercase) -> Option<NamedOrAbleVariable> {
+        if let Some(nav) = self
+            .named
             .iter()
             .find(|nv| &nv.name == name)
-            .map(|nv| &nv.variable)
+            .map(NamedOrAbleVariable::Named)
-    }
+        {
-
+            return Some(nav);
-    pub fn name_by_var(&self, var: Variable) -> Option<&Lowercase> {
+        }
-        self.named
+        self.able
             .iter()
-            .find(|nv| nv.variable == var)
+            .find(|av| &av.name == name)
-            .map(|nv| &nv.name)
+            .map(NamedOrAbleVariable::Able)
-    }
-
-    pub fn named_var_by_name(&self, name: &Lowercase) -> Option<&NamedVariable> {
-        self.named.iter().find(|nv| &nv.name == name)
     }
 }
 
@@ -147,13 +207,6 @@ pub fn canonicalize_annotation(
     }
 }
 
-#[derive(Clone, Debug)]
-pub struct HasClause {
-    pub var_name: Lowercase,
-    pub var: Variable,
-    pub ability: Symbol,
-}
-
 pub fn canonicalize_annotation_with_possible_clauses(
     env: &mut Env,
     scope: &mut Scope,
@@ -161,16 +214,17 @@ pub fn canonicalize_annotation_with_possible_clauses(
     region: Region,
     var_store: &mut VarStore,
     abilities_in_scope: &[Symbol],
-) -> (Annotation, Vec<Loc<HasClause>>) {
+) -> Annotation {
     let mut introduced_variables = IntroducedVariables::default();
     let mut references = MutSet::default();
     let mut aliases = SendMap::default();
 
-    let (annotation, region, clauses) = match annotation {
+    let (annotation, region) = match annotation {
         TypeAnnotation::Where(annotation, clauses) => {
-            let mut can_clauses = Vec::with_capacity(clauses.len());
+            // Add each "has" clause. The association of a variable to an ability will be saved on
+            // `introduced_variables`, which we'll process later.
             for clause in clauses.iter() {
-                match canonicalize_has_clause(
+                let opt_err = canonicalize_has_clause(
                     env,
                     scope,
                     var_store,
@@ -178,24 +232,19 @@ pub fn canonicalize_annotation_with_possible_clauses(
                     clause,
                     abilities_in_scope,
                     &mut references,
-                ) {
+                );
-                    Ok(result) => can_clauses.push(Loc::at(clause.region, result)),
+                if let Err(err_type) = opt_err {
-                    Err(err_type) => {
+                    return Annotation {
-                        return (
+                        typ: err_type,
-                            Annotation {
+                        introduced_variables,
-                                typ: err_type,
+                        references,
-                                introduced_variables,
+                        aliases,
-                                references,
+                    };
-                                aliases,
+                }
-                            },
-                            can_clauses,
-                        )
-                    }
-                };
             }
-            (&annotation.value, annotation.region, can_clauses)
+            (&annotation.value, annotation.region)
         }
-        annot => (annot, region, vec![]),
+        annot => (annot, region),
     };
 
     let typ = can_annotation_help(
@@ -209,14 +258,12 @@ pub fn canonicalize_annotation_with_possible_clauses(
         &mut references,
     );
 
-    let annot = Annotation {
+    Annotation {
         typ,
         introduced_variables,
         references,
         aliases,
-    };
+    }
-
-    (annot, clauses)
 }
 
 fn make_apply_symbol(
@@ -502,7 +549,7 @@ fn can_annotation_help(
             let name = Lowercase::from(*v);
 
             match introduced_variables.var_by_name(&name) {
-                Some(var) => Type::Variable(*var),
+                Some(var) => Type::Variable(var),
                 None => {
                     let var = var_store.fresh();
 
@@ -566,8 +613,8 @@ fn can_annotation_help(
                 let var_name = Lowercase::from(var);
 
                 if let Some(var) = introduced_variables.var_by_name(&var_name) {
-                    vars.push((var_name.clone(), Type::Variable(*var)));
+                    vars.push((var_name.clone(), Type::Variable(var)));
-                    lowercase_vars.push(Loc::at(loc_var.region, (var_name, *var)));
+                    lowercase_vars.push(Loc::at(loc_var.region, (var_name, var)));
                 } else {
                     let var = var_store.fresh();
 
@@ -799,7 +846,7 @@ fn canonicalize_has_clause(
     clause: &Loc<roc_parse::ast::HasClause<'_>>,
     abilities_in_scope: &[Symbol],
     references: &mut MutSet<Symbol>,
-) -> Result<HasClause, Type> {
+) -> Result<(), Type> {
     let Loc {
         region,
         value: roc_parse::ast::HasClause { var, ability },
@@ -836,25 +883,21 @@ fn canonicalize_has_clause(
         let var_name_ident = var_name.to_string().into();
         let shadow = Loc::at(region, var_name_ident);
         env.problem(roc_problem::can::Problem::Shadowing {
-            original_region: shadowing.first_seen,
+            original_region: shadowing.first_seen(),
             shadow: shadow.clone(),
             kind: ShadowKind::Variable,
         });
         return Err(Type::Erroneous(Problem::Shadowed(
-            shadowing.first_seen,
+            shadowing.first_seen(),
             shadow,
         )));
     }
 
     let var = var_store.fresh();
 
-    introduced_variables.insert_named(var_name.clone(), Loc::at(region, var));
+    introduced_variables.insert_able(var_name, Loc::at(region, var), ability);
 
-    Ok(HasClause {
+    Ok(())
-        var_name,
-        var,
-        ability,
-    })
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -1105,7 +1148,7 @@ fn can_assigned_fields<'a>(
                     let field_name = Lowercase::from(loc_field_name.value);
                     let field_type = {
                         if let Some(var) = introduced_variables.var_by_name(&field_name) {
-                            Type::Variable(*var)
+                            Type::Variable(var)
                         } else {
                             let field_var = var_store.fresh();
                             introduced_variables.insert_named(

compiler/can/src/def.rs

@@ -1,4 +1,3 @@
-use crate::abilities::AbilitiesStore;
 use crate::annotation::canonicalize_annotation;
 use crate::annotation::canonicalize_annotation_with_possible_clauses;
 use crate::annotation::IntroducedVariables;
@@ -430,12 +429,11 @@ pub fn canonicalize_defs<'a>(
     }
 
     // Now we can go through and resolve all pending abilities, to add them to scope.
-    let mut abilities_store = AbilitiesStore::default();
     for (loc_ability_name, members) in abilities.into_values() {
         let mut can_members = Vec::with_capacity(members.len());
 
         for member in members {
-            let (member_annot, clauses) = canonicalize_annotation_with_possible_clauses(
+            let member_annot = canonicalize_annotation_with_possible_clauses(
                 env,
                 &mut scope,
                 &member.typ.value,
@@ -450,13 +448,14 @@ pub fn canonicalize_defs<'a>(
                 output.references.referenced_type_defs.insert(symbol);
             }
 
+            let name_region = member.name.region;
             let member_name = member.name.extract_spaces().item;
 
             let member_sym = match scope.introduce(
                 member_name.into(),
                 &env.exposed_ident_ids,
                 &mut env.ident_ids,
-                member.name.region,
+                name_region,
             ) {
                 Ok(sym) => sym,
                 Err((original_region, shadow, _new_symbol)) => {
@@ -473,9 +472,11 @@ pub fn canonicalize_defs<'a>(
             // What variables in the annotation are bound to the parent ability, and what variables
             // are bound to some other ability?
             let (variables_bound_to_ability, variables_bound_to_other_abilities): (Vec<_>, Vec<_>) =
-                clauses
+                member_annot
-                    .into_iter()
+                    .introduced_variables
-                    .partition(|has_clause| has_clause.value.ability == loc_ability_name.value);
+                    .able
+                    .iter()
+                    .partition(|av| av.ability == loc_ability_name.value);
 
             let mut bad_has_clauses = false;
 
@@ -485,18 +486,38 @@ pub fn canonicalize_defs<'a>(
                 env.problem(Problem::AbilityMemberMissingHasClause {
                     member: member_sym,
                     ability: loc_ability_name.value,
-                    region: member.name.region,
+                    region: name_region,
+                });
+                bad_has_clauses = true;
+            }
+
+            if variables_bound_to_ability.len() > 1 {
+                // There is more than one variable bound to the member signature, so something like
+                //   Eq has eq : a, b -> Bool | a has Eq, b has Eq
+                // We have no way of telling what type implements a particular instance of Eq in
+                // this case (a or b?), so disallow it.
+                let span_has_clauses =
+                    Region::across_all(variables_bound_to_ability.iter().map(|v| &v.first_seen));
+                let bound_var_names = variables_bound_to_ability
+                    .iter()
+                    .map(|v| v.name.clone())
+                    .collect();
+                env.problem(Problem::AbilityMemberMultipleBoundVars {
+                    member: member_sym,
+                    ability: loc_ability_name.value,
+                    span_has_clauses,
+                    bound_var_names,
                 });
                 bad_has_clauses = true;
             }
 
             if !variables_bound_to_other_abilities.is_empty() {
                 // Disallow variables bound to other abilities, for now.
-                for bad_clause in variables_bound_to_other_abilities.iter() {
+                for bad_variable in variables_bound_to_other_abilities.iter() {
                     env.problem(Problem::AbilityMemberBindsExternalAbility {
                         member: member_sym,
                         ability: loc_ability_name.value,
-                        region: bad_clause.region,
+                        region: bad_variable.first_seen,
                     });
                 }
                 bad_has_clauses = true;
@@ -507,15 +528,18 @@ pub fn canonicalize_defs<'a>(
                 continue;
             }
 
-            let has_clauses = variables_bound_to_ability
+            // The introduced variables are good; add them to the output.
-                .into_iter()
+            output
-                .map(|clause| clause.value)
+                .introduced_variables
-                .collect();
+                .union(&member_annot.introduced_variables);
-            can_members.push((member_sym, member_annot.typ, has_clauses));
+
+            can_members.push((member_sym, name_region, member_annot.typ));
         }
 
         // Store what symbols a type must define implementations for to have this ability.
-        abilities_store.register_ability(loc_ability_name.value, can_members);
+        scope
+            .abilities_store
+            .register_ability(loc_ability_name.value, can_members);
     }
 
     // Now that we have the scope completely assembled, and shadowing resolved,
@@ -526,14 +550,7 @@ pub fn canonicalize_defs<'a>(
     // once we've finished assembling the entire scope.
     let mut pending_value_defs = Vec::with_capacity(value_defs.len());
     for loc_def in value_defs.into_iter() {
-        match to_pending_value_def(
+        match to_pending_value_def(env, var_store, loc_def.value, &mut scope, pattern_type) {
-            env,
-            var_store,
-            loc_def.value,
-            &mut scope,
-            &abilities_store,
-            pattern_type,
-        ) {
             None => { /* skip */ }
             Some((new_output, pending_def)) => {
                 // store the top-level defs, used to ensure that closures won't capture them
@@ -1561,7 +1578,9 @@ pub fn can_defs_with_return<'a>(
     // Now that we've collected all the references, check to see if any of the new idents
     // we defined went unused by the return expression. If any were unused, report it.
     for (symbol, region) in symbols_introduced {
-        if !output.references.has_value_lookup(symbol) && !output.references.has_type_lookup(symbol)
+        if !output.references.has_value_lookup(symbol)
+            && !output.references.has_type_lookup(symbol)
+            && !scope.abilities_store.is_specialization_name(symbol)
         {
             env.problem(Problem::UnusedDef(symbol, region));
         }
@@ -1772,7 +1791,6 @@ fn to_pending_value_def<'a>(
     var_store: &mut VarStore,
     def: &'a ast::ValueDef<'a>,
     scope: &mut Scope,
-    abilities_store: &AbilitiesStore,
     pattern_type: PatternType,
 ) -> Option<(Output, PendingValueDef<'a>)> {
     use ast::ValueDef::*;
@@ -1784,7 +1802,6 @@ fn to_pending_value_def<'a>(
                 env,
                 var_store,
                 scope,
-                abilities_store,
                 pattern_type,
                 &loc_pattern.value,
                 loc_pattern.region,
@@ -1801,7 +1818,6 @@ fn to_pending_value_def<'a>(
                 env,
                 var_store,
                 scope,
-                abilities_store,
                 pattern_type,
                 &loc_pattern.value,
                 loc_pattern.region,
@@ -1832,7 +1848,6 @@ fn to_pending_value_def<'a>(
                     env,
                     var_store,
                     scope,
-                    abilities_store,
                     pattern_type,
                     &body_pattern.value,
                     body_pattern.region,

compiler/can/src/expr.rs

@@ -1083,6 +1083,7 @@ fn canonicalize_when_branch<'a>(
             && !branch_output.references.has_value_lookup(symbol)
             && !branch_output.references.has_type_lookup(symbol)
             && !original_scope.contains_symbol(symbol)
+            && !scope.abilities_store.is_specialization_name(symbol)
         {
             env.problem(Problem::UnusedDef(symbol, *region));
         }

compiler/can/src/module.rs

@@ -1,3 +1,4 @@
+use crate::abilities::AbilitiesStore;
 use crate::def::{canonicalize_defs, sort_can_defs, Declaration, Def};
 use crate::effect_module::HostedGeneratedFunctions;
 use crate::env::Env;
@@ -28,11 +29,13 @@ pub struct Module {
     /// all aliases. `bool` indicates whether it is exposed
     pub aliases: MutMap<Symbol, (bool, Alias)>,
     pub rigid_variables: RigidVariables,
+    pub abilities_store: AbilitiesStore,
 }
 
 #[derive(Debug, Default)]
 pub struct RigidVariables {
     pub named: MutMap<Variable, Lowercase>,
+    pub able: MutMap<Variable, (Lowercase, Symbol)>,
     pub wildcards: MutSet<Variable>,
 }
 
@@ -250,6 +253,7 @@ pub fn canonicalize_module_defs<'a>(
         if !output.references.has_value_lookup(symbol)
             && !output.references.has_type_lookup(symbol)
             && !exposed_symbols.contains(&symbol)
+            && !scope.abilities_store.is_specialization_name(symbol)
         {
             env.problem(Problem::UnusedDef(symbol, region));
         }
@@ -259,6 +263,12 @@ pub fn canonicalize_module_defs<'a>(
         rigid_variables.named.insert(named.variable, named.name);
     }
 
+    for able in output.introduced_variables.able {
+        rigid_variables
+            .able
+            .insert(able.variable, (able.name, able.ability));
+    }
+
     for var in output.introduced_variables.wildcards {
         rigid_variables.wildcards.insert(var.value);
     }
@@ -444,6 +454,10 @@ pub fn canonicalize_module_defs<'a>(
                 aliases.insert(symbol, alias);
             }
 
+            for member in scope.abilities_store.root_ability_members().keys() {
+                exposed_but_not_defined.remove(member);
+            }
+
             // By this point, all exposed symbols should have been removed from
             // exposed_symbols and added to exposed_vars_by_symbol. If any were
             // not, that means they were declared as exposed but there was

compiler/can/src/pattern.rs

@@ -1,4 +1,3 @@
-use crate::abilities::AbilitiesStore;
 use crate::annotation::freshen_opaque_def;
 use crate::env::Env;
 use crate::expr::{canonicalize_expr, unescape_char, Expr, IntValue, Output};
@@ -157,7 +156,6 @@ pub fn canonicalize_def_header_pattern<'a>(
     env: &mut Env<'a>,
     var_store: &mut VarStore,
     scope: &mut Scope,
-    abilities_store: &AbilitiesStore,
     pattern_type: PatternType,
     pattern: &ast::Pattern<'a>,
     region: Region,
@@ -172,7 +170,6 @@ pub fn canonicalize_def_header_pattern<'a>(
             &env.exposed_ident_ids,
             &mut env.ident_ids,
             region,
-            abilities_store,
         ) {
             Ok((symbol, shadowing_ability_member)) => {
                 output.references.bound_symbols.insert(symbol);

compiler/can/src/scope.rs

@@ -22,7 +22,7 @@ pub struct Scope {
     pub aliases: SendMap<Symbol, Alias>,
 
     /// The abilities currently in scope, and their implementors.
-    pub abilities: SendMap<Symbol, Region>,
+    pub abilities_store: AbilitiesStore,
 
     /// The current module being processed. This will be used to turn
     /// unqualified idents into Symbols.
@@ -68,7 +68,7 @@ impl Scope {
             symbols: SendMap::default(),
             aliases,
             // TODO(abilities): default abilities in scope
-            abilities: SendMap::default(),
+            abilities_store: AbilitiesStore::default(),
         }
     }
 
@@ -247,7 +247,6 @@ impl Scope {
         exposed_ident_ids: &IdentIds,
         all_ident_ids: &mut IdentIds,
         region: Region,
-        abilities_store: &AbilitiesStore,
     ) -> Result<(Symbol, Option<Symbol>), (Region, Loc<Ident>, Symbol)> {
         match self.idents.get(&ident) {
             Some(&(original_symbol, original_region)) => {
@@ -256,7 +255,9 @@ impl Scope {
 
                 self.symbols.insert(shadow_symbol, region);
 
-                if abilities_store.is_ability_member_name(original_symbol) {
+                if self.abilities_store.is_ability_member_name(original_symbol) {
+                    self.abilities_store
+                        .register_specializing_symbol(shadow_symbol, original_symbol);
                     // Add a symbol for the shadow, but don't re-associate the member name.
                     Ok((shadow_symbol, Some(original_symbol)))
                 } else {

compiler/constrain/src/module.rs

@@ -1,4 +1,5 @@
 use roc_builtins::std::StdLib;
+use roc_can::abilities::AbilitiesStore;
 use roc_can::constraint::{Constraint, Constraints};
 use roc_can::def::Declaration;
 use roc_collections::all::MutMap;
@@ -100,10 +101,26 @@ pub enum ExposedModuleTypes {
 
 pub fn constrain_module(
     constraints: &mut Constraints,
+    abilities_store: &AbilitiesStore,
     declarations: &[Declaration],
     home: ModuleId,
 ) -> Constraint {
-    crate::expr::constrain_decls(constraints, home, declarations)
+    let mut constraint = crate::expr::constrain_decls(constraints, home, declarations);
+
+    for (member_name, member_data) in abilities_store.root_ability_members().iter() {
+        constraint = constraints.let_constraint(
+            [],
+            [],
+            [(*member_name, Loc::at_zero(member_data.signature.clone()))],
+            Constraint::True,
+            constraint,
+        );
+    }
+
+    // The module constraint should always save the environment at the end.
+    debug_assert!(constraints.contains_save_the_environment(&constraint));
+
+    constraint
 }
 
 #[derive(Debug, Clone)]

compiler/constrain/src/pattern.rs

@@ -188,9 +188,23 @@ pub fn constrain_pattern(
             // Erroneous patterns don't add any constraints.
         }
 
-        Identifier(symbol) | Shadowed(_, _, symbol)
+        Identifier(symbol) | Shadowed(_, _, symbol) => {
-        // TODO(abilities): handle linking the member def to the specialization ident
+            if could_be_a_tag_union(expected.get_type_ref()) {
-        | AbilityMemberSpecialization {
+                state
+                    .constraints
+                    .push(constraints.is_open_type(expected.get_type_ref().clone()));
+            }
+
+            state.headers.insert(
+                *symbol,
+                Loc {
+                    region,
+                    value: expected.get_type(),
+                },
+            );
+        }
+
+        AbilityMemberSpecialization {
             ident: symbol,
             specializes: _,
         } => {

compiler/load/Cargo.toml

@@ -13,10 +13,12 @@ roc_constrain= { path = "../constrain" }
 roc_types = { path = "../types" }
 roc_module = { path = "../module" }
 roc_collections = { path = "../collections" }
+roc_reporting = { path = "../../reporting" }
 
 [build-dependencies]
 roc_load_internal = { path = "../load_internal" }
 roc_builtins = { path = "../builtins" }
 roc_module = { path = "../module" }
+roc_reporting = { path = "../../reporting" }
 roc_target = { path = "../roc_target" }
 bumpalo = { version = "3.8.0", features = ["collections"] }

compiler/load/build.rs

@@ -36,6 +36,7 @@ fn write_subs_for_module(module_id: ModuleId, filename: &str) {
         &src_dir,
         Default::default(),
         target_info,
+        roc_reporting::report::RenderTarget::ColorTerminal,
     );
 
     let module = res_module.unwrap();

compiler/load/src/lib.rs

@@ -2,6 +2,7 @@ use bumpalo::Bump;
 use roc_collections::all::MutMap;
 use roc_constrain::module::ExposedByModule;
 use roc_module::symbol::{ModuleId, Symbol};
+use roc_reporting::report::RenderTarget;
 use roc_target::TargetInfo;
 use roc_types::subs::{Subs, Variable};
 use std::path::{Path, PathBuf};
@@ -18,6 +19,7 @@ fn load<'a>(
     exposed_types: ExposedByModule,
     goal_phase: Phase,
     target_info: TargetInfo,
+    render: RenderTarget,
 ) -> Result<LoadResult<'a>, LoadingProblem<'a>> {
     let cached_subs = read_cached_subs();
 
@@ -29,6 +31,7 @@ fn load<'a>(
         goal_phase,
         target_info,
         cached_subs,
+        render,
     )
 }
 
@@ -39,6 +42,7 @@ pub fn load_and_monomorphize_from_str<'a>(
     src_dir: &Path,
     exposed_types: ExposedByModule,
     target_info: TargetInfo,
+    render: RenderTarget,
 ) -> Result<MonomorphizedModule<'a>, LoadingProblem<'a>> {
     use LoadResult::*;
 
@@ -51,6 +55,7 @@ pub fn load_and_monomorphize_from_str<'a>(
         exposed_types,
         Phase::MakeSpecializations,
         target_info,
+        render,
     )? {
         Monomorphized(module) => Ok(module),
         TypeChecked(_) => unreachable!(""),
@@ -63,10 +68,11 @@ pub fn load_and_monomorphize<'a>(
     src_dir: &Path,
     exposed_types: ExposedByModule,
     target_info: TargetInfo,
+    render: RenderTarget,
 ) -> Result<MonomorphizedModule<'a>, LoadingProblem<'a>> {
     use LoadResult::*;
 
-    let load_start = LoadStart::from_path(arena, filename)?;
+    let load_start = LoadStart::from_path(arena, filename, render)?;
 
     match load(
         arena,
@@ -75,6 +81,7 @@ pub fn load_and_monomorphize<'a>(
         exposed_types,
         Phase::MakeSpecializations,
         target_info,
+        render,
     )? {
         Monomorphized(module) => Ok(module),
         TypeChecked(_) => unreachable!(""),
@@ -87,10 +94,11 @@ pub fn load_and_typecheck<'a>(
     src_dir: &Path,
     exposed_types: ExposedByModule,
     target_info: TargetInfo,
+    render: RenderTarget,
 ) -> Result<LoadedModule, LoadingProblem<'a>> {
     use LoadResult::*;
 
-    let load_start = LoadStart::from_path(arena, filename)?;
+    let load_start = LoadStart::from_path(arena, filename, render)?;
 
     match load(
         arena,
@@ -99,6 +107,7 @@ pub fn load_and_typecheck<'a>(
         exposed_types,
         Phase::SolveTypes,
         target_info,
+        render,
     )? {
         Monomorphized(_) => unreachable!(""),
         TypeChecked(module) => Ok(module),

compiler/load_internal/Cargo.toml

@@ -33,4 +33,3 @@ tempfile = "3.2.0"
 pretty_assertions = "1.0.0"
 maplit = "1.0.2"
 indoc = "1.0.3"
-strip-ansi-escapes = "0.1.1"

compiler/load_internal/src/file.rs

@@ -5,6 +5,7 @@ use crossbeam::deque::{Injector, Stealer, Worker};
 use crossbeam::thread;
 use parking_lot::Mutex;
 use roc_builtins::std::borrow_stdlib;
+use roc_can::abilities::AbilitiesStore;
 use roc_can::constraint::{Constraint as ConstraintSoa, Constraints};
 use roc_can::def::Declaration;
 use roc_can::module::{canonicalize_module_defs, Module};
@@ -31,6 +32,7 @@ use roc_parse::ident::UppercaseIdent;
 use roc_parse::module::module_defs;
 use roc_parse::parser::{FileError, Parser, SyntaxError};
 use roc_region::all::{LineInfo, Loc, Region};
+use roc_reporting::report::RenderTarget;
 use roc_solve::module::SolvedModule;
 use roc_solve::solve;
 use roc_target::TargetInfo;
@@ -347,6 +349,7 @@ pub struct LoadedModule {
     pub sources: MutMap<ModuleId, (PathBuf, Box<str>)>,
     pub timings: MutMap<ModuleId, ModuleTiming>,
     pub documentation: MutMap<ModuleId, ModuleDocumentation>,
+    pub abilities_store: AbilitiesStore,
 }
 
 impl LoadedModule {
@@ -508,6 +511,7 @@ enum Msg<'a> {
         decls: Vec<Declaration>,
         dep_idents: MutMap<ModuleId, IdentIds>,
         module_timing: ModuleTiming,
+        abilities_store: AbilitiesStore,
     },
     FinishedAllTypeChecking {
         solved_subs: Solved<Subs>,
@@ -515,6 +519,7 @@ enum Msg<'a> {
         exposed_aliases_by_symbol: MutMap<Symbol, (bool, Alias)>,
         dep_idents: MutMap<ModuleId, IdentIds>,
         documentation: MutMap<ModuleId, ModuleDocumentation>,
+        abilities_store: AbilitiesStore,
     },
     FoundSpecializations {
         module_id: ModuleId,
@@ -604,6 +609,8 @@ struct State<'a> {
     // (Granted, this has not been attempted or measured!)
     pub layout_caches: std::vec::Vec<LayoutCache<'a>>,
 
+    pub render: RenderTarget,
+
     // cached subs (used for builtin modules, could include packages in the future too)
     cached_subs: CachedSubs,
 }
@@ -611,6 +618,7 @@ struct State<'a> {
 type CachedSubs = Arc<Mutex<MutMap<ModuleId, (Subs, Vec<(Symbol, Variable)>)>>>;
 
 impl<'a> State<'a> {
+    #[allow(clippy::too_many_arguments)]
     fn new(
         root_id: ModuleId,
         target_info: TargetInfo,
@@ -619,6 +627,7 @@ impl<'a> State<'a> {
         arc_modules: Arc<Mutex<PackageModuleIds<'a>>>,
         ident_ids_by_module: Arc<Mutex<MutMap<ModuleId, IdentIds>>>,
         cached_subs: MutMap<ModuleId, (Subs, Vec<(Symbol, Variable)>)>,
+        render: RenderTarget,
     ) -> Self {
         let arc_shorthands = Arc::new(Mutex::new(MutMap::default()));
 
@@ -643,6 +652,7 @@ impl<'a> State<'a> {
             timings: MutMap::default(),
             layout_caches: std::vec::Vec::with_capacity(num_cpus::get()),
             cached_subs: Arc::new(Mutex::new(cached_subs)),
+            render,
         }
     }
 }
@@ -824,6 +834,7 @@ pub fn load_and_typecheck_str<'a>(
     src_dir: &Path,
     exposed_types: ExposedByModule,
     target_info: TargetInfo,
+    render: RenderTarget,
 ) -> Result<LoadedModule, LoadingProblem<'a>> {
     use LoadResult::*;
 
@@ -841,12 +852,19 @@ pub fn load_and_typecheck_str<'a>(
         Phase::SolveTypes,
         target_info,
         cached_subs,
+        render,
     )? {
         Monomorphized(_) => unreachable!(""),
         TypeChecked(module) => Ok(module),
     }
 }
 
+#[derive(Clone, Copy)]
+pub enum PrintTarget {
+    ColorTerminal,
+    Generic,
+}
+
 pub struct LoadStart<'a> {
     arc_modules: Arc<Mutex<PackageModuleIds<'a>>>,
     ident_ids_by_module: Arc<Mutex<MutMap<ModuleId, IdentIds>>>,
@@ -855,7 +873,11 @@ pub struct LoadStart<'a> {
 }
 
 impl<'a> LoadStart<'a> {
-    pub fn from_path(arena: &'a Bump, filename: PathBuf) -> Result<Self, LoadingProblem<'a>> {
+    pub fn from_path(
+        arena: &'a Bump,
+        filename: PathBuf,
+        render: RenderTarget,
+    ) -> Result<Self, LoadingProblem<'a>> {
         let arc_modules = Arc::new(Mutex::new(PackageModuleIds::default()));
         let root_exposed_ident_ids = IdentIds::exposed_builtins(0);
         let ident_ids_by_module = Arc::new(Mutex::new(root_exposed_ident_ids));
@@ -887,7 +909,12 @@ impl<'a> LoadStart<'a> {
 
                     // if parsing failed, this module did not add any identifiers
                     let root_exposed_ident_ids = IdentIds::exposed_builtins(0);
-                    let buf = to_parse_problem_report(problem, module_ids, root_exposed_ident_ids);
+                    let buf = to_parse_problem_report(
+                        problem,
+                        module_ids,
+                        root_exposed_ident_ids,
+                        render,
+                    );
                     return Err(LoadingProblem::FormattedReport(buf));
                 }
                 Err(LoadingProblem::FileProblem { filename, error }) => {
@@ -995,6 +1022,7 @@ pub fn load<'a>(
     goal_phase: Phase,
     target_info: TargetInfo,
     cached_subs: MutMap<ModuleId, (Subs, Vec<(Symbol, Variable)>)>,
+    render: RenderTarget,
 ) -> Result<LoadResult<'a>, LoadingProblem<'a>> {
     // When compiling to wasm, we cannot spawn extra threads
     // so we have a single-threaded implementation
@@ -1007,6 +1035,7 @@ pub fn load<'a>(
             goal_phase,
             target_info,
             cached_subs,
+            render,
         )
     } else {
         load_multi_threaded(
@@ -1017,6 +1046,7 @@ pub fn load<'a>(
             goal_phase,
             target_info,
             cached_subs,
+            render,
         )
     }
 }
@@ -1031,12 +1061,14 @@ fn load_single_threaded<'a>(
     goal_phase: Phase,
     target_info: TargetInfo,
     cached_subs: MutMap<ModuleId, (Subs, Vec<(Symbol, Variable)>)>,
+    render: RenderTarget,
 ) -> Result<LoadResult<'a>, LoadingProblem<'a>> {
     let LoadStart {
         arc_modules,
         ident_ids_by_module,
         root_id,
         root_msg,
+        ..
     } = load_start;
 
     let (msg_tx, msg_rx) = bounded(1024);
@@ -1053,6 +1085,7 @@ fn load_single_threaded<'a>(
         arc_modules,
         ident_ids_by_module,
         cached_subs,
+        render,
     );
 
     // We'll add tasks to this, and then worker threads will take tasks from it.
@@ -1115,6 +1148,7 @@ fn state_thread_step<'a>(
                     exposed_aliases_by_symbol,
                     dep_idents,
                     documentation,
+                    abilities_store,
                 } => {
                     // We're done! There should be no more messages pending.
                     debug_assert!(msg_rx.is_empty());
@@ -1131,6 +1165,7 @@ fn state_thread_step<'a>(
                         exposed_vars_by_symbol,
                         dep_idents,
                         documentation,
+                        abilities_store,
                     );
 
                     Ok(ControlFlow::Break(LoadResult::TypeChecked(typechecked)))
@@ -1153,8 +1188,12 @@ fn state_thread_step<'a>(
 
                 Msg::FailedToParse(problem) => {
                     let module_ids = (*state.arc_modules).lock().clone().into_module_ids();
-                    let buf =
+                    let buf = to_parse_problem_report(
-                        to_parse_problem_report(problem, module_ids, state.constrained_ident_ids);
+                        problem,
+                        module_ids,
+                        state.constrained_ident_ids,
+                        state.render,
+                    );
                     Err(LoadingProblem::FormattedReport(buf))
                 }
                 msg => {
@@ -1164,6 +1203,8 @@ fn state_thread_step<'a>(
                     let constrained_ident_ids = state.constrained_ident_ids.clone();
                     let arc_modules = state.arc_modules.clone();
 
+                    let render = state.render;
+
                     let res_state = update(
                         state,
                         msg,
@@ -1185,8 +1226,12 @@ fn state_thread_step<'a>(
                                 .into_inner()
                                 .into_module_ids();
 
-                            let buf =
+                            let buf = to_parse_problem_report(
-                                to_parse_problem_report(problem, module_ids, constrained_ident_ids);
+                                problem,
+                                module_ids,
+                                constrained_ident_ids,
+                                render,
+                            );
                             Err(LoadingProblem::FormattedReport(buf))
                         }
                         Err(e) => Err(e),
@@ -1210,12 +1255,14 @@ fn load_multi_threaded<'a>(
     goal_phase: Phase,
     target_info: TargetInfo,
     cached_subs: MutMap<ModuleId, (Subs, Vec<(Symbol, Variable)>)>,
+    render: RenderTarget,
 ) -> Result<LoadResult<'a>, LoadingProblem<'a>> {
     let LoadStart {
         arc_modules,
         ident_ids_by_module,
         root_id,
         root_msg,
+        ..
     } = load_start;
 
     let mut state = State::new(
@@ -1226,6 +1273,7 @@ fn load_multi_threaded<'a>(
         arc_modules,
         ident_ids_by_module,
         cached_subs,
+        render,
     );
 
     let (msg_tx, msg_rx) = bounded(1024);
@@ -1746,6 +1794,7 @@ fn update<'a>(
             decls,
             dep_idents,
             mut module_timing,
+            abilities_store,
         } => {
             log!("solved types for {:?}", module_id);
             module_timing.end_time = SystemTime::now();
@@ -1798,6 +1847,7 @@ fn update<'a>(
                         exposed_aliases_by_symbol: solved_module.aliases,
                         dep_idents,
                         documentation,
+                        abilities_store,
                     })
                     .map_err(|_| LoadingProblem::MsgChannelDied)?;
 
@@ -2126,6 +2176,7 @@ fn finish(
     exposed_vars_by_symbol: Vec<(Symbol, Variable)>,
     dep_idents: MutMap<ModuleId, IdentIds>,
     documentation: MutMap<ModuleId, ModuleDocumentation>,
+    abilities_store: AbilitiesStore,
 ) -> LoadedModule {
     let module_ids = Arc::try_unwrap(state.arc_modules)
         .unwrap_or_else(|_| panic!("There were still outstanding Arc references to module_ids"))
@@ -2160,6 +2211,7 @@ fn finish(
         sources,
         timings: state.timings,
         documentation,
+        abilities_store,
     }
 }
 
@@ -3102,6 +3154,10 @@ fn add_imports(
                     rigid_vars.extend(copied_import.rigid);
                     rigid_vars.extend(copied_import.flex);
 
+                    // Rigid vars bound to abilities are also treated like rigids.
+                    rigid_vars.extend(copied_import.rigid_able);
+                    rigid_vars.extend(copied_import.flex_able);
+
                     import_variables.extend(copied_import.registered);
 
                     def_types.push((
@@ -3119,6 +3175,7 @@ fn add_imports(
     import_variables
 }
 
+#[allow(clippy::complexity)]
 fn run_solve_solve(
     imported_builtins: Vec<Symbol>,
     exposed_for_module: ExposedForModule,
@@ -3126,11 +3183,17 @@ fn run_solve_solve(
     constraint: ConstraintSoa,
     mut var_store: VarStore,
     module: Module,
-) -> (Solved<Subs>, Vec<(Symbol, Variable)>, Vec<solve::TypeError>) {
+) -> (
+    Solved<Subs>,
+    Vec<(Symbol, Variable)>,
+    Vec<solve::TypeError>,
+    AbilitiesStore,
+) {
     let Module {
         exposed_symbols,
         aliases,
         rigid_variables,
+        abilities_store,
         ..
     } = module;
 
@@ -3155,12 +3218,13 @@ fn run_solve_solve(
         solve_aliases.insert(*name, alias.clone());
     }
 
-    let (solved_subs, solved_env, problems) = roc_solve::module::run_solve(
+    let (solved_subs, solved_env, problems, abilities_store) = roc_solve::module::run_solve(
         &constraints,
         actual_constraint,
         rigid_variables,
         subs,
         solve_aliases,
+        abilities_store,
     );
 
     let solved_subs = if true {
@@ -3179,7 +3243,12 @@ fn run_solve_solve(
         .filter(|(k, _)| exposed_symbols.contains(k))
         .collect();
 
-    (solved_subs, exposed_vars_by_symbol, problems)
+    (
+        solved_subs,
+        exposed_vars_by_symbol,
+        problems,
+        abilities_store,
+    )
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -3203,7 +3272,7 @@ fn run_solve<'a>(
     // TODO remove when we write builtins in roc
     let aliases = module.aliases.clone();
 
-    let (solved_subs, exposed_vars_by_symbol, problems) = {
+    let (solved_subs, exposed_vars_by_symbol, problems, abilities_store) = {
         if module_id.is_builtin() {
             match cached_subs.lock().remove(&module_id) {
                 None => {
@@ -3217,9 +3286,13 @@ fn run_solve<'a>(
                         module,
                     )
                 }
-                Some((subs, exposed_vars_by_symbol)) => {
+                Some((subs, exposed_vars_by_symbol)) => (
-                    (Solved(subs), exposed_vars_by_symbol.to_vec(), vec![])
+                    Solved(subs),
-                }
+                    exposed_vars_by_symbol.to_vec(),
+                    vec![],
+                    // TODO(abilities) replace when we have abilities for builtins
+                    AbilitiesStore::default(),
+                ),
             }
         } else {
             run_solve_solve(
@@ -3258,6 +3331,7 @@ fn run_solve<'a>(
         dep_idents,
         solved_module,
         module_timing,
+        abilities_store,
     }
 }
 
@@ -3385,8 +3459,12 @@ fn canonicalize_and_constrain<'a>(
 
             let mut constraints = Constraints::new();
 
-            let constraint =
+            let constraint = constrain_module(
-                constrain_module(&mut constraints, &module_output.declarations, module_id);
+                &mut constraints,
+                &module_output.scope.abilities_store,
+                &module_output.declarations,
+                module_id,
+            );
 
             let after = roc_types::types::get_type_clone_count();
 
@@ -3426,6 +3504,7 @@ fn canonicalize_and_constrain<'a>(
                 referenced_types: module_output.referenced_types,
                 aliases,
                 rigid_variables: module_output.rigid_variables,
+                abilities_store: module_output.scope.abilities_store,
             };
 
             let constrained_module = ConstrainedModule {
@@ -4069,6 +4148,7 @@ fn to_parse_problem_report<'a>(
     problem: FileError<'a, SyntaxError<'a>>,
     mut module_ids: ModuleIds,
     all_ident_ids: MutMap<ModuleId, IdentIds>,
+    render: RenderTarget,
 ) -> String {
     use roc_reporting::report::{parse_problem, RocDocAllocator, DEFAULT_PALETTE};
 
@@ -4103,7 +4183,7 @@ fn to_parse_problem_report<'a>(
     let mut buf = String::new();
     let palette = DEFAULT_PALETTE;
 
-    report.render_color_terminal(&mut buf, &alloc, &palette);
+    report.render(render, &mut buf, &alloc, &palette);
 
     buf
 }

compiler/load_internal/tests/test_load.rs

@@ -25,6 +25,7 @@ mod test_load {
     use roc_problem::can::Problem;
     use roc_region::all::LineInfo;
     use roc_reporting::report::can_problem;
+    use roc_reporting::report::RenderTarget;
     use roc_reporting::report::RocDocAllocator;
     use roc_target::TargetInfo;
     use roc_types::pretty_print::{content_to_string, name_all_type_vars};
@@ -41,7 +42,7 @@ mod test_load {
     ) -> Result<LoadedModule, LoadingProblem<'a>> {
         use LoadResult::*;
 
-        let load_start = LoadStart::from_path(arena, filename)?;
+        let load_start = LoadStart::from_path(arena, filename, RenderTarget::Generic)?;
 
         match roc_load_internal::file::load(
             arena,
@@ -51,6 +52,7 @@ mod test_load {
             Phase::SolveTypes,
             target_info,
             Default::default(), // these tests will re-compile the builtins
+            RenderTarget::Generic,
         )? {
             Monomorphized(_) => unreachable!(""),
             TypeChecked(module) => Ok(module),
@@ -88,8 +90,6 @@ mod test_load {
     }
 
     fn multiple_modules(files: Vec<(&str, &str)>) -> Result<LoadedModule, String> {
-        use roc_load_internal::file::LoadingProblem;
-
         let arena = Bump::new();
         let arena = &arena;
 
@@ -589,18 +589,18 @@ mod test_load {
                 report,
                 indoc!(
                     "
-            \u{1b}[36m── UNFINISHED LIST ─────────────────────────────────────────────────────────────\u{1b}[0m
+                    ── UNFINISHED LIST ─────────────────────────────────────────────────────────────
 
-            I cannot find the end of this list:
+                    I cannot find the end of this list:
 
-            \u{1b}[36m3\u{1b}[0m\u{1b}[36m│\u{1b}[0m  \u{1b}[37mmain = [\u{1b}[0m
+                    3│  main = [
-                        \u{1b}[31m^\u{1b}[0m
+                                ^
 
-            You could change it to something like \u{1b}[33m[ 1, 2, 3 ]\u{1b}[0m or even just \u{1b}[33m[]\u{1b}[0m.
+                    You could change it to something like [ 1, 2, 3 ] or even just [].
-            Anything where there is an open and a close square bracket, and where
+                    Anything where there is an open and a close square bracket, and where
-            the elements of the list are separated by commas.
+                    the elements of the list are separated by commas.
 
-            \u{1b}[4mNote\u{1b}[0m: I may be confused by indentation"
+                    Note: I may be confused by indentation"
                 )
             ),
             Ok(_) => unreachable!("we expect failure here"),
@@ -769,8 +769,6 @@ mod test_load {
         ];
 
         let err = multiple_modules(modules).unwrap_err();
-        let err = strip_ansi_escapes::strip(err).unwrap();
-        let err = String::from_utf8(err).unwrap();
         assert_eq!(
             err,
             indoc!(

compiler/mono/src/layout.rs

@@ -3,6 +3,7 @@ use bumpalo::collections::Vec;
 use bumpalo::Bump;
 use roc_builtins::bitcode::{FloatWidth, IntWidth};
 use roc_collections::all::{default_hasher, MutMap};
+use roc_error_macros::todo_abilities;
 use roc_module::ident::{Lowercase, TagName};
 use roc_module::symbol::{Interns, Symbol};
 use roc_problem::can::RuntimeError;
@@ -72,6 +73,7 @@ impl<'a> RawFunctionLayout<'a> {
         use roc_types::subs::Content::*;
         match content {
             FlexVar(_) | RigidVar(_) => Err(LayoutProblem::UnresolvedTypeVar(var)),
+            FlexAbleVar(_, _) | RigidAbleVar(_, _) => todo_abilities!("Not reachable yet"),
             RecursionVar { structure, .. } => {
                 let structure_content = env.subs.get_content_without_compacting(structure);
                 Self::new_help(env, structure, *structure_content)
@@ -952,6 +954,7 @@ impl<'a> Layout<'a> {
         use roc_types::subs::Content::*;
         match content {
             FlexVar(_) | RigidVar(_) => Err(LayoutProblem::UnresolvedTypeVar(var)),
+            FlexAbleVar(_, _) | RigidAbleVar(_, _) => todo_abilities!("Not reachable yet"),
             RecursionVar { structure, .. } => {
                 let structure_content = env.subs.get_content_without_compacting(structure);
                 Self::new_help(env, structure, *structure_content)
@@ -2661,6 +2664,7 @@ fn layout_from_num_content<'a>(
             // (e.g. for (5 + 5) assume both 5s are 64-bit integers.)
             Ok(Layout::default_integer())
         }
+        FlexAbleVar(_, _) | RigidAbleVar(_, _) => todo_abilities!("Not reachable yet"),
         Structure(Apply(symbol, args)) => match *symbol {
             // Ints
             Symbol::NUM_NAT => Ok(Layout::usize(target_info)),

compiler/mono/src/layout_soa.rs

@@ -137,8 +137,10 @@ impl FunctionLayout {
         use LayoutError::*;
 
         match content {
-            Content::FlexVar(_) => Err(UnresolvedVariable(var)),
+            Content::FlexVar(_)
-            Content::RigidVar(_) => Err(UnresolvedVariable(var)),
+            | Content::RigidVar(_)
+            | Content::FlexAbleVar(_, _)
+            | Content::RigidAbleVar(_, _) => Err(UnresolvedVariable(var)),
             Content::RecursionVar { .. } => Err(TypeError(())),
             Content::Structure(flat_type) => Self::from_flat_type(layouts, subs, flat_type),
             Content::Alias(_, _, actual, _) => Self::from_var_help(layouts, subs, *actual),
@@ -243,8 +245,10 @@ impl LambdaSet {
         use LayoutError::*;
 
         match content {
-            Content::FlexVar(_) => Err(UnresolvedVariable(var)),
+            Content::FlexVar(_)
-            Content::RigidVar(_) => Err(UnresolvedVariable(var)),
+            | Content::RigidVar(_)
+            | Content::FlexAbleVar(_, _)
+            | Content::RigidAbleVar(_, _) => Err(UnresolvedVariable(var)),
             Content::RecursionVar { .. } => {
                 unreachable!("lambda sets cannot currently be recursive")
             }
@@ -627,8 +631,10 @@ impl Layout {
         use LayoutError::*;
 
         match content {
-            Content::FlexVar(_) => Err(UnresolvedVariable(var)),
+            Content::FlexVar(_)
-            Content::RigidVar(_) => Err(UnresolvedVariable(var)),
+            | Content::RigidVar(_)
+            | Content::FlexAbleVar(_, _)
+            | Content::RigidAbleVar(_, _) => Err(UnresolvedVariable(var)),
             Content::RecursionVar {
                 structure,
                 opt_name: _,

compiler/problem/src/can.rs

@@ -119,6 +119,13 @@ pub enum Problem {
         ability: Symbol,
         region: Region,
     },
+    AbilityMemberMultipleBoundVars {
+        member: Symbol,
+        ability: Symbol,
+        span_has_clauses: Region,
+        bound_var_names: Vec<Lowercase>,
+    },
+    // TODO(abilities): remove me when ability hierarchies are supported
     AbilityMemberBindsExternalAbility {
         member: Symbol,
         ability: Symbol,

compiler/solve/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 
 [dependencies]
 roc_collections = { path = "../collections" }
+roc_error_macros = { path = "../../error_macros" }
 roc_region = { path = "../region" }
 roc_module = { path = "../module" }
 roc_types = { path = "../types" }
@@ -22,6 +23,7 @@ roc_problem = { path = "../problem" }
 roc_parse = { path = "../parse" }
 roc_solve = { path = "../solve" }
 roc_target = { path = "../roc_target" }
+roc_reporting = { path = "../../reporting" }
 pretty_assertions = "1.0.0"
 indoc = "1.0.3"
 tempfile = "3.2.0"

compiler/solve/src/module.rs

@@ -1,4 +1,5 @@
 use crate::solve::{self, Aliases};
+use roc_can::abilities::AbilitiesStore;
 use roc_can::constraint::{Constraint as ConstraintSoa, Constraints};
 use roc_can::module::RigidVariables;
 use roc_collections::all::MutMap;
@@ -32,13 +33,23 @@ pub fn run_solve(
     rigid_variables: RigidVariables,
     mut subs: Subs,
     mut aliases: Aliases,
-) -> (Solved<Subs>, solve::Env, Vec<solve::TypeError>) {
+    mut abilities_store: AbilitiesStore,
+) -> (
+    Solved<Subs>,
+    solve::Env,
+    Vec<solve::TypeError>,
+    AbilitiesStore,
+) {
     let env = solve::Env::default();
 
     for (var, name) in rigid_variables.named {
         subs.rigid_var(var, name);
     }
 
+    for (var, (name, ability)) in rigid_variables.able {
+        subs.rigid_able_var(var, name, ability);
+    }
+
     for var in rigid_variables.wildcards {
         subs.rigid_var(var, "*".into());
     }
@@ -55,9 +66,10 @@ pub fn run_solve(
         subs,
         &mut aliases,
         &constraint,
+        &mut abilities_store,
     );
 
-    (solved_subs, solved_env, problems)
+    (solved_subs, solved_env, problems, abilities_store)
 }
 
 pub fn exposed_types_storage_subs(

compiler/solve/src/solve.rs

@@ -1,4 +1,5 @@
 use bumpalo::Bump;
+use roc_can::abilities::{AbilitiesStore, MemberSpecialization};
 use roc_can::constraint::Constraint::{self, *};
 use roc_can::constraint::{Constraints, LetConstraint};
 use roc_can::expected::{Expected, PExpected};
@@ -14,9 +15,9 @@ use roc_types::subs::{
 use roc_types::types::Type::{self, *};
 use roc_types::types::{
     gather_fields_unsorted_iter, AliasCommon, AliasKind, Category, ErrorType, PatternCategory,
-    TypeExtension,
+    Reason, TypeExtension,
 };
-use roc_unify::unify::{unify, Mode, Unified::*};
+use roc_unify::unify::{unify, Mode, MustImplementAbility, Unified::*};
 
 // Type checking system adapted from Elm by Evan Czaplicki, BSD-3-Clause Licensed
 // https://github.com/elm/compiler
@@ -75,6 +76,13 @@ pub enum TypeError {
     CircularType(Region, Symbol, ErrorType),
     BadType(roc_types::types::Problem),
     UnexposedLookup(Symbol),
+    IncompleteAbilityImplementation {
+        // TODO(abilities): have general types here, not just opaques
+        typ: Symbol,
+        ability: Symbol,
+        specialized_members: Vec<Loc<Symbol>>,
+        missing_members: Vec<Loc<Symbol>>,
+    },
 }
 
 use roc_types::types::Alias;
@@ -515,8 +523,17 @@ pub fn run(
     mut subs: Subs,
     aliases: &mut Aliases,
     constraint: &Constraint,
+    abilities_store: &mut AbilitiesStore,
 ) -> (Solved<Subs>, Env) {
-    let env = run_in_place(constraints, env, problems, &mut subs, aliases, constraint);
+    let env = run_in_place(
+        constraints,
+        env,
+        problems,
+        &mut subs,
+        aliases,
+        constraint,
+        abilities_store,
+    );
 
     (Solved(subs), env)
 }
@@ -529,6 +546,7 @@ pub fn run_in_place(
     subs: &mut Subs,
     aliases: &mut Aliases,
     constraint: &Constraint,
+    abilities_store: &mut AbilitiesStore,
 ) -> Env {
     let mut pools = Pools::default();
 
@@ -540,6 +558,8 @@ pub fn run_in_place(
 
     let arena = Bump::new();
 
+    let mut deferred_must_implement_abilities = Vec::new();
+
     let state = solve(
         &arena,
         constraints,
@@ -551,8 +571,39 @@ pub fn run_in_place(
         aliases,
         subs,
         constraint,
+        abilities_store,
+        &mut deferred_must_implement_abilities,
     );
 
+    // Now that the module has been solved, we can run through and check all
+    // types claimed to implement abilities.
+    deferred_must_implement_abilities.dedup();
+    for MustImplementAbility { typ, ability } in deferred_must_implement_abilities.into_iter() {
+        let members_of_ability = abilities_store.members_of_ability(ability).unwrap();
+        let mut specialized_members = Vec::with_capacity(members_of_ability.len());
+        let mut missing_members = Vec::with_capacity(members_of_ability.len());
+        for &member in members_of_ability {
+            match abilities_store.get_specialization(member, typ) {
+                None => {
+                    let root_data = abilities_store.member_def(member).unwrap();
+                    missing_members.push(Loc::at(root_data.region, member));
+                }
+                Some(specialization) => {
+                    specialized_members.push(Loc::at(specialization.region, member));
+                }
+            }
+        }
+
+        if !missing_members.is_empty() {
+            problems.push(TypeError::IncompleteAbilityImplementation {
+                typ,
+                ability,
+                specialized_members,
+                missing_members,
+            });
+        }
+    }
+
     state.env
 }
 
@@ -604,6 +655,8 @@ fn solve(
     aliases: &mut Aliases,
     subs: &mut Subs,
     constraint: &Constraint,
+    abilities_store: &mut AbilitiesStore,
+    deferred_must_implement_abilities: &mut Vec<MustImplementAbility>,
 ) -> State {
     let initial = Work::Constraint {
         env,
@@ -752,6 +805,19 @@ fn solve(
 
                 let mut new_env = env.clone();
                 for (symbol, loc_var) in local_def_vars.iter() {
+                    check_ability_specialization(
+                        arena,
+                        subs,
+                        &new_env,
+                        pools,
+                        rank,
+                        abilities_store,
+                        problems,
+                        deferred_must_implement_abilities,
+                        *symbol,
+                        *loc_var,
+                    );
+
                     new_env.insert_symbol_var_if_vacant(*symbol, loc_var.value);
                 }
 
@@ -796,12 +862,15 @@ fn solve(
                 let expected = type_to_var(subs, rank, pools, aliases, expectation.get_type_ref());
 
                 match unify(subs, actual, expected, Mode::EQ) {
-                    Success(vars) => {
+                    Success {
+                        vars,
+                        must_implement_ability: _,
+                    } => {
                         introduce(subs, rank, pools, &vars);
 
                         state
                     }
-                    Failure(vars, actual_type, expected_type) => {
+                    Failure(vars, actual_type, expected_type, _bad_impls) => {
                         introduce(subs, rank, pools, &vars);
 
                         let problem = TypeError::BadExpr(
@@ -838,12 +907,15 @@ fn solve(
                 let target = *target;
 
                 match unify(subs, actual, target, Mode::EQ) {
-                    Success(vars) => {
+                    Success {
+                        vars,
+                        must_implement_ability: _,
+                    } => {
                         introduce(subs, rank, pools, &vars);
 
                         state
                     }
-                    Failure(vars, _actual_type, _expected_type) => {
+                    Failure(vars, _actual_type, _expected_type, _bad_impls) => {
                         introduce(subs, rank, pools, &vars);
 
                         // ERROR NOT REPORTED
@@ -890,13 +962,16 @@ fn solve(
                             type_to_var(subs, rank, pools, aliases, expectation.get_type_ref());
 
                         match unify(subs, actual, expected, Mode::EQ) {
-                            Success(vars) => {
+                            Success {
+                                vars,
+                                must_implement_ability: _,
+                            } => {
                                 introduce(subs, rank, pools, &vars);
 
                                 state
                             }
 
-                            Failure(vars, actual_type, expected_type) => {
+                            Failure(vars, actual_type, expected_type, _bad_impls) => {
                                 introduce(subs, rank, pools, &vars);
 
                                 let problem = TypeError::BadExpr(
@@ -954,12 +1029,15 @@ fn solve(
                 };
 
                 match unify(subs, actual, expected, mode) {
-                    Success(vars) => {
+                    Success {
+                        vars,
+                        must_implement_ability: _,
+                    } => {
                         introduce(subs, rank, pools, &vars);
 
                         state
                     }
-                    Failure(vars, actual_type, expected_type) => {
+                    Failure(vars, actual_type, expected_type, _bad_impls) => {
                         introduce(subs, rank, pools, &vars);
 
                         let problem = TypeError::BadPattern(
@@ -1123,12 +1201,15 @@ fn solve(
                 let includes = type_to_var(subs, rank, pools, aliases, &tag_ty);
 
                 match unify(subs, actual, includes, Mode::PRESENT) {
-                    Success(vars) => {
+                    Success {
+                        vars,
+                        must_implement_ability: _,
+                    } => {
                         introduce(subs, rank, pools, &vars);
 
                         state
                     }
-                    Failure(vars, actual_type, expected_to_include_type) => {
+                    Failure(vars, actual_type, expected_to_include_type, _bad_impls) => {
                         introduce(subs, rank, pools, &vars);
 
                         let problem = TypeError::BadPattern(
@@ -1156,6 +1237,136 @@ fn solve(
     state
 }
 
+/// If a symbol claims to specialize an ability member, check that its solved type in fact
+/// does specialize the ability, and record the specialization.
+#[allow(clippy::too_many_arguments)]
+// Aggressive but necessary - there aren't many usages.
+#[inline(always)]
+fn check_ability_specialization(
+    arena: &Bump,
+    subs: &mut Subs,
+    env: &Env,
+    pools: &mut Pools,
+    rank: Rank,
+    abilities_store: &mut AbilitiesStore,
+    problems: &mut Vec<TypeError>,
+    deferred_must_implement_abilities: &mut Vec<MustImplementAbility>,
+    symbol: Symbol,
+    symbol_loc_var: Loc<Variable>,
+) {
+    // If the symbol specializes an ability member, we need to make sure that the
+    // inferred type for the specialization actually aligns with the expected
+    // implementation.
+    if let Some((root_symbol, root_data)) = abilities_store.root_name_and_def(symbol) {
+        let root_signature_var = env
+            .get_var_by_symbol(&root_symbol)
+            .expect("Ability should be registered in env by now!");
+
+        // Check if they unify - if they don't, then the claimed specialization isn't really one,
+        // and that's a type error!
+        // This also fixes any latent type variables that need to be specialized to exactly what
+        // the ability signature expects.
+
+        // We need to freshly instantiate the root signature so that all unifications are reflected
+        // in the specialization type, but not the original signature type.
+        let root_signature_var =
+            deep_copy_var_in(subs, Rank::toplevel(), pools, root_signature_var, arena);
+        let snapshot = subs.snapshot();
+        let unified = unify(subs, symbol_loc_var.value, root_signature_var, Mode::EQ);
+
+        match unified {
+            Success {
+                vars: _,
+                must_implement_ability,
+            } if must_implement_ability.is_empty() => {
+                // This can happen when every ability constriant on a type variable went
+                // through only another type variable. That means this def is not specialized
+                // for one type - for now, we won't admit this.
+
+                // Rollback the snapshot so we unlink the root signature with the specialization,
+                // so we can have two separate error types.
+                subs.rollback_to(snapshot);
+
+                let (expected_type, _problems) = subs.var_to_error_type(root_signature_var);
+                let (actual_type, _problems) = subs.var_to_error_type(symbol_loc_var.value);
+
+                let reason = Reason::GeneralizedAbilityMemberSpecialization {
+                    member_name: root_symbol,
+                    def_region: root_data.region,
+                };
+
+                let problem = TypeError::BadExpr(
+                    symbol_loc_var.region,
+                    Category::AbilityMemberSpecialization(root_symbol),
+                    actual_type,
+                    Expected::ForReason(reason, expected_type, symbol_loc_var.region),
+                );
+
+                problems.push(problem);
+            }
+
+            Success {
+                vars,
+                must_implement_ability,
+            } => {
+                subs.commit_snapshot(snapshot);
+                introduce(subs, rank, pools, &vars);
+
+                // First, figure out and register for what type does this symbol specialize
+                // the ability member.
+                let mut ability_implementations_for_specialization = must_implement_ability
+                    .iter()
+                    .filter(|mia| mia.ability == root_data.parent_ability)
+                    .collect::<Vec<_>>();
+                ability_implementations_for_specialization.dedup();
+
+                debug_assert!(ability_implementations_for_specialization.len() == 1, "Multiple variables bound to an ability - this is ambiguous and should have been caught in canonicalization");
+
+                // This is a valid specialization! Record it.
+                let specialization_type = ability_implementations_for_specialization[0].typ;
+                let specialization = MemberSpecialization {
+                    symbol,
+                    region: symbol_loc_var.region,
+                };
+                abilities_store.register_specialization_for_type(
+                    root_symbol,
+                    specialization_type,
+                    specialization,
+                );
+
+                // Store the checks for what abilities must be implemented to be checked after the
+                // whole module is complete.
+                deferred_must_implement_abilities.extend(must_implement_ability);
+            }
+            Failure(vars, actual_type, expected_type, unimplemented_abilities) => {
+                subs.commit_snapshot(snapshot);
+                introduce(subs, rank, pools, &vars);
+
+                let reason = Reason::InvalidAbilityMemberSpecialization {
+                    member_name: root_symbol,
+                    def_region: root_data.region,
+                    unimplemented_abilities,
+                };
+
+                let problem = TypeError::BadExpr(
+                    symbol_loc_var.region,
+                    Category::AbilityMemberSpecialization(root_symbol),
+                    actual_type,
+                    Expected::ForReason(reason, expected_type, symbol_loc_var.region),
+                );
+
+                problems.push(problem);
+            }
+            BadType(vars, problem) => {
+                subs.commit_snapshot(snapshot);
+                introduce(subs, rank, pools, &vars);
+
+                problems.push(TypeError::BadType(problem));
+            }
+        }
+    }
+}
+
 #[derive(Debug)]
 enum LocalDefVarsVec<T> {
     Stack(arrayvec::ArrayVec<T, 32>),
@@ -1288,7 +1499,7 @@ impl RegisterVariable {
         use RegisterVariable::*;
 
         match typ {
-            Variable(var) => Direct(*var),
+            Type::Variable(var) => Direct(*var),
             EmptyRec => Direct(Variable::EMPTY_RECORD),
             EmptyTagUnion => Direct(Variable::EMPTY_TAG_UNION),
             Type::DelayedAlias(AliasCommon { symbol, .. }) => {
@@ -2180,7 +2391,7 @@ fn adjust_rank_content(
     use roc_types::subs::FlatType::*;
 
     match content {
-        FlexVar(_) | RigidVar(_) | Error => group_rank,
+        FlexVar(_) | RigidVar(_) | FlexAbleVar(_, _) | RigidAbleVar(_, _) | Error => group_rank,
 
         RecursionVar { .. } => group_rank,
 
@@ -2396,7 +2607,14 @@ fn instantiate_rigids_help(subs: &mut Subs, max_rank: Rank, initial: Variable) {
                 desc.mark = Mark::NONE;
                 desc.copy = OptVariable::NONE;
             }
-            FlexVar(_) | Error => (),
+            &RigidAbleVar(name, ability) => {
+                // Same as `RigidVar` above
+                desc.content = FlexAbleVar(Some(name), ability);
+                desc.rank = max_rank;
+                desc.mark = Mark::NONE;
+                desc.copy = OptVariable::NONE;
+            }
+            FlexVar(_) | FlexAbleVar(_, _) | Error => (),
 
             RecursionVar { structure, .. } => {
                 stack.push(*structure);
@@ -2684,7 +2902,7 @@ fn deep_copy_var_help(
             copy
         }
 
-        FlexVar(_) | Error => copy,
+        FlexVar(_) | FlexAbleVar(_, _) | Error => copy,
 
         RecursionVar {
             opt_name,
@@ -2709,6 +2927,12 @@ fn deep_copy_var_help(
             copy
         }
 
+        RigidAbleVar(name, ability) => {
+            subs.set(copy, make_descriptor(FlexAbleVar(Some(name), ability)));
+
+            copy
+        }
+
         Alias(symbol, arguments, real_type_var, kind) => {
             let new_variables =
                 SubsSlice::reserve_into_subs(subs, arguments.all_variables_len as _);

compiler/solve/tests/solve_expr.rs

@@ -10,20 +10,12 @@ mod helpers;
 #[cfg(test)]
 mod solve_expr {
     use crate::helpers::with_larger_debug_stack;
+    use roc_load::LoadedModule;
     use roc_types::pretty_print::{content_to_string, name_all_type_vars};
 
     // HELPERS
 
-    fn infer_eq_help(
+    fn run_load_and_infer(src: &str) -> Result<LoadedModule, std::io::Error> {
-        src: &str,
-    ) -> Result<
-        (
-            Vec<roc_solve::solve::TypeError>,
-            Vec<roc_problem::can::Problem>,
-            String,
-        ),
-        std::io::Error,
-    > {
         use bumpalo::Bump;
         use std::fs::File;
         use std::io::Write;
@@ -58,6 +50,7 @@ mod solve_expr {
                 dir.path(),
                 exposed_types,
                 roc_target::TargetInfo::default_x86_64(),
+                roc_reporting::report::RenderTarget::Generic,
             );
 
             dir.close()?;
@@ -66,8 +59,19 @@ mod solve_expr {
         };
 
         let loaded = loaded.expect("failed to load module");
+        Ok(loaded)
+    }
 
-        use roc_load::LoadedModule;
+    fn infer_eq_help(
+        src: &str,
+    ) -> Result<
+        (
+            Vec<roc_solve::solve::TypeError>,
+            Vec<roc_problem::can::Problem>,
+            String,
+        ),
+        std::io::Error,
+    > {
         let LoadedModule {
             module_id: home,
             mut can_problems,
@@ -76,7 +80,7 @@ mod solve_expr {
             mut solved,
             exposed_to_host,
             ..
-        } = loaded;
+        } = run_load_and_infer(src)?;
 
         let mut can_problems = can_problems.remove(&home).unwrap_or_default();
         let type_problems = type_problems.remove(&home).unwrap_or_default();
@@ -155,6 +159,51 @@ mod solve_expr {
         assert_eq!(actual, expected.to_string());
     }
 
+    fn check_inferred_abilities<'a, I>(src: &'a str, expected_specializations: I)
+    where
+        I: IntoIterator<Item = (&'a str, &'a str)>,
+    {
+        let LoadedModule {
+            module_id: home,
+            mut can_problems,
+            mut type_problems,
+            interns,
+            abilities_store,
+            ..
+        } = run_load_and_infer(src).unwrap();
+
+        let can_problems = can_problems.remove(&home).unwrap_or_default();
+        let type_problems = type_problems.remove(&home).unwrap_or_default();
+
+        assert_eq!(can_problems, Vec::new(), "Canonicalization problems: ");
+
+        if !type_problems.is_empty() {
+            eprintln!("{:?}", type_problems);
+            panic!();
+        }
+
+        let known_specializations = abilities_store.get_known_specializations();
+        use std::collections::HashSet;
+        let pretty_specializations = known_specializations
+            .into_iter()
+            .map(|(typ, member)| {
+                (
+                    typ.ident_str(&interns).as_str(),
+                    member.ident_str(&interns).as_str(),
+                )
+            })
+            .collect::<HashSet<_>>();
+
+        for expected_spec in expected_specializations.into_iter() {
+            assert!(
+                pretty_specializations.contains(&expected_spec),
+                "{:#?} not in {:#?}",
+                expected_spec,
+                pretty_specializations,
+            );
+        }
+    }
+
     #[test]
     fn int_literal() {
         infer_eq("5", "Num *");
@@ -5710,4 +5759,85 @@ mod solve_expr {
             "a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, aa, bb -> { a : a, aa : aa, b : b, bb : bb, c : c, d : d, e : e, f : f, g : g, h : h, i : i, j : j, k : k, l : l, m : m, n : n, o : o, p : p, q : q, r : r, s : s, t : t, u : u, v : v, w : w, x : x, y : y, z : z }",
         )
     }
+
+    #[test]
+    fn exposed_ability_name() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                app "test" provides [ hash ] to "./platform"
+
+                Hash has hash : a -> U64 | a has Hash
+                "#
+            ),
+            "a -> U64 | a has Hash",
+        )
+    }
+
+    #[test]
+    fn single_ability_single_member_specializations() {
+        check_inferred_abilities(
+            indoc!(
+                r#"
+                app "test" provides [ hash ] to "./platform"
+
+                Hash has hash : a -> U64 | a has Hash
+
+                Id := U64
+
+                hash = \$Id n -> n
+                "#
+            ),
+            [("hash", "Id")],
+        )
+    }
+
+    #[test]
+    fn single_ability_multiple_members_specializations() {
+        check_inferred_abilities(
+            indoc!(
+                r#"
+                app "test" provides [ hash, hash32 ] to "./platform"
+
+                Hash has
+                    hash : a -> U64 | a has Hash
+                    hash32 : a -> U32 | a has Hash
+
+                Id := U64
+
+                hash = \$Id n -> n
+                hash32 = \$Id n -> Num.toU32 n
+                "#
+            ),
+            [("hash", "Id"), ("hash32", "Id")],
+        )
+    }
+
+    #[test]
+    fn multiple_abilities_multiple_members_specializations() {
+        check_inferred_abilities(
+            indoc!(
+                r#"
+                app "test" provides [ hash, hash32, eq, le ] to "./platform"
+
+                Hash has
+                    hash : a -> U64 | a has Hash
+                    hash32 : a -> U32 | a has Hash
+
+                Ord has
+                    eq : a, a -> Bool | a has Ord
+                    le : a, a -> Bool | a has Ord
+
+                Id := U64
+
+                hash = \$Id n -> n
+                hash32 = \$Id n -> Num.toU32 n
+
+                eq = \$Id m, $Id n -> m == n
+                le = \$Id m, $Id n -> m < n
+                "#
+            ),
+            [("hash", "Id"), ("hash32", "Id"), ("eq", "Id"), ("le", "Id")],
+        )
+    }
 }

compiler/test_gen/src/helpers/dev.rs

@@ -54,6 +54,7 @@ pub fn helper(
         src_dir,
         Default::default(),
         roc_target::TargetInfo::default_x86_64(),
+        roc_reporting::report::RenderTarget::ColorTerminal,
     );
 
     let mut loaded = loaded.expect("failed to load module");

compiler/test_gen/src/helpers/llvm.rs

@@ -7,6 +7,7 @@ use roc_collections::all::MutSet;
 use roc_gen_llvm::llvm::externs::add_default_roc_externs;
 use roc_mono::ir::OptLevel;
 use roc_region::all::LineInfo;
+use roc_reporting::report::RenderTarget;
 use target_lexicon::Triple;
 
 fn promote_expr_to_module(src: &str) -> String {
@@ -57,6 +58,7 @@ fn create_llvm_module<'a>(
         src_dir,
         Default::default(),
         target_info,
+        RenderTarget::ColorTerminal,
     );
 
     let mut loaded = match loaded {

compiler/test_gen/src/helpers/wasm.rs

@@ -91,6 +91,7 @@ fn compile_roc_to_wasm_bytes<'a, T: Wasm32Result>(
         src_dir,
         Default::default(),
         roc_target::TargetInfo::default_wasm32(),
+        roc_reporting::report::RenderTarget::ColorTerminal,
     );
 
     let loaded = loaded.expect("failed to load module");

compiler/test_mono/Cargo.toml

@@ -17,6 +17,7 @@ roc_load = { path = "../load" }
 roc_can = { path = "../can" }
 roc_mono = { path = "../mono" }
 roc_target = { path = "../roc_target" }
+roc_reporting = { path = "../../reporting" }
 test_mono_macros = { path = "../test_mono_macros" }
 pretty_assertions = "1.0.0"
 bumpalo = { version = "3.8.0", features = ["collections"] }

compiler/test_mono/src/tests.rs

@@ -101,6 +101,7 @@ fn compiles_to_ir(test_name: &str, src: &str) {
         src_dir,
         Default::default(),
         TARGET_INFO,
+        roc_reporting::report::RenderTarget::Generic,
     );
 
     let mut loaded = match loaded {

compiler/types/src/pretty_print.rs

@@ -116,7 +116,7 @@ fn find_names_needed(
     }
 
     match &subs.get_content_without_compacting(variable).clone() {
-        RecursionVar { opt_name: None, .. } | FlexVar(None) => {
+        RecursionVar { opt_name: None, .. } | FlexVar(None) | FlexAbleVar(None, _) => {
             let root = subs.get_root_key_without_compacting(variable);
 
             // If this var is *not* its own root, then the
@@ -139,7 +139,8 @@ fn find_names_needed(
             opt_name: Some(name_index),
             ..
         }
-        | FlexVar(Some(name_index)) => {
+        | FlexVar(Some(name_index))
+        | FlexAbleVar(Some(name_index), _) => {
             // This root already has a name. Nothing more to do here!
 
             // User-defined names are already taken.
@@ -147,7 +148,7 @@ fn find_names_needed(
             let name = subs.field_names[name_index.index as usize].clone();
             names_taken.insert(name);
         }
-        RigidVar(name_index) => {
+        RigidVar(name_index) | RigidAbleVar(name_index, _) => {
             // User-defined names are already taken.
             // We must not accidentally generate names that collide with them!
             let name = subs.field_names[name_index.index as usize].clone();
@@ -289,6 +290,11 @@ fn set_root_name(root: Variable, name: Lowercase, subs: &mut Subs) {
     }
 }
 
+#[derive(Default)]
+struct Context<'a> {
+    able_variables: Vec<(&'a str, Symbol)>,
+}
+
 pub fn content_to_string(
     content: &Content,
     subs: &Subs,
@@ -297,8 +303,16 @@ pub fn content_to_string(
 ) -> String {
     let mut buf = String::new();
     let env = Env { home, interns };
+    let mut ctx = Context::default();
 
-    write_content(&env, content, subs, &mut buf, Parens::Unnecessary);
+    write_content(&env, &mut ctx, content, subs, &mut buf, Parens::Unnecessary);
+
+    for (i, (var, ability)) in ctx.able_variables.into_iter().enumerate() {
+        buf.push_str(if i == 0 { " | " } else { ", " });
+        buf.push_str(var);
+        buf.push_str(" has ");
+        write_symbol(&env, ability, &mut buf);
+    }
 
     buf
 }
@@ -314,7 +328,14 @@ pub fn get_single_arg<'a>(subs: &'a Subs, args: &'a AliasVariables) -> &'a Conte
     subs.get_content_without_compacting(arg_var)
 }
 
-fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, parens: Parens) {
+fn write_content<'a>(
+    env: &Env,
+    ctx: &mut Context<'a>,
+    content: &Content,
+    subs: &'a Subs,
+    buf: &mut String,
+    parens: Parens,
+) {
     use crate::subs::Content::*;
 
     match content {
@@ -327,6 +348,18 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
             let name = &subs.field_names[name_index.index as usize];
             buf.push_str(name.as_str())
         }
+        FlexAbleVar(opt_name_index, ability) => {
+            let name = opt_name_index
+                .map(|name_index| subs.field_names[name_index.index as usize].as_str())
+                .unwrap_or(WILDCARD);
+            ctx.able_variables.push((name, *ability));
+            buf.push_str(name);
+        }
+        RigidAbleVar(name_index, ability) => {
+            let name = subs.field_names[name_index.index as usize].as_str();
+            ctx.able_variables.push((name, *ability));
+            buf.push_str(name);
+        }
         RecursionVar { opt_name, .. } => match opt_name {
             Some(name_index) => {
                 let name = &subs.field_names[name_index.index as usize];
@@ -334,7 +367,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
             }
             None => buf.push_str(WILDCARD),
         },
-        Structure(flat_type) => write_flat_type(env, flat_type, subs, buf, parens),
+        Structure(flat_type) => write_flat_type(env, ctx, flat_type, subs, buf, parens),
         Alias(symbol, args, _actual, _kind) => {
             let write_parens = parens == Parens::InTypeParam && !args.is_empty();
 
@@ -346,6 +379,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
                             Symbol::NUM_INTEGER => {
                                 write_integer(
                                     env,
+                                    ctx,
                                     get_single_arg(subs, &args),
                                     subs,
                                     buf,
@@ -357,13 +391,13 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
 
                             _ => write_parens!(write_parens, buf, {
                                 buf.push_str("Num ");
-                                write_content(env, content, subs, buf, parens);
+                                write_content(env, ctx, content, subs, buf, parens);
                             }),
                         },
 
                         _ => write_parens!(write_parens, buf, {
                             buf.push_str("Num ");
-                            write_content(env, content, subs, buf, parens);
+                            write_content(env, ctx, content, subs, buf, parens);
                         }),
                     }
                 }
@@ -371,7 +405,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
                 Symbol::NUM_INT => {
                     let content = get_single_arg(subs, args);
 
-                    write_integer(env, content, subs, buf, parens, write_parens)
+                    write_integer(env, ctx, content, subs, buf, parens, write_parens)
                 }
 
                 Symbol::NUM_FLOAT => {
@@ -390,7 +424,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
                         Alias(Symbol::NUM_DECIMAL, _, _, _) => buf.push_str("Dec"),
                         _ => write_parens!(write_parens, buf, {
                             buf.push_str("Float ");
-                            write_content(env, content, subs, buf, parens);
+                            write_content(env, ctx, content, subs, buf, parens);
                         }),
                     }
                 }
@@ -403,6 +437,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
                         buf.push(' ');
                         write_content(
                             env,
+                            ctx,
                             subs.get_content_without_compacting(var),
                             subs,
                             buf,
@@ -414,7 +449,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
                     if false {
                         buf.push_str("[[ but really ");
                         let content = subs.get_content_without_compacting(*_actual);
-                        write_content(env, content, subs, buf, parens);
+                        write_content(env, ctx, content, subs, buf, parens);
                         buf.push_str("]]");
                     }
                 }),
@@ -422,6 +457,7 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
         }
         RangedNumber(typ, _range_vars) => write_content(
             env,
+            ctx,
             subs.get_content_without_compacting(*typ),
             subs,
             buf,
@@ -431,10 +467,11 @@ fn write_content(env: &Env, content: &Content, subs: &Subs, buf: &mut String, pa
     }
 }
 
-fn write_integer(
+fn write_integer<'a>(
     env: &Env,
+    ctx: &mut Context<'a>,
     content: &Content,
-    subs: &Subs,
+    subs: &'a Subs,
     buf: &mut String,
     parens: Parens,
     write_parens: bool,
@@ -454,7 +491,7 @@ fn write_integer(
                     )*
                     actual => {
                         buf.push_str("Int ");
-                        write_content(env, actual, subs, buf, parens);
+                        write_content(env, ctx, actual, subs, buf, parens);
                     }
                 }
             )
@@ -497,6 +534,7 @@ impl<'a> ExtContent<'a> {
 
 fn write_ext_content<'a>(
     env: &Env,
+    ctx: &mut Context<'a>,
     subs: &'a Subs,
     buf: &mut String,
     ext_content: ExtContent<'a>,
@@ -508,12 +546,13 @@ fn write_ext_content<'a>(
         //
         // e.g. the "*" at the end of `{ x: I64 }*`
         // or the "r" at the end of `{ x: I64 }r`
-        write_content(env, content, subs, buf, parens)
+        write_content(env, ctx, content, subs, buf, parens)
     }
 }
 
 fn write_sorted_tags2<'a>(
     env: &Env,
+    ctx: &mut Context<'a>,
     subs: &'a Subs,
     buf: &mut String,
     tags: &UnionTags,
@@ -546,6 +585,7 @@ fn write_sorted_tags2<'a>(
             buf.push(' ');
             write_content(
                 env,
+                ctx,
                 subs.get_content_without_compacting(*var),
                 subs,
                 buf,
@@ -559,6 +599,7 @@ fn write_sorted_tags2<'a>(
 
 fn write_sorted_tags<'a>(
     env: &Env,
+    ctx: &mut Context<'a>,
     subs: &'a Subs,
     buf: &mut String,
     tags: &MutMap<TagName, Vec<Variable>>,
@@ -603,6 +644,7 @@ fn write_sorted_tags<'a>(
             buf.push(' ');
             write_content(
                 env,
+                ctx,
                 subs.get_content_without_compacting(*var),
                 subs,
                 buf,
@@ -614,18 +656,37 @@ fn write_sorted_tags<'a>(
     ExtContent::from_var(subs, ext_var)
 }
 
-fn write_flat_type(env: &Env, flat_type: &FlatType, subs: &Subs, buf: &mut String, parens: Parens) {
+fn write_flat_type<'a>(
+    env: &Env,
+    ctx: &mut Context<'a>,
+    flat_type: &FlatType,
+    subs: &'a Subs,
+    buf: &mut String,
+    parens: Parens,
+) {
     use crate::subs::FlatType::*;
 
     match flat_type {
-        Apply(symbol, args) => {
+        Apply(symbol, args) => write_apply(
-            write_apply(env, *symbol, subs.get_subs_slice(*args), subs, buf, parens)
+            env,
-        }
+            ctx,
+            *symbol,
+            subs.get_subs_slice(*args),
+            subs,
+            buf,
+            parens,
+        ),
         EmptyRecord => buf.push_str(EMPTY_RECORD),
         EmptyTagUnion => buf.push_str(EMPTY_TAG_UNION),
-        Func(args, _closure, ret) => {
+        Func(args, _closure, ret) => write_fn(
-            write_fn(env, subs.get_subs_slice(*args), *ret, subs, buf, parens)
+            env,
-        }
+            ctx,
+            subs.get_subs_slice(*args),
+            *ret,
+            subs,
+            buf,
+            parens,
+        ),
         Record(fields, ext_var) => {
             use crate::types::{gather_fields, RecordStructure};
 
@@ -664,6 +725,7 @@ fn write_flat_type(env: &Env, flat_type: &FlatType, subs: &Subs, buf: &mut Strin
 
                     write_content(
                         env,
+                        ctx,
                         subs.get_content_without_compacting(var),
                         subs,
                         buf,
@@ -684,18 +746,18 @@ fn write_flat_type(env: &Env, flat_type: &FlatType, subs: &Subs, buf: &mut Strin
                     //
                     // e.g. the "*" at the end of `{ x: I64 }*`
                     // or the "r" at the end of `{ x: I64 }r`
-                    write_content(env, content, subs, buf, parens)
+                    write_content(env, ctx, content, subs, buf, parens)
                 }
             }
         }
         TagUnion(tags, ext_var) => {
             buf.push_str("[ ");
 
-            let ext_content = write_sorted_tags2(env, subs, buf, tags, *ext_var);
+            let ext_content = write_sorted_tags2(env, ctx, subs, buf, tags, *ext_var);
 
             buf.push_str(" ]");
 
-            write_ext_content(env, subs, buf, ext_content, parens)
+            write_ext_content(env, ctx, subs, buf, ext_content, parens)
         }
 
         FunctionOrTagUnion(tag_name, _, ext_var) => {
@@ -703,25 +765,26 @@ fn write_flat_type(env: &Env, flat_type: &FlatType, subs: &Subs, buf: &mut Strin
 
             let mut tags: MutMap<TagName, _> = MutMap::default();
             tags.insert(subs[*tag_name].clone(), vec![]);
-            let ext_content = write_sorted_tags(env, subs, buf, &tags, *ext_var);
+            let ext_content = write_sorted_tags(env, ctx, subs, buf, &tags, *ext_var);
 
             buf.push_str(" ]");
 
-            write_ext_content(env, subs, buf, ext_content, parens)
+            write_ext_content(env, ctx, subs, buf, ext_content, parens)
         }
 
         RecursiveTagUnion(rec_var, tags, ext_var) => {
             buf.push_str("[ ");
 
-            let ext_content = write_sorted_tags2(env, subs, buf, tags, *ext_var);
+            let ext_content = write_sorted_tags2(env, ctx, subs, buf, tags, *ext_var);
 
             buf.push_str(" ]");
 
-            write_ext_content(env, subs, buf, ext_content, parens);
+            write_ext_content(env, ctx, subs, buf, ext_content, parens);
 
             buf.push_str(" as ");
             write_content(
                 env,
+                ctx,
                 subs.get_content_without_compacting(*rec_var),
                 subs,
                 buf,
@@ -777,11 +840,12 @@ pub fn chase_ext_tag_union<'a>(
     }
 }
 
-fn write_apply(
+fn write_apply<'a>(
     env: &Env,
+    ctx: &mut Context<'a>,
     symbol: Symbol,
     args: &[Variable],
-    subs: &Subs,
+    subs: &'a Subs,
     buf: &mut String,
     parens: Parens,
 ) {
@@ -805,7 +869,7 @@ fn write_apply(
                     buf.push('(');
                 }
 
-                write_content(env, content, subs, &mut arg_param, Parens::InTypeParam);
+                write_content(env, ctx, content, subs, &mut arg_param, Parens::InTypeParam);
                 buf.push_str("Num ");
                 buf.push_str(&arg_param);
 
@@ -838,6 +902,7 @@ fn write_apply(
                 buf.push(' ');
                 write_content(
                     env,
+                    ctx,
                     subs.get_content_without_compacting(*arg),
                     subs,
                     buf,
@@ -852,11 +917,12 @@ fn write_apply(
     }
 }
 
-fn write_fn(
+fn write_fn<'a>(
     env: &Env,
+    ctx: &mut Context<'a>,
     args: &[Variable],
     ret: Variable,
-    subs: &Subs,
+    subs: &'a Subs,
     buf: &mut String,
     parens: Parens,
 ) {
@@ -876,6 +942,7 @@ fn write_fn(
 
         write_content(
             env,
+            ctx,
             subs.get_content_without_compacting(*arg),
             subs,
             buf,
@@ -886,6 +953,7 @@ fn write_fn(
     buf.push_str(" -> ");
     write_content(
         env,
+        ctx,
         subs.get_content_without_compacting(ret),
         subs,
         buf,

compiler/types/src/subs.rs

@@ -756,7 +756,9 @@ impl<'a> fmt::Debug for SubsFmtContent<'a> {
 fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt::Result {
     match this {
         Content::FlexVar(name) => write!(f, "Flex({:?})", name),
+        Content::FlexAbleVar(name, symbol) => write!(f, "FlexAble({:?}, {:?})", name, symbol),
         Content::RigidVar(name) => write!(f, "Rigid({:?})", name),
+        Content::RigidAbleVar(name, symbol) => write!(f, "RigidAble({:?}, {:?})", name, symbol),
         Content::RecursionVar {
             structure,
             opt_name,
@@ -796,7 +798,19 @@ fn subs_fmt_flat_type(this: &FlatType, subs: &Subs, f: &mut fmt::Formatter) -> f
         }
         FlatType::Func(arguments, lambda_set, result) => {
             let slice = subs.get_subs_slice(*arguments);
-            write!(f, "Func({:?}, {:?}, {:?})", slice, lambda_set, result)
+            write!(f, "Func([")?;
+            for var in slice {
+                let content = subs.get_content_without_compacting(*var);
+                write!(f, "<{:?}>{:?},", *var, SubsFmtContent(content, subs))?;
+            }
+            let result_content = subs.get_content_without_compacting(*result);
+            write!(
+                f,
+                "], {:?}, <{:?}>{:?})",
+                lambda_set,
+                *result,
+                SubsFmtContent(result_content, subs)
+            )
         }
         FlatType::Record(fields, ext) => {
             write!(f, "{{ ")?;
@@ -1739,6 +1753,14 @@ impl Subs {
         self.set(var, desc);
     }
 
+    pub fn rigid_able_var(&mut self, var: Variable, name: Lowercase, ability: Symbol) {
+        let name_index = SubsIndex::push_new(&mut self.field_names, name);
+        let content = Content::RigidAbleVar(name_index, ability);
+        let desc = Descriptor::from(content);
+
+        self.set(var, desc);
+    }
+
     /// Unions two keys without the possibility of failure.
     pub fn union(&mut self, left: Variable, right: Variable, desc: Descriptor) {
         let l_root = self.utable.inlined_get_root_key(left);
@@ -2120,6 +2142,12 @@ pub enum Content {
     FlexVar(Option<SubsIndex<Lowercase>>),
     /// name given in a user-written annotation
     RigidVar(SubsIndex<Lowercase>),
+    /// Like a [Self::FlexVar], but is also bound to an ability.
+    /// This can only happen when unified with a [Self::RigidAbleVar].
+    FlexAbleVar(Option<SubsIndex<Lowercase>>, Symbol),
+    /// Like a [Self::RigidVar], but is also bound to an ability.
+    /// For example, "a has Hash".
+    RigidAbleVar(SubsIndex<Lowercase>, Symbol),
     /// name given to a recursion variable
     RecursionVar {
         structure: Variable,
@@ -2840,7 +2868,12 @@ fn occurs(
         Err((root_var, vec![]))
     } else {
         match subs.get_content_without_compacting(root_var) {
-            FlexVar(_) | RigidVar(_) | RecursionVar { .. } | Error => Ok(()),
+            FlexVar(_)
+            | RigidVar(_)
+            | FlexAbleVar(_, _)
+            | RigidAbleVar(_, _)
+            | RecursionVar { .. }
+            | Error => Ok(()),
 
             Structure(flat_type) => {
                 let mut new_seen = seen.to_owned();
@@ -2968,7 +3001,12 @@ fn explicit_substitute(
             to
         } else {
             match subs.get(in_var).content {
-                FlexVar(_) | RigidVar(_) | RecursionVar { .. } | Error => in_var,
+                FlexVar(_)
+                | RigidVar(_)
+                | FlexAbleVar(_, _)
+                | RigidAbleVar(_, _)
+                | RecursionVar { .. }
+                | Error => in_var,
 
                 Structure(flat_type) => {
                     match flat_type {
@@ -3136,9 +3174,9 @@ fn get_var_names(
         subs.set_mark(var, Mark::GET_VAR_NAMES);
 
         match desc.content {
-            Error | FlexVar(None) => taken_names,
+            Error | FlexVar(None) | FlexAbleVar(None, _) => taken_names,
 
-            FlexVar(Some(name_index)) => add_name(
+            FlexVar(Some(name_index)) | FlexAbleVar(Some(name_index), _) => add_name(
                 subs,
                 0,
                 name_index,
@@ -3165,7 +3203,9 @@ fn get_var_names(
                 None => taken_names,
             },
 
-            RigidVar(name_index) => add_name(subs, 0, name_index, var, RigidVar, taken_names),
+            RigidVar(name_index) | RigidAbleVar(name_index, _) => {
+                add_name(subs, 0, name_index, var, RigidVar, taken_names)
+            }
 
             Alias(_, args, _, _) => args.into_iter().fold(taken_names, |answer, arg_var| {
                 get_var_names(subs, subs[arg_var], answer)
@@ -3331,11 +3371,6 @@ fn content_to_err_type(
     match content {
         Structure(flat_type) => flat_type_to_err_type(subs, state, flat_type),
 
-        FlexVar(Some(name_index)) => {
-            let name = subs.field_names[name_index.index as usize].clone();
-            ErrorType::FlexVar(name)
-        }
-
         FlexVar(opt_name) => {
             let name = match opt_name {
                 Some(name_index) => subs.field_names[name_index.index as usize].clone(),
@@ -3358,6 +3393,28 @@ fn content_to_err_type(
             ErrorType::RigidVar(name)
         }
 
+        FlexAbleVar(opt_name, ability) => {
+            let name = match opt_name {
+                Some(name_index) => subs.field_names[name_index.index as usize].clone(),
+                None => {
+                    // set the name so when this variable occurs elsewhere in the type it gets the same name
+                    let name = get_fresh_var_name(state);
+                    let name_index = SubsIndex::push_new(&mut subs.field_names, name.clone());
+
+                    subs.set_content(var, FlexVar(Some(name_index)));
+
+                    name
+                }
+            };
+
+            ErrorType::FlexAbleVar(name, ability)
+        }
+
+        RigidAbleVar(name_index, ability) => {
+            let name = subs.field_names[name_index.index as usize].clone();
+            ErrorType::RigidAbleVar(name, ability)
+        }
+
         RecursionVar { opt_name, .. } => {
             let name = match opt_name {
                 Some(name_index) => subs.field_names[name_index.index as usize].clone(),
@@ -3630,7 +3687,7 @@ fn restore_help(subs: &mut Subs, initial: Variable) {
             use FlatType::*;
 
             match &desc.content {
-                FlexVar(_) | RigidVar(_) | Error => (),
+                FlexVar(_) | RigidVar(_) | FlexAbleVar(_, _) | RigidAbleVar(_, _) | Error => (),
 
                 RecursionVar { structure, .. } => {
                     stack.push(*structure);
@@ -3859,6 +3916,8 @@ impl StorageSubs {
         match content {
             FlexVar(opt_name) => FlexVar(*opt_name),
             RigidVar(name) => RigidVar(*name),
+            FlexAbleVar(opt_name, ability) => FlexAbleVar(*opt_name, *ability),
+            RigidAbleVar(name, ability) => RigidAbleVar(*name, *ability),
             RecursionVar {
                 structure,
                 opt_name,
@@ -4255,6 +4314,29 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
             copy
         }
 
+        FlexAbleVar(opt_name_index, ability) => {
+            let new_name_index = opt_name_index.map(|name_index| {
+                let name = env.source.field_names[name_index.index as usize].clone();
+                SubsIndex::push_new(&mut env.target.field_names, name)
+            });
+
+            let content = FlexAbleVar(new_name_index, ability);
+            env.target.set_content(copy, content);
+
+            copy
+        }
+
+        RigidAbleVar(name_index, ability) => {
+            let name = env.source.field_names[name_index.index as usize].clone();
+            let new_name_index = SubsIndex::push_new(&mut env.target.field_names, name);
+            env.target.set(
+                copy,
+                make_descriptor(FlexAbleVar(Some(new_name_index), ability)),
+            );
+
+            copy
+        }
+
         Alias(symbol, arguments, real_type_var, kind) => {
             let new_variables =
                 SubsSlice::reserve_into_subs(env.target, arguments.all_variables_len as _);
@@ -4314,6 +4396,8 @@ pub struct CopiedImport {
     pub variable: Variable,
     pub flex: Vec<Variable>,
     pub rigid: Vec<Variable>,
+    pub flex_able: Vec<Variable>,
+    pub rigid_able: Vec<Variable>,
     pub translations: Vec<(Variable, Variable)>,
     pub registered: Vec<Variable>,
 }
@@ -4324,6 +4408,8 @@ struct CopyImportEnv<'a> {
     target: &'a mut Subs,
     flex: Vec<Variable>,
     rigid: Vec<Variable>,
+    flex_able: Vec<Variable>,
+    rigid_able: Vec<Variable>,
     translations: Vec<(Variable, Variable)>,
     registered: Vec<Variable>,
 }
@@ -4345,6 +4431,8 @@ pub fn copy_import_to(
             target,
             flex: Vec::new(),
             rigid: Vec::new(),
+            flex_able: Vec::new(),
+            rigid_able: Vec::new(),
             translations: Vec::new(),
             registered: Vec::new(),
         };
@@ -4356,6 +4444,8 @@ pub fn copy_import_to(
             source,
             flex,
             rigid,
+            flex_able,
+            rigid_able,
             translations,
             registered,
             target: _,
@@ -4378,6 +4468,8 @@ pub fn copy_import_to(
             variable: copy,
             flex,
             rigid,
+            flex_able,
+            rigid_able,
             translations,
             registered,
         }
@@ -4395,7 +4487,10 @@ pub fn copy_import_to(
 /// standard variables
 fn is_registered(content: &Content) -> bool {
     match content {
-        Content::FlexVar(_) | Content::RigidVar(_) => false,
+        Content::FlexVar(_)
+        | Content::RigidVar(_)
+        | Content::FlexAbleVar(..)
+        | Content::RigidAbleVar(..) => false,
         Content::Structure(FlatType::EmptyRecord | FlatType::EmptyTagUnion) => false,
 
         Content::Structure(_)
@@ -4633,6 +4728,20 @@ fn copy_import_to_help(env: &mut CopyImportEnv<'_>, max_rank: Rank, var: Variabl
             copy
         }
 
+        FlexAbleVar(opt_name_index, ability) => {
+            if let Some(name_index) = opt_name_index {
+                let name = env.source.field_names[name_index.index as usize].clone();
+                let new_name_index = SubsIndex::push_new(&mut env.target.field_names, name);
+
+                let content = FlexAbleVar(Some(new_name_index), ability);
+                env.target.set_content(copy, content);
+            }
+
+            env.flex_able.push(copy);
+
+            copy
+        }
+
         Error => {
             // Open question: should this return Error, or a Flex var?
 
@@ -4655,6 +4764,20 @@ fn copy_import_to_help(env: &mut CopyImportEnv<'_>, max_rank: Rank, var: Variabl
             copy
         }
 
+        RigidAbleVar(name_index, ability) => {
+            let name = env.source.field_names[name_index.index as usize].clone();
+            let new_name_index = SubsIndex::push_new(&mut env.target.field_names, name);
+
+            env.target
+                .set(copy, make_descriptor(RigidAbleVar(new_name_index, ability)));
+
+            env.rigid_able.push(copy);
+
+            env.translations.push((var, copy));
+
+            copy
+        }
+
         RecursionVar {
             opt_name,
             structure,
@@ -4748,7 +4871,7 @@ where
         use Content::*;
         use FlatType::*;
         match content {
-            FlexVar(_) | RigidVar(_) => {}
+            FlexVar(_) | RigidVar(_) | FlexAbleVar(_, _) | RigidAbleVar(_, _) => {}
             RecursionVar {
                 structure,
                 opt_name: _,

compiler/types/src/types.rs

@@ -1744,6 +1744,15 @@ pub enum Reason {
     RecordUpdateKeys(Symbol, SendMap<Lowercase, Region>),
     RecordDefaultField(Lowercase),
     NumericLiteralSuffix,
+    InvalidAbilityMemberSpecialization {
+        member_name: Symbol,
+        def_region: Region,
+        unimplemented_abilities: DoesNotImplementAbility,
+    },
+    GeneralizedAbilityMemberSpecialization {
+        member_name: Symbol,
+        def_region: Region,
+    },
 }
 
 #[derive(PartialEq, Debug, Clone)]
@@ -1783,6 +1792,8 @@ pub enum Category {
     Accessor(Lowercase),
     Access(Lowercase),
     DefaultValue(Lowercase), // for setting optional fields
+
+    AbilityMemberSpecialization(Symbol),
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -1867,14 +1878,19 @@ pub enum Mismatch {
     InconsistentWhenBranches,
     CanonicalizationProblem,
     TypeNotInRange,
+    DoesNotImplementAbiity(Variable, Symbol),
 }
 
+pub type DoesNotImplementAbility = Vec<(ErrorType, Symbol)>;
+
 #[derive(PartialEq, Eq, Clone, Hash)]
 pub enum ErrorType {
     Infinite,
     Type(Symbol, Vec<ErrorType>),
     FlexVar(Lowercase),
     RigidVar(Lowercase),
+    FlexAbleVar(Lowercase, Symbol),
+    RigidAbleVar(Lowercase, Symbol),
     Record(SendMap<Lowercase, RecordField<ErrorType>>, TypeExt),
     TagUnion(SendMap<TagName, Vec<ErrorType>>, TypeExt),
     RecursiveTagUnion(Box<ErrorType>, SendMap<TagName, Vec<ErrorType>>, TypeExt),
@@ -1905,10 +1921,7 @@ impl ErrorType {
         match self {
             Infinite => {}
             Type(_, ts) => ts.iter().for_each(|t| t.add_names(taken)),
-            FlexVar(v) => {
+            FlexVar(v) | RigidVar(v) | FlexAbleVar(v, _) | RigidAbleVar(v, _) => {
-                taken.insert(v.clone());
-            }
-            RigidVar(v) => {
                 taken.insert(v.clone());
             }
             Record(fields, ext) => {
@@ -2087,8 +2100,18 @@ fn write_debug_error_type_help(error_type: ErrorType, buf: &mut String, parens:
     match error_type {
         Infinite => buf.push('∞'),
         Error => buf.push('?'),
-        FlexVar(name) => buf.push_str(name.as_str()),
+        FlexVar(name) | RigidVar(name) => buf.push_str(name.as_str()),
-        RigidVar(name) => buf.push_str(name.as_str()),
+        FlexAbleVar(name, symbol) | RigidAbleVar(name, symbol) => {
+            let write_parens = parens == Parens::InTypeParam;
+            if write_parens {
+                buf.push('(');
+            }
+            buf.push_str(name.as_str());
+            buf.push_str(&format!(" has {:?}", symbol));
+            if write_parens {
+                buf.push(')');
+            }
+        }
         Type(symbol, arguments) => {
             let write_parens = parens == Parens::InTypeParam && !arguments.is_empty();
 

compiler/unify/Cargo.toml

@@ -11,6 +11,9 @@ bitflags = "1.3.2"
 [dependencies.roc_collections]
 path = "../collections"
 
+[dependencies.roc_error_macros]
+path = "../../error_macros"
+
 [dependencies.roc_module]
 path = "../module"
 

compiler/unify/src/unify.rs

@@ -1,4 +1,5 @@
 use bitflags::bitflags;
+use roc_error_macros::todo_abilities;
 use roc_module::ident::{Lowercase, TagName};
 use roc_module::symbol::Symbol;
 use roc_types::subs::Content::{self, *};
@@ -6,7 +7,7 @@ use roc_types::subs::{
     AliasVariables, Descriptor, ErrorTypeContext, FlatType, GetSubsSlice, Mark, OptVariable,
     RecordFields, Subs, SubsIndex, SubsSlice, UnionTags, Variable, VariableSubsSlice,
 };
-use roc_types::types::{AliasKind, ErrorType, Mismatch, RecordField};
+use roc_types::types::{AliasKind, DoesNotImplementAbility, ErrorType, Mismatch, RecordField};
 
 macro_rules! mismatch {
     () => {{
@@ -19,7 +20,10 @@ macro_rules! mismatch {
             );
         }
 
-        vec![Mismatch::TypeMismatch]
+        Outcome {
+            mismatches: vec![Mismatch::TypeMismatch],
+            ..Outcome::default()
+        }
     }};
     ($msg:expr) => {{
         if cfg!(debug_assertions) && std::env::var("ROC_PRINT_MISMATCHES").is_ok() {
@@ -34,7 +38,10 @@ macro_rules! mismatch {
         }
 
 
-        vec![Mismatch::TypeMismatch]
+        Outcome {
+            mismatches: vec![Mismatch::TypeMismatch],
+            ..Outcome::default()
+        }
     }};
     ($msg:expr,) => {{
         mismatch!($msg)
@@ -51,8 +58,28 @@ macro_rules! mismatch {
             println!("");
         }
 
-        vec![Mismatch::TypeMismatch]
+        Outcome {
+            mismatches: vec![Mismatch::TypeMismatch],
+            ..Outcome::default()
+        }
     }};
+    (%not_able, $var:expr, $ability:expr, $msg:expr, $($arg:tt)*) => {{
+        if cfg!(debug_assertions) && std::env::var("ROC_PRINT_MISMATCHES").is_ok() {
+            println!(
+                "Mismatch in {} Line {} Column {}",
+                file!(),
+                line!(),
+                column!()
+            );
+            println!($msg, $($arg)*);
+            println!("");
+        }
+
+        Outcome {
+            mismatches: vec![Mismatch::TypeMismatch, Mismatch::DoesNotImplementAbiity($var, $ability)],
+            ..Outcome::default()
+        }
+    }}
 }
 
 type Pool = Vec<Variable>;
@@ -105,20 +132,52 @@ pub struct Context {
 
 #[derive(Debug)]
 pub enum Unified {
-    Success(Pool),
+    Success {
-    Failure(Pool, ErrorType, ErrorType),
+        vars: Pool,
+        must_implement_ability: Vec<MustImplementAbility>,
+    },
+    Failure(Pool, ErrorType, ErrorType, DoesNotImplementAbility),
     BadType(Pool, roc_types::types::Problem),
 }
 
-type Outcome = Vec<Mismatch>;
+/// Specifies that `type` must implement the ability `ability`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct MustImplementAbility {
+    // This only points to opaque type names currently.
+    // TODO(abilities) support structural types in general
+    pub typ: Symbol,
+    pub ability: Symbol,
+}
+
+#[derive(Debug, Default)]
+pub struct Outcome {
+    mismatches: Vec<Mismatch>,
+    /// We defer these checks until the end of a solving phase.
+    /// NOTE: this vector is almost always empty!
+    must_implement_ability: Vec<MustImplementAbility>,
+}
+
+impl Outcome {
+    fn union(&mut self, other: Self) {
+        self.mismatches.extend(other.mismatches);
+        self.must_implement_ability
+            .extend(other.must_implement_ability);
+    }
+}
 
 #[inline(always)]
 pub fn unify(subs: &mut Subs, var1: Variable, var2: Variable, mode: Mode) -> Unified {
     let mut vars = Vec::new();
-    let mismatches = unify_pool(subs, &mut vars, var1, var2, mode);
+    let Outcome {
+        mismatches,
+        must_implement_ability,
+    } = unify_pool(subs, &mut vars, var1, var2, mode);
 
     if mismatches.is_empty() {
-        Unified::Success(vars)
+        Unified::Success {
+            vars,
+            must_implement_ability,
+        }
     } else {
         let error_context = if mismatches.contains(&Mismatch::TypeNotInRange) {
             ErrorTypeContext::ExpandRanges
@@ -136,7 +195,19 @@ pub fn unify(subs: &mut Subs, var1: Variable, var2: Variable, mode: Mode) -> Uni
         if !problems.is_empty() {
             Unified::BadType(vars, problems.remove(0))
         } else {
-            Unified::Failure(vars, type1, type2)
+            let do_not_implement_ability = mismatches
+                .into_iter()
+                .filter_map(|mismatch| match mismatch {
+                    Mismatch::DoesNotImplementAbiity(var, ab) => {
+                        let (err_type, _new_problems) =
+                            subs.var_to_error_type_contextual(var, error_context);
+                        Some((err_type, ab))
+                    }
+                    _ => None,
+                })
+                .collect();
+
+            Unified::Failure(vars, type1, type2, do_not_implement_ability)
         }
     }
 }
@@ -150,7 +221,7 @@ pub fn unify_pool(
     mode: Mode,
 ) -> Outcome {
     if subs.equivalent(var1, var2) {
-        Vec::new()
+        Outcome::default()
     } else {
         let ctx = Context {
             first: var1,
@@ -191,7 +262,14 @@ fn unify_context(subs: &mut Subs, pool: &mut Pool, ctx: Context) -> Outcome {
         );
     }
     match &ctx.first_desc.content {
-        FlexVar(opt_name) => unify_flex(subs, &ctx, opt_name, &ctx.second_desc.content),
+        FlexVar(opt_name) => unify_flex(subs, &ctx, opt_name, None, &ctx.second_desc.content),
+        FlexAbleVar(opt_name, ability) => unify_flex(
+            subs,
+            &ctx,
+            opt_name,
+            Some(*ability),
+            &ctx.second_desc.content,
+        ),
         RecursionVar {
             opt_name,
             structure,
@@ -203,7 +281,10 @@ fn unify_context(subs: &mut Subs, pool: &mut Pool, ctx: Context) -> Outcome {
             *structure,
             &ctx.second_desc.content,
         ),
-        RigidVar(name) => unify_rigid(subs, &ctx, name, &ctx.second_desc.content),
+        RigidVar(name) => unify_rigid(subs, &ctx, name, None, &ctx.second_desc.content),
+        RigidAbleVar(name, ability) => {
+            unify_rigid(subs, &ctx, name, Some(*ability), &ctx.second_desc.content)
+        }
         Structure(flat_type) => {
             unify_structure(subs, pool, &ctx, flat_type, &ctx.second_desc.content)
         }
@@ -238,7 +319,7 @@ fn unify_ranged_number(
         }
         &RangedNumber(other_real_var, other_range_vars) => {
             let outcome = unify_pool(subs, pool, real_var, other_real_var, ctx.mode);
-            if outcome.is_empty() {
+            if outcome.mismatches.is_empty() {
                 check_valid_range(subs, pool, ctx.first, other_range_vars, ctx.mode)
             } else {
                 outcome
@@ -246,9 +327,12 @@ fn unify_ranged_number(
             // TODO: We should probably check that "range_vars" and "other_range_vars" intersect
         }
         Error => merge(subs, ctx, Error),
+        FlexAbleVar(..) | RigidAbleVar(..) => {
+            todo_abilities!("I don't think this can be reached yet")
+        }
     };
 
-    if !outcome.is_empty() {
+    if !outcome.mismatches.is_empty() {
         return outcome;
     }
 
@@ -269,11 +353,11 @@ fn check_valid_range(
         let snapshot = subs.snapshot();
         let old_pool = pool.clone();
         let outcome = unify_pool(subs, pool, var, possible_var, mode | Mode::RIGID_AS_FLEX);
-        if outcome.is_empty() {
+        if outcome.mismatches.is_empty() {
             // Okay, we matched some type in the range.
             subs.rollback_to(snapshot);
             *pool = old_pool;
-            return vec![];
+            return Outcome::default();
         } else if it.peek().is_some() {
             // We failed to match something in the range, but there are still things we can try.
             subs.rollback_to(snapshot);
@@ -283,7 +367,10 @@ fn check_valid_range(
         }
     }
 
-    return vec![Mismatch::TypeNotInRange];
+    Outcome {
+        mismatches: vec![Mismatch::TypeNotInRange],
+        ..Outcome::default()
+    }
 }
 
 #[inline(always)]
@@ -310,13 +397,19 @@ fn unify_alias(
             unify_pool(subs, pool, real_var, *structure, ctx.mode)
         }
         RigidVar(_) => unify_pool(subs, pool, real_var, ctx.second, ctx.mode),
+        RigidAbleVar (_, ability) |  FlexAbleVar(_, ability) if kind == AliasKind::Opaque && args.is_empty() => {
+            // Opaque type wins
+            let mut outcome = merge(subs, ctx, Alias(symbol, args, real_var, kind));
+            outcome.must_implement_ability.push(MustImplementAbility { typ: symbol, ability: *ability });
+            outcome
+        }
         Alias(other_symbol, other_args, other_real_var, _)
             // Opaques types are only equal if the opaque symbols are equal!
             if !either_is_opaque || symbol == *other_symbol =>
         {
             if symbol == *other_symbol {
                 if args.len() == other_args.len() {
-                    let mut problems = Vec::new();
+                    let mut outcome = Outcome::default();
                     let it = args
                         .all_variables()
                         .into_iter()
@@ -327,23 +420,23 @@ fn unify_alias(
                     for (l, r) in it {
                         let l_var = subs[l];
                         let r_var = subs[r];
-                        problems.extend(unify_pool(subs, pool, l_var, r_var, ctx.mode));
+                        outcome.union(unify_pool(subs, pool, l_var, r_var, ctx.mode));
                     }
 
-                    if problems.is_empty() {
+                    if outcome.mismatches.is_empty() {
-                        problems.extend(merge(subs, ctx, *other_content));
+                        outcome.union(merge(subs, ctx, *other_content));
                     }
 
                     let args_unification_had_changes = !subs.vars_since_snapshot(&args_unification_snapshot).is_empty();
                     subs.commit_snapshot(args_unification_snapshot);
 
-                    if !args.is_empty() && args_unification_had_changes && problems.is_empty() {
+                    if !args.is_empty() && args_unification_had_changes && outcome.mismatches.is_empty() {
                         // We need to unify the real vars because unification of type variables
                         // may have made them larger, which then needs to be reflected in the `real_var`.
-                        problems.extend(unify_pool(subs, pool, real_var, *other_real_var, ctx.mode));
+                        outcome.union(unify_pool(subs, pool, real_var, *other_real_var, ctx.mode));
                     }
 
-                    problems
+                    outcome
                 } else {
                     dbg!(args.len(), other_args.len());
                     mismatch!("{:?}", symbol)
@@ -355,7 +448,7 @@ fn unify_alias(
         Structure(_) if !either_is_opaque => unify_pool(subs, pool, real_var, ctx.second, ctx.mode),
         RangedNumber(other_real_var, other_range_vars) if !either_is_opaque => {
             let outcome = unify_pool(subs, pool, real_var, *other_real_var, ctx.mode);
-            if outcome.is_empty() {
+            if outcome.mismatches.is_empty() {
                 check_valid_range(subs, pool, real_var, *other_range_vars, ctx.mode)
             } else {
                 outcome
@@ -448,13 +541,31 @@ fn unify_structure(
         },
         RangedNumber(other_real_var, other_range_vars) => {
             let outcome = unify_pool(subs, pool, ctx.first, *other_real_var, ctx.mode);
-            if outcome.is_empty() {
+            if outcome.mismatches.is_empty() {
                 check_valid_range(subs, pool, ctx.first, *other_range_vars, ctx.mode)
             } else {
                 outcome
             }
         }
         Error => merge(subs, ctx, Error),
+
+        FlexAbleVar(_, ability) => {
+            // TODO(abilities) support structural types in ability bounds
+            mismatch!(
+                %not_able, ctx.first, *ability,
+                "trying to unify {:?} with FlexAble {:?}",
+                &flat_type,
+                &other
+            )
+        }
+        RigidAbleVar(_, ability) => {
+            mismatch!(
+                %not_able, ctx.first, *ability,
+                "trying to unify {:?} with RigidAble {:?}",
+                &flat_type,
+                &other
+            )
+        }
     }
 }
 
@@ -474,29 +585,29 @@ fn unify_record(
     if separate.only_in_1.is_empty() {
         if separate.only_in_2.is_empty() {
             // these variable will be the empty record, but we must still unify them
-            let ext_problems = unify_pool(subs, pool, ext1, ext2, ctx.mode);
+            let ext_outcome = unify_pool(subs, pool, ext1, ext2, ctx.mode);
 
-            if !ext_problems.is_empty() {
+            if !ext_outcome.mismatches.is_empty() {
-                return ext_problems;
+                return ext_outcome;
             }
 
-            let mut field_problems =
+            let mut field_outcome =
                 unify_shared_fields(subs, pool, ctx, shared_fields, OtherFields::None, ext1);
 
-            field_problems.extend(ext_problems);
+            field_outcome.union(ext_outcome);
 
-            field_problems
+            field_outcome
         } else {
             let only_in_2 = RecordFields::insert_into_subs(subs, separate.only_in_2);
             let flat_type = FlatType::Record(only_in_2, ext2);
             let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
-            let ext_problems = unify_pool(subs, pool, ext1, sub_record, ctx.mode);
+            let ext_outcome = unify_pool(subs, pool, ext1, sub_record, ctx.mode);
 
-            if !ext_problems.is_empty() {
+            if !ext_outcome.mismatches.is_empty() {
-                return ext_problems;
+                return ext_outcome;
             }
 
-            let mut field_problems = unify_shared_fields(
+            let mut field_outcome = unify_shared_fields(
                 subs,
                 pool,
                 ctx,
@@ -505,21 +616,21 @@ fn unify_record(
                 sub_record,
             );
 
-            field_problems.extend(ext_problems);
+            field_outcome.union(ext_outcome);
 
-            field_problems
+            field_outcome
         }
     } else if separate.only_in_2.is_empty() {
         let only_in_1 = RecordFields::insert_into_subs(subs, separate.only_in_1);
         let flat_type = FlatType::Record(only_in_1, ext1);
         let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
-        let ext_problems = unify_pool(subs, pool, sub_record, ext2, ctx.mode);
+        let ext_outcome = unify_pool(subs, pool, sub_record, ext2, ctx.mode);
 
-        if !ext_problems.is_empty() {
+        if !ext_outcome.mismatches.is_empty() {
-            return ext_problems;
+            return ext_outcome;
         }
 
-        let mut field_problems = unify_shared_fields(
+        let mut field_outcome = unify_shared_fields(
             subs,
             pool,
             ctx,
@@ -528,9 +639,9 @@ fn unify_record(
             sub_record,
         );
 
-        field_problems.extend(ext_problems);
+        field_outcome.union(ext_outcome);
 
-        field_problems
+        field_outcome
     } else {
         let only_in_1 = RecordFields::insert_into_subs(subs, separate.only_in_1);
         let only_in_2 = RecordFields::insert_into_subs(subs, separate.only_in_2);
@@ -544,24 +655,26 @@ fn unify_record(
         let sub1 = fresh(subs, pool, ctx, Structure(flat_type1));
         let sub2 = fresh(subs, pool, ctx, Structure(flat_type2));
 
-        let rec1_problems = unify_pool(subs, pool, ext1, sub2, ctx.mode);
+        let rec1_outcome = unify_pool(subs, pool, ext1, sub2, ctx.mode);
-        if !rec1_problems.is_empty() {
+        if !rec1_outcome.mismatches.is_empty() {
-            return rec1_problems;
+            return rec1_outcome;
         }
 
-        let rec2_problems = unify_pool(subs, pool, sub1, ext2, ctx.mode);
+        let rec2_outcome = unify_pool(subs, pool, sub1, ext2, ctx.mode);
-        if !rec2_problems.is_empty() {
+        if !rec2_outcome.mismatches.is_empty() {
-            return rec2_problems;
+            return rec2_outcome;
         }
 
-        let mut field_problems =
+        let mut field_outcome =
             unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, ext);
 
-        field_problems.reserve(rec1_problems.len() + rec2_problems.len());
+        field_outcome
-        field_problems.extend(rec1_problems);
+            .mismatches
-        field_problems.extend(rec2_problems);
+            .reserve(rec1_outcome.mismatches.len() + rec2_outcome.mismatches.len());
+        field_outcome.union(rec1_outcome);
+        field_outcome.union(rec2_outcome);
 
-        field_problems
+        field_outcome
     }
 }
 
@@ -584,7 +697,7 @@ fn unify_shared_fields(
     let num_shared_fields = shared_fields.len();
 
     for (name, (actual, expected)) in shared_fields {
-        let local_problems = unify_pool(
+        let local_outcome = unify_pool(
             subs,
             pool,
             actual.into_inner(),
@@ -592,7 +705,7 @@ fn unify_shared_fields(
             ctx.mode,
         );
 
-        if local_problems.is_empty() {
+        if local_outcome.mismatches.is_empty() {
             use RecordField::*;
 
             // Unification of optional fields
@@ -856,18 +969,18 @@ fn unify_tag_union_new(
 
     if separate.only_in_1.is_empty() {
         if separate.only_in_2.is_empty() {
-            let ext_problems = if ctx.mode.is_eq() {
+            let ext_outcome = if ctx.mode.is_eq() {
                 unify_pool(subs, pool, ext1, ext2, ctx.mode)
             } else {
                 // In a presence context, we don't care about ext2 being equal to ext1
-                vec![]
+                Outcome::default()
             };
 
-            if !ext_problems.is_empty() {
+            if !ext_outcome.mismatches.is_empty() {
-                return ext_problems;
+                return ext_outcome;
             }
 
-            let mut tag_problems = unify_shared_tags_new(
+            let mut shared_tags_outcome = unify_shared_tags_new(
                 subs,
                 pool,
                 ctx,
@@ -877,20 +990,20 @@ fn unify_tag_union_new(
                 recursion_var,
             );
 
-            tag_problems.extend(ext_problems);
+            shared_tags_outcome.union(ext_outcome);
 
-            tag_problems
+            shared_tags_outcome
         } else {
             let unique_tags2 = UnionTags::insert_slices_into_subs(subs, separate.only_in_2);
             let flat_type = FlatType::TagUnion(unique_tags2, ext2);
             let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
-            let ext_problems = unify_pool(subs, pool, ext1, sub_record, ctx.mode);
+            let ext_outcome = unify_pool(subs, pool, ext1, sub_record, ctx.mode);
 
-            if !ext_problems.is_empty() {
+            if !ext_outcome.mismatches.is_empty() {
-                return ext_problems;
+                return ext_outcome;
             }
 
-            let mut tag_problems = unify_shared_tags_new(
+            let mut shared_tags_outcome = unify_shared_tags_new(
                 subs,
                 pool,
                 ctx,
@@ -900,9 +1013,9 @@ fn unify_tag_union_new(
                 recursion_var,
             );
 
-            tag_problems.extend(ext_problems);
+            shared_tags_outcome.union(ext_outcome);
 
-            tag_problems
+            shared_tags_outcome
         }
     } else if separate.only_in_2.is_empty() {
         let unique_tags1 = UnionTags::insert_slices_into_subs(subs, separate.only_in_1);
@@ -911,10 +1024,10 @@ fn unify_tag_union_new(
 
         // In a presence context, we don't care about ext2 being equal to tags1
         if ctx.mode.is_eq() {
-            let ext_problems = unify_pool(subs, pool, sub_record, ext2, ctx.mode);
+            let ext_outcome = unify_pool(subs, pool, sub_record, ext2, ctx.mode);
 
-            if !ext_problems.is_empty() {
+            if !ext_outcome.mismatches.is_empty() {
-                return ext_problems;
+                return ext_outcome;
             }
         }
 
@@ -961,17 +1074,17 @@ fn unify_tag_union_new(
 
         let snapshot = subs.snapshot();
 
-        let ext1_problems = unify_pool(subs, pool, ext1, sub2, ctx.mode);
+        let ext1_outcome = unify_pool(subs, pool, ext1, sub2, ctx.mode);
-        if !ext1_problems.is_empty() {
+        if !ext1_outcome.mismatches.is_empty() {
             subs.rollback_to(snapshot);
-            return ext1_problems;
+            return ext1_outcome;
         }
 
         if ctx.mode.is_eq() {
-            let ext2_problems = unify_pool(subs, pool, sub1, ext2, ctx.mode);
+            let ext2_outcome = unify_pool(subs, pool, sub1, ext2, ctx.mode);
-            if !ext2_problems.is_empty() {
+            if !ext2_outcome.mismatches.is_empty() {
                 subs.rollback_to(snapshot);
-                return ext2_problems;
+                return ext2_outcome;
             }
         }
 
@@ -1063,17 +1176,17 @@ fn unify_shared_tags_new(
             maybe_mark_tag_union_recursive(subs, actual);
             maybe_mark_tag_union_recursive(subs, expected);
 
-            let mut problems = Vec::new();
+            let mut outcome = Outcome::default();
 
-            problems.extend(unify_pool(subs, pool, actual, expected, ctx.mode));
+            outcome.union(unify_pool(subs, pool, actual, expected, ctx.mode));
 
             // clearly, this is very suspicious: these variables have just been unified. And yet,
             // not doing this leads to stack overflows
             if let Rec::Right(_) = recursion_var {
-                if problems.is_empty() {
+                if outcome.mismatches.is_empty() {
                     matching_vars.push(expected);
                 }
-            } else if problems.is_empty() {
+            } else if outcome.mismatches.is_empty() {
                 matching_vars.push(actual);
             }
         }
@@ -1215,39 +1328,43 @@ fn unify_flat_type(
             debug_assert!(is_recursion_var(subs, *rec2));
 
             let rec = Rec::Both(*rec1, *rec2);
-            let mut problems =
+            let mut outcome =
                 unify_tag_union_new(subs, pool, ctx, *tags1, *ext1, *tags2, *ext2, rec);
-            problems.extend(unify_pool(subs, pool, *rec1, *rec2, ctx.mode));
+            outcome.union(unify_pool(subs, pool, *rec1, *rec2, ctx.mode));
 
-            problems
+            outcome
         }
 
         (Apply(l_symbol, l_args), Apply(r_symbol, r_args)) if l_symbol == r_symbol => {
-            let problems = unify_zip_slices(subs, pool, *l_args, *r_args);
+            let mut outcome = unify_zip_slices(subs, pool, *l_args, *r_args);
 
-            if problems.is_empty() {
+            if outcome.mismatches.is_empty() {
-                merge(subs, ctx, Structure(Apply(*r_symbol, *r_args)))
+                outcome.union(merge(subs, ctx, Structure(Apply(*r_symbol, *r_args))));
-            } else {
-                problems
             }
+
+            outcome
         }
         (Func(l_args, l_closure, l_ret), Func(r_args, r_closure, r_ret))
             if l_args.len() == r_args.len() =>
         {
-            let arg_problems = unify_zip_slices(subs, pool, *l_args, *r_args);
+            let arg_outcome = unify_zip_slices(subs, pool, *l_args, *r_args);
-            let ret_problems = unify_pool(subs, pool, *l_ret, *r_ret, ctx.mode);
+            let ret_outcome = unify_pool(subs, pool, *l_ret, *r_ret, ctx.mode);
-            let closure_problems = unify_pool(subs, pool, *l_closure, *r_closure, ctx.mode);
+            let closure_outcome = unify_pool(subs, pool, *l_closure, *r_closure, ctx.mode);
 
-            if arg_problems.is_empty() && closure_problems.is_empty() && ret_problems.is_empty() {
+            let mut outcome = ret_outcome;
-                merge(subs, ctx, Structure(Func(*r_args, *r_closure, *r_ret)))
-            } else {
-                let mut problems = ret_problems;
 
-                problems.extend(closure_problems);
+            outcome.union(closure_outcome);
-                problems.extend(arg_problems);
+            outcome.union(arg_outcome);
 
-                problems
+            if outcome.mismatches.is_empty() {
+                outcome.union(merge(
+                    subs,
+                    ctx,
+                    Structure(Func(*r_args, *r_closure, *r_ret)),
+                ));
             }
+
+            outcome
         }
         (FunctionOrTagUnion(tag_name, tag_symbol, ext), Func(args, closure, ret)) => {
             unify_function_or_tag_union_and_func(
@@ -1282,12 +1399,12 @@ fn unify_flat_type(
             let tag_name_2_ref = &subs[*tag_name_2];
 
             if tag_name_1_ref == tag_name_2_ref {
-                let problems = unify_pool(subs, pool, *ext1, *ext2, ctx.mode);
+                let outcome = unify_pool(subs, pool, *ext1, *ext2, ctx.mode);
-                if problems.is_empty() {
+                if outcome.mismatches.is_empty() {
                     let content = *subs.get_content_without_compacting(ctx.second);
                     merge(subs, ctx, content)
                 } else {
-                    problems
+                    outcome
                 }
             } else {
                 let tags1 = UnionTags::from_tag_name_index(*tag_name_1);
@@ -1343,7 +1460,7 @@ fn unify_zip_slices(
     left: SubsSlice<Variable>,
     right: SubsSlice<Variable>,
 ) -> Outcome {
-    let mut problems = Vec::new();
+    let mut outcome = Outcome::default();
 
     let it = left.into_iter().zip(right.into_iter());
 
@@ -1351,10 +1468,10 @@ fn unify_zip_slices(
         let l_var = subs[l_index];
         let r_var = subs[r_index];
 
-        problems.extend(unify_pool(subs, pool, l_var, r_var, Mode::EQ));
+        outcome.union(unify_pool(subs, pool, l_var, r_var, Mode::EQ));
     }
 
-    problems
+    outcome
 }
 
 #[inline(always)]
@@ -1362,6 +1479,7 @@ fn unify_rigid(
     subs: &mut Subs,
     ctx: &Context,
     name: &SubsIndex<Lowercase>,
+    opt_able_bound: Option<Symbol>,
     other: &Content,
 ) -> Outcome {
     match other {
@@ -1369,16 +1487,76 @@ fn unify_rigid(
             // If the other is flex, rigid wins!
             merge(subs, ctx, RigidVar(*name))
         }
-        RigidVar(_) | RecursionVar { .. } | Structure(_) | Alias(_, _, _, _) | RangedNumber(..) => {
+        FlexAbleVar(_, other_ability) => {
-            if !ctx.mode.contains(Mode::RIGID_AS_FLEX) {
+            match opt_able_bound {
-                // Type mismatch! Rigid can only unify with flex, even if the
+                Some(ability) => {
-                // rigid names are the same.
+                    if ability == *other_ability {
-                mismatch!("Rigid {:?} with {:?}", ctx.first, &other)
+                        // The ability bounds are the same, so rigid wins!
-            } else {
+                        merge(subs, ctx, RigidAbleVar(*name, ability))
-                // We are treating rigid vars as flex vars; admit this
+                    } else {
-                merge(subs, ctx, *other)
+                        // Mismatch for now.
+                        // TODO check ability hierarchies.
+                        mismatch!(
+                            %not_able, ctx.second, ability,
+                            "RigidAble {:?} with ability {:?} not compatible with ability {:?}",
+                            ctx.first,
+                            ability,
+                            other_ability
+                        )
+                    }
+                }
+                None => {
+                    // Mismatch - Rigid can unify with FlexAble only when the Rigid has an ability
+                    // bound as well, otherwise the user failed to correctly annotate the bound.
+                    mismatch!(
+                        %not_able, ctx.first, *other_ability,
+                        "Rigid {:?} with FlexAble {:?}", ctx.first, other
+                    )
+                }
             }
         }
+
+        RigidVar(_) | RecursionVar { .. } | Structure(_) | Alias(_, _, _, _) | RangedNumber(..)
+            if ctx.mode.contains(Mode::RIGID_AS_FLEX) =>
+        {
+            // Usually rigids can only unify with flex, but the mode indicates we are treating
+            // rigid vars as flex, so admit this.
+            match (opt_able_bound, other) {
+                (None, other) => merge(subs, ctx, *other),
+                (Some(ability), Alias(opaque_name, vars, _real_var, AliasKind::Opaque))
+                    if vars.is_empty() =>
+                {
+                    let mut output = merge(subs, ctx, *other);
+                    let must_implement_ability = MustImplementAbility {
+                        typ: *opaque_name,
+                        ability,
+                    };
+                    output.must_implement_ability.push(must_implement_ability);
+                    output
+                }
+                (Some(ability), other) => {
+                    // For now, only allow opaque types with no type variables to implement abilities.
+                    mismatch!(
+                        %not_able, ctx.second, ability,
+                        "RigidAble {:?} with non-opaque or opaque with type variables {:?}",
+                        ctx.first,
+                        &other
+                    )
+                }
+            }
+        }
+
+        RigidVar(_)
+        | RigidAbleVar(..)
+        | RecursionVar { .. }
+        | Structure(_)
+        | Alias(..)
+        | RangedNumber(..) => {
+            // Type mismatch! Rigid can only unify with flex, even if the
+            // rigid names are the same.
+            mismatch!("Rigid {:?} with {:?}", ctx.first, &other)
+        }
+
         Error => {
             // Error propagates.
             merge(subs, ctx, Error)
@@ -1391,16 +1569,49 @@ fn unify_flex(
     subs: &mut Subs,
     ctx: &Context,
     opt_name: &Option<SubsIndex<Lowercase>>,
+    opt_able_bound: Option<Symbol>,
     other: &Content,
 ) -> Outcome {
     match other {
         FlexVar(None) => {
             // If both are flex, and only left has a name, keep the name around.
-            merge(subs, ctx, FlexVar(*opt_name))
+            match opt_able_bound {
+                Some(ability) => merge(subs, ctx, FlexAbleVar(*opt_name, ability)),
+                None => merge(subs, ctx, FlexVar(*opt_name)),
+            }
+        }
+
+        FlexAbleVar(opt_other_name, other_ability) => {
+            // Prefer the right's name when possible.
+            let opt_name = (opt_other_name).or(*opt_name);
+
+            match opt_able_bound {
+                Some(ability) => {
+                    if ability == *other_ability {
+                        // The ability bounds are the same! Keep the name around if it exists.
+                        merge(subs, ctx, FlexAbleVar(opt_name, ability))
+                    } else {
+                        // Ability names differ; mismatch for now.
+                        // TODO check ability hierarchies.
+                        mismatch!(
+                            %not_able, ctx.second, ability,
+                            "FlexAble {:?} with ability {:?} not compatible with ability {:?}",
+                            ctx.first,
+                            ability,
+                            other_ability
+                        )
+                    }
+                }
+                None => {
+                    // Right has an ability bound, but left might have the name. Combine them.
+                    merge(subs, ctx, FlexAbleVar(opt_name, *other_ability))
+                }
+            }
         }
 
         FlexVar(Some(_))
         | RigidVar(_)
+        | RigidAbleVar(_, _)
         | RecursionVar { .. }
         | Structure(_)
         | Alias(_, _, _, _)
@@ -1446,7 +1657,13 @@ fn unify_recursion(
             // unify the structure variable with this Structure
             unify_pool(subs, pool, structure, ctx.second, ctx.mode)
         }
-        RigidVar(_) => mismatch!("RecursionVar {:?} with rigid {:?}", ctx.first, &other),
+        RigidVar(_) => {
+            mismatch!("RecursionVar {:?} with rigid {:?}", ctx.first, &other)
+        }
+
+        FlexAbleVar(..) | RigidAbleVar(..) => {
+            mismatch!("RecursionVar {:?} with able var {:?}", ctx.first, &other)
+        }
 
         FlexVar(_) => merge(
             subs,
@@ -1492,7 +1709,7 @@ pub fn merge(subs: &mut Subs, ctx: &Context, content: Content) -> Outcome {
 
     subs.union(ctx.first, ctx.second, desc);
 
-    Vec::new()
+    Outcome::default()
 }
 
 fn register(subs: &mut Subs, desc: Descriptor, pool: &mut Pool) -> Variable {
@@ -1543,7 +1760,7 @@ fn unify_function_or_tag_union_and_func(
 
     let new_tag_union_var = fresh(subs, pool, ctx, content);
 
-    let mut problems = if left {
+    let mut outcome = if left {
         unify_pool(subs, pool, new_tag_union_var, function_return, ctx.mode)
     } else {
         unify_pool(subs, pool, function_return, new_tag_union_var, ctx.mode)
@@ -1567,16 +1784,16 @@ fn unify_function_or_tag_union_and_func(
             pool,
         );
 
-        let closure_problems = if left {
+        let closure_outcome = if left {
             unify_pool(subs, pool, tag_lambda_set, function_lambda_set, ctx.mode)
         } else {
             unify_pool(subs, pool, function_lambda_set, tag_lambda_set, ctx.mode)
         };
 
-        problems.extend(closure_problems);
+        outcome.union(closure_outcome);
     }
 
-    if problems.is_empty() {
+    if outcome.mismatches.is_empty() {
         let desc = if left {
             subs.get(ctx.second)
         } else {
@@ -1586,5 +1803,5 @@ fn unify_function_or_tag_union_and_func(
         subs.union(ctx.first, ctx.second, desc);
     }
 
-    problems
+    outcome
 }

docs/Cargo.toml

@@ -21,6 +21,7 @@ roc_parse = { path = "../compiler/parse" }
 roc_target = { path = "../compiler/roc_target" }
 roc_collections = { path = "../compiler/collections" }
 roc_highlight = { path = "../highlight"}
+roc_reporting = { path = "../reporting"}
 bumpalo = { version = "3.8.0", features = ["collections"] }
 snafu = { version = "0.6.10", features = ["backtraces"] }
 peg = "0.8.0"

docs/src/lib.rs

@@ -424,6 +424,7 @@ pub fn load_modules_for_files(filenames: Vec<PathBuf>) -> Vec<LoadedModule> {
             src_dir.as_path(),
             Default::default(),
             roc_target::TargetInfo::default_x86_64(), // This is just type-checking for docs, so "target" doesn't matter
+            roc_reporting::report::RenderTarget::ColorTerminal,
         ) {
             Ok(loaded) => modules.push(loaded),
             Err(LoadingProblem::FormattedReport(report)) => {

repl_eval/src/gen.rs

@@ -60,6 +60,7 @@ pub fn compile_to_mono<'a>(
         src_dir,
         exposed_types,
         target_info,
+        roc_reporting::report::RenderTarget::ColorTerminal,
     );
 
     let mut loaded = match loaded {

reporting/Cargo.toml

@@ -23,9 +23,11 @@ bumpalo = { version = "3.8.0", features = ["collections"] }
 [dev-dependencies]
 roc_constrain = { path = "../compiler/constrain" }
 roc_builtins = { path = "../compiler/builtins" }
+roc_load = { path = "../compiler/load" }
 roc_problem = { path = "../compiler/problem" }
 roc_parse = { path = "../compiler/parse" }
 roc_target = { path = "../compiler/roc_target" }
 roc_test_utils = { path = "../test_utils" }
 pretty_assertions = "1.0.0"
 indoc = "1.0.3"
+tempfile = "3.2.0"

reporting/src/error/canonicalize.rs

@@ -44,6 +44,7 @@ const ALIAS_USES_ABILITY: &str = "ALIAS USES ABILITY";
 const ILLEGAL_HAS_CLAUSE: &str = "ILLEGAL HAS CLAUSE";
 const ABILITY_MEMBER_MISSING_HAS_CLAUSE: &str = "ABILITY MEMBER MISSING HAS CLAUSE";
 const ABILITY_MEMBER_HAS_EXTRANEOUS_HAS_CLAUSE: &str = "ABILITY MEMBER HAS EXTRANEOUS HAS CLAUSE";
+const ABILITY_MEMBER_BINDS_MULTIPLE_VARIABLES: &str = "ABILITY MEMBER BINDS MULTIPLE VARIABLES";
 
 pub fn can_problem<'b>(
     alloc: &'b RocDocAllocator<'b>,
@@ -684,6 +685,34 @@ pub fn can_problem<'b>(
             severity = Severity::RuntimeError;
         }
 
+        Problem::AbilityMemberMultipleBoundVars {
+            member,
+            ability,
+            span_has_clauses,
+            mut bound_var_names,
+        } => {
+            doc = alloc.stack(vec![
+                alloc.concat(vec![
+                    alloc.reflow("The definition of the ability member "),
+                    alloc.symbol_unqualified(member),
+                    alloc.reflow(" includes multiple variables bound to the "),
+                    alloc.symbol_unqualified(ability),
+                    alloc.keyword(" ability:"),
+                ]),
+                alloc.region(lines.convert_region(span_has_clauses)),
+                alloc.reflow("Ability members can only bind one type variable to their parent ability. Otherwise, I wouldn't know what type implements an ability by looking at specializations!"),
+                alloc.concat(vec![
+                    alloc.hint("Did you mean to only bind "),
+                    alloc.type_variable(bound_var_names.swap_remove(0)),
+                    alloc.reflow(" to "),
+                    alloc.symbol_unqualified(ability),
+                    alloc.reflow("?"),
+                ])
+            ]);
+            title = ABILITY_MEMBER_BINDS_MULTIPLE_VARIABLES.to_string();
+            severity = Severity::RuntimeError;
+        }
+
         Problem::AbilityMemberBindsExternalAbility {
             member,
             ability,

reporting/src/error/type.rs

@@ -118,6 +118,53 @@ pub fn type_problem<'b>(
                 other => panic!("unhandled bad type: {:?}", other),
             }
         }
+        IncompleteAbilityImplementation {
+            typ,
+            ability,
+            specialized_members,
+            missing_members,
+        } => {
+            let title = "INCOMPLETE ABILITY IMPLEMENTATION".to_string();
+
+            let mut stack = vec![alloc.concat(vec![
+                alloc.reflow("The type "),
+                alloc.symbol_unqualified(typ),
+                alloc.reflow(" does not fully implement the ability "),
+                alloc.symbol_unqualified(ability),
+                alloc.reflow(". The following specializations are missing:"),
+            ])];
+
+            for member in missing_members.into_iter() {
+                stack.push(alloc.concat(vec![
+                    alloc.reflow("A specialization for "),
+                    alloc.symbol_unqualified(member.value),
+                    alloc.reflow(", which is defined here:"),
+                ]));
+                stack.push(alloc.region(lines.convert_region(member.region)));
+            }
+
+            debug_assert!(!specialized_members.is_empty());
+
+            stack.push(alloc.concat(vec![
+                alloc.note(""),
+                alloc.symbol_unqualified(typ),
+                alloc.reflow(" specializes the following members of "),
+                alloc.symbol_unqualified(ability),
+                alloc.reflow(":"),
+            ]));
+
+            for spec in specialized_members {
+                stack.push(alloc.concat(vec![
+                    alloc.symbol_unqualified(spec.value),
+                    alloc.reflow(", specialized here:"),
+                ]));
+                stack.push(alloc.region(lines.convert_region(spec.region)));
+            }
+
+            let doc = alloc.stack(stack);
+
+            report(title, doc, filename)
+        }
     }
 }
 
@@ -950,6 +997,102 @@ fn to_expr_report<'b>(
                 None,
             ),
 
+            Reason::InvalidAbilityMemberSpecialization {
+                member_name,
+                def_region: _,
+                unimplemented_abilities,
+            } => {
+                let problem = alloc.concat(vec![
+                    alloc.reflow("Something is off with this specialization of "),
+                    alloc.symbol_unqualified(member_name),
+                    alloc.reflow(":"),
+                ]);
+                let this_is = alloc.reflow("This value is");
+                let instead_of = alloc.concat(vec![
+                    alloc.reflow("But the type annotation on "),
+                    alloc.symbol_unqualified(member_name),
+                    alloc.reflow(" says it must match:"),
+                ]);
+
+                let hint = if unimplemented_abilities.is_empty() {
+                    None
+                } else {
+                    let mut stack = Vec::with_capacity(unimplemented_abilities.len());
+                    for (err_type, ability) in unimplemented_abilities.into_iter() {
+                        stack.push(alloc.concat(vec![
+                            to_doc(alloc, Parens::Unnecessary, err_type).0,
+                            alloc.reflow(" does not implement "),
+                            alloc.symbol_unqualified(ability),
+                        ]));
+                    }
+
+                    let hint = alloc.stack(vec![
+                        alloc.concat(vec![
+                            alloc.note(""),
+                            alloc.reflow("Some types in this specialization don't implement the abilities they are expected to. I found the following missing implementations:"),
+                        ]),
+                        alloc.type_block(alloc.stack(stack)),
+                    ]);
+
+                    Some(hint)
+                };
+
+                report_mismatch(
+                    alloc,
+                    lines,
+                    filename,
+                    &category,
+                    found,
+                    expected_type,
+                    region,
+                    Some(expr_region),
+                    problem,
+                    this_is,
+                    instead_of,
+                    hint,
+                )
+            }
+
+            Reason::GeneralizedAbilityMemberSpecialization {
+                member_name,
+                def_region: _,
+            } => {
+                let problem = alloc.concat(vec![
+                    alloc.reflow("This specialization of "),
+                    alloc.symbol_unqualified(member_name),
+                    alloc.reflow(" is overly general:"),
+                ]);
+                let this_is = alloc.reflow("This value is");
+                let instead_of = alloc.concat(vec![
+                    alloc.reflow("But the type annotation on "),
+                    alloc.symbol_unqualified(member_name),
+                    alloc.reflow(" says it must match:"),
+                ]);
+
+                let note = alloc.stack(vec![
+                    alloc.concat(vec![
+                        alloc.note(""),
+                        alloc.reflow("The specialized type is too general, and does not provide a concrete type where a type variable is bound to an ability."),
+                    ]),
+                    alloc.reflow("Specializations can only be made for concrete types. If you have a generic implementation for this value, perhaps you don't need an ability?"),
+                ]);
+
+                report_mismatch(
+                    alloc,
+                    lines,
+                    filename,
+                    &category,
+                    found,
+                    expected_type,
+                    region,
+                    Some(expr_region),
+                    problem,
+                    this_is,
+                    instead_of,
+                    Some(note),
+                )
+            }
+
             Reason::LowLevelOpArg { op, arg_index } => {
                 panic!(
                     "Compiler bug: argument #{} to low-level operation {:?} was the wrong type!",
@@ -1281,6 +1424,10 @@ fn format_category<'b>(
             alloc.concat(vec![this_is, alloc.text(" a default field")]),
             alloc.text(" of type:"),
         ),
+        AbilityMemberSpecialization(_ability_member) => (
+            alloc.concat(vec![this_is, alloc.text(" a declared specialization")]),
+            alloc.text(" of type:"),
+        ),
     }
 }
 
@@ -1526,7 +1673,7 @@ fn to_circular_report<'b>(
                         You will see ∞ for parts of the type that repeat \
                         something already printed out infinitely.",
                     ),
-                    alloc.type_block(to_doc(alloc, Parens::Unnecessary, overall_type)),
+                    alloc.type_block(to_doc(alloc, Parens::Unnecessary, overall_type).0),
                 ]),
             ])
         },
@@ -1627,10 +1774,12 @@ fn to_comparison<'b>(
     expected: ErrorType,
 ) -> Comparison<'b> {
     let diff = to_diff(alloc, Parens::Unnecessary, actual, expected);
+    let actual = type_with_able_vars(alloc, diff.left, diff.left_able);
+    let expected = type_with_able_vars(alloc, diff.right, diff.right_able);
 
     Comparison {
-        actual: alloc.type_block(diff.left),
+        actual: alloc.type_block(actual),
-        expected: alloc.type_block(diff.right),
+        expected: alloc.type_block(expected),
         problems: match diff.status {
             Status::Similar => vec![],
             Status::Different(problems) => problems,
@@ -1683,6 +1832,9 @@ pub struct Diff<T> {
     left: T,
     right: T,
     status: Status,
+    // idea: lift "able" type variables so they are shown at the top of a type.
+    left_able: AbleVariables,
+    right_able: AbleVariables,
 }
 
 fn ext_to_doc<'b>(alloc: &'b RocDocAllocator<'b>, ext: TypeExt) -> Option<RocDocBuilder<'b>> {
@@ -1694,10 +1846,30 @@ fn ext_to_doc<'b>(alloc: &'b RocDocAllocator<'b>, ext: TypeExt) -> Option<RocDoc
     }
 }
 
+type AbleVariables = Vec<(Lowercase, Symbol)>;
+
+#[derive(Default)]
+struct Context {
+    able_variables: AbleVariables,
+}
+
 pub fn to_doc<'b>(
     alloc: &'b RocDocAllocator<'b>,
     parens: Parens,
     tipe: ErrorType,
+) -> (RocDocBuilder<'b>, AbleVariables) {
+    let mut ctx = Context::default();
+
+    let doc = to_doc_help(&mut ctx, alloc, parens, tipe);
+
+    (doc, ctx.able_variables)
+}
+
+fn to_doc_help<'b>(
+    ctx: &mut Context,
+    alloc: &'b RocDocAllocator<'b>,
+    parens: Parens,
+    tipe: ErrorType,
 ) -> RocDocBuilder<'b> {
     use ErrorType::*;
 
@@ -1706,22 +1878,26 @@ pub fn to_doc<'b>(
             alloc,
             parens,
             args.into_iter()
-                .map(|arg| to_doc(alloc, Parens::InFn, arg))
+                .map(|arg| to_doc_help(ctx, alloc, Parens::InFn, arg))
                 .collect(),
-            to_doc(alloc, Parens::InFn, *ret),
+            to_doc_help(ctx, alloc, Parens::InFn, *ret),
         ),
         Infinite => alloc.text("∞"),
         Error => alloc.text("?"),
 
-        FlexVar(lowercase) => alloc.type_variable(lowercase),
+        FlexVar(lowercase) | RigidVar(lowercase) => alloc.type_variable(lowercase),
-        RigidVar(lowercase) => alloc.type_variable(lowercase),
+        FlexAbleVar(lowercase, ability) | RigidAbleVar(lowercase, ability) => {
+            // TODO we should be putting able variables on the toplevel of the type, not here
+            ctx.able_variables.push((lowercase.clone(), ability));
+            alloc.type_variable(lowercase)
+        }
 
         Type(symbol, args) => report_text::apply(
             alloc,
             parens,
             alloc.symbol_foreign_qualified(symbol),
             args.into_iter()
-                .map(|arg| to_doc(alloc, Parens::InTypeParam, arg))
+                .map(|arg| to_doc_help(ctx, alloc, Parens::InTypeParam, arg))
                 .collect(),
         ),
 
@@ -1730,7 +1906,7 @@ pub fn to_doc<'b>(
             parens,
             alloc.symbol_foreign_qualified(symbol),
             args.into_iter()
-                .map(|arg| to_doc(alloc, Parens::InTypeParam, arg))
+                .map(|arg| to_doc_help(ctx, alloc, Parens::InTypeParam, arg))
                 .collect(),
         ),
 
@@ -1746,15 +1922,24 @@ pub fn to_doc<'b>(
                         (
                             alloc.string(k.as_str().to_string()),
                             match value {
-                                RecordField::Optional(v) => {
+                                RecordField::Optional(v) => RecordField::Optional(to_doc_help(
-                                    RecordField::Optional(to_doc(alloc, Parens::Unnecessary, v))
+                                    ctx,
-                                }
+                                    alloc,
-                                RecordField::Required(v) => {
+                                    Parens::Unnecessary,
-                                    RecordField::Required(to_doc(alloc, Parens::Unnecessary, v))
+                                    v,
-                                }
+                                )),
-                                RecordField::Demanded(v) => {
+                                RecordField::Required(v) => RecordField::Required(to_doc_help(
-                                    RecordField::Demanded(to_doc(alloc, Parens::Unnecessary, v))
+                                    ctx,
-                                }
+                                    alloc,
+                                    Parens::Unnecessary,
+                                    v,
+                                )),
+                                RecordField::Demanded(v) => RecordField::Demanded(to_doc_help(
+                                    ctx,
+                                    alloc,
+                                    Parens::Unnecessary,
+                                    v,
+                                )),
                             },
                         )
                     })
@@ -1770,7 +1955,7 @@ pub fn to_doc<'b>(
                     (
                         name,
                         args.into_iter()
-                            .map(|arg| to_doc(alloc, Parens::InTypeParam, arg))
+                            .map(|arg| to_doc_help(ctx, alloc, Parens::InTypeParam, arg))
                             .collect::<Vec<_>>(),
                     )
                 })
@@ -1793,7 +1978,7 @@ pub fn to_doc<'b>(
                     (
                         name,
                         args.into_iter()
-                            .map(|arg| to_doc(alloc, Parens::InTypeParam, arg))
+                            .map(|arg| to_doc_help(ctx, alloc, Parens::InTypeParam, arg))
                             .collect::<Vec<_>>(),
                     )
                 })
@@ -1802,7 +1987,7 @@ pub fn to_doc<'b>(
 
             report_text::recursive_tag_union(
                 alloc,
-                to_doc(alloc, Parens::Unnecessary, *rec_var),
+                to_doc_help(ctx, alloc, Parens::Unnecessary, *rec_var),
                 tags.into_iter()
                     .map(|(k, v)| (alloc.tag_name(k), v))
                     .collect(),
@@ -1811,10 +1996,10 @@ pub fn to_doc<'b>(
         }
 
         Range(typ, range_types) => {
-            let typ = to_doc(alloc, parens, *typ);
+            let typ = to_doc_help(ctx, alloc, parens, *typ);
             let range_types = range_types
                 .into_iter()
-                .map(|arg| to_doc(alloc, Parens::Unnecessary, arg))
+                .map(|arg| to_doc_help(ctx, alloc, Parens::Unnecessary, arg))
                 .collect();
             report_text::range(alloc, typ, range_types)
         }
@@ -1826,15 +2011,42 @@ fn same<'b>(
     parens: Parens,
     tipe: ErrorType,
 ) -> Diff<RocDocBuilder<'b>> {
-    let doc = to_doc(alloc, parens, tipe);
+    let (doc, able) = to_doc(alloc, parens, tipe);
 
     Diff {
         left: doc.clone(),
         right: doc,
         status: Status::Similar,
+        left_able: able.clone(),
+        right_able: able,
     }
 }
 
+fn type_with_able_vars<'b>(
+    alloc: &'b RocDocAllocator<'b>,
+    typ: RocDocBuilder<'b>,
+    able: AbleVariables,
+) -> RocDocBuilder<'b> {
+    if able.is_empty() {
+        // fast path: taken the vast majority of the time
+        return typ;
+    }
+
+    let mut doc = Vec::with_capacity(1 + 6 * able.len());
+    doc.push(typ);
+
+    for (i, (var, ability)) in able.into_iter().enumerate() {
+        doc.push(alloc.string(if i == 0 { " | " } else { ", " }.to_string()));
+        doc.push(alloc.type_variable(var));
+        doc.push(alloc.space());
+        doc.push(alloc.keyword("has"));
+        doc.push(alloc.space());
+        doc.push(alloc.symbol_foreign_qualified(ability));
+    }
+
+    alloc.concat(doc)
+}
+
 fn to_diff<'b>(
     alloc: &'b RocDocAllocator<'b>,
     parens: Parens,
@@ -1863,15 +2075,21 @@ fn to_diff<'b>(
 
                 let left = report_text::function(alloc, parens, arg_diff.left, ret_diff.left);
                 let right = report_text::function(alloc, parens, arg_diff.right, ret_diff.right);
+                let mut left_able = arg_diff.left_able;
+                left_able.extend(ret_diff.left_able);
+                let mut right_able = arg_diff.right_able;
+                right_able.extend(ret_diff.right_able);
 
                 Diff {
                     left,
                     right,
                     status,
+                    left_able,
+                    right_able,
                 }
             } else {
-                let left = to_doc(alloc, Parens::InFn, type1);
+                let (left, left_able) = to_doc(alloc, Parens::InFn, type1);
-                let right = to_doc(alloc, Parens::InFn, type2);
+                let (right, right_able) = to_doc(alloc, Parens::InFn, type2);
 
                 Diff {
                     left,
@@ -1880,6 +2098,8 @@ fn to_diff<'b>(
                         args1.len(),
                         args2.len(),
                     )]),
+                    left_able,
+                    right_able,
                 }
             }
         }
@@ -1902,6 +2122,8 @@ fn to_diff<'b>(
                 left,
                 right,
                 status: args_diff.status,
+                left_able: args_diff.left_able,
+                right_able: args_diff.right_able,
             }
         }
 
@@ -1924,17 +2146,21 @@ fn to_diff<'b>(
                 left,
                 right,
                 status: args_diff.status,
+                left_able: args_diff.left_able,
+                right_able: args_diff.right_able,
             }
         }
 
         (Alias(_, _, _, AliasKind::Opaque), _) | (_, Alias(_, _, _, AliasKind::Opaque)) => {
-            let left = to_doc(alloc, Parens::InFn, type1);
+            let (left, left_able) = to_doc(alloc, Parens::InFn, type1);
-            let right = to_doc(alloc, Parens::InFn, type2);
+            let (right, right_able) = to_doc(alloc, Parens::InFn, type2);
 
             Diff {
                 left,
                 right,
                 status: Status::Different(vec![Problem::OpaqueComparedToNonOpaque]),
+                left_able,
+                right_able,
             }
         }
 
@@ -1961,20 +2187,22 @@ fn to_diff<'b>(
 
         (RecursiveTagUnion(_rec1, _tags1, _ext1), RecursiveTagUnion(_rec2, _tags2, _ext2)) => {
             // TODO do a better job here
-            let left = to_doc(alloc, Parens::Unnecessary, type1);
+            let (left, left_able) = to_doc(alloc, Parens::Unnecessary, type1);
-            let right = to_doc(alloc, Parens::Unnecessary, type2);
+            let (right, right_able) = to_doc(alloc, Parens::Unnecessary, type2);
 
             Diff {
                 left,
                 right,
                 status: Status::Similar,
+                left_able,
+                right_able,
             }
         }
 
         pair => {
             // We hit none of the specific cases where we give more detailed information
-            let left = to_doc(alloc, parens, type1);
+            let (left, left_able) = to_doc(alloc, parens, type1);
-            let right = to_doc(alloc, parens, type2);
+            let (right, right_able) = to_doc(alloc, parens, type2);
 
             let is_int = |t: &ErrorType| match t {
                 ErrorType::Type(Symbol::NUM_INT, _) => true,
@@ -2030,6 +2258,8 @@ fn to_diff<'b>(
                 left,
                 right,
                 status: Status::Different(problems),
+                left_able,
+                right_able,
             }
         }
     }
@@ -2049,6 +2279,8 @@ where
     // TODO use ExactSizeIterator to pre-allocate here
     let mut left = Vec::new();
     let mut right = Vec::new();
+    let mut left_able = Vec::new();
+    let mut right_able = Vec::new();
 
     for (arg1, arg2) in args1.into_iter().zip(args2.into_iter()) {
         let diff = to_diff(alloc, parens, arg1, arg2);
@@ -2056,12 +2288,16 @@ where
         left.push(diff.left);
         right.push(diff.right);
         status.merge(diff.status);
+        left_able.extend(diff.left_able);
+        right_able.extend(diff.right_able);
     }
 
     Diff {
         left,
         right,
         status,
+        left_able,
+        right_able,
     }
 }
 
@@ -2128,6 +2364,8 @@ fn diff_record<'b>(
                     _ => diff.status,
                 }
             },
+            left_able: diff.left_able,
+            right_able: diff.right_able,
         }
     };
 
@@ -2135,7 +2373,7 @@ fn diff_record<'b>(
         (
             field.clone(),
             alloc.string(field.as_str().to_string()),
-            tipe.map(|t| to_doc(alloc, Parens::Unnecessary, t.clone())),
+            tipe.map(|t| to_doc(alloc, Parens::Unnecessary, t.clone()).0),
         )
     };
     let shared_keys = fields1
@@ -2193,12 +2431,16 @@ fn diff_record<'b>(
             left: vec![],
             right: vec![],
             status: Status::Similar,
+            left_able: vec![],
+            right_able: vec![],
         };
 
     for diff in both {
         fields_diff.left.push(diff.left);
         fields_diff.right.push(diff.right);
         fields_diff.status.merge(diff.status);
+        fields_diff.left_able.extend(diff.left_able);
+        fields_diff.right_able.extend(diff.right_able);
     }
 
     if !all_fields_shared {
@@ -2236,6 +2478,8 @@ fn diff_record<'b>(
         left: doc1,
         right: doc2,
         status: fields_diff.status,
+        left_able: fields_diff.left_able,
+        right_able: fields_diff.right_able,
     }
 }
 
@@ -2253,16 +2497,26 @@ fn diff_tag_union<'b>(
             left: (field.clone(), alloc.tag_name(field.clone()), diff.left),
             right: (field.clone(), alloc.tag_name(field), diff.right),
             status: diff.status,
+            left_able: diff.left_able,
+            right_able: diff.right_able,
         }
     };
-    let to_unknown_docs = |(field, args): (&TagName, &Vec<ErrorType>)| {
+    let to_unknown_docs = |(field, args): (&TagName, &Vec<ErrorType>)| -> (
-        (
+        TagName,
-            field.clone(),
+        RocDocBuilder<'b>,
-            alloc.tag_name(field.clone()),
+        Vec<RocDocBuilder<'b>>,
+        AbleVariables,
+    ) {
+        let (args, able): (_, Vec<AbleVariables>) =
             // TODO add spaces between args
             args.iter()
                 .map(|arg| to_doc(alloc, Parens::InTypeParam, arg.clone()))
-                .collect(),
+                .unzip();
+        (
+            field.clone(),
+            alloc.tag_name(field.clone()),
+            args,
+            able.into_iter().flatten().collect(),
         )
     };
     let shared_keys = fields1
@@ -2280,7 +2534,7 @@ fn diff_tag_union<'b>(
 
     let status = match (ext_has_fixed_fields(&ext1), ext_has_fixed_fields(&ext2)) {
         (true, true) => match left.peek() {
-            Some((f, _, _)) => Status::Different(vec![Problem::TagTypo(
+            Some((f, _, _, _)) => Status::Different(vec![Problem::TagTypo(
                 f.clone(),
                 fields2.keys().cloned().collect(),
             )]),
@@ -2297,14 +2551,14 @@ fn diff_tag_union<'b>(
             }
         },
         (false, true) => match left.peek() {
-            Some((f, _, _)) => Status::Different(vec![Problem::TagTypo(
+            Some((f, _, _, _)) => Status::Different(vec![Problem::TagTypo(
                 f.clone(),
                 fields2.keys().cloned().collect(),
             )]),
             None => Status::Similar,
         },
         (true, false) => match right.peek() {
-            Some((f, _, _)) => Status::Different(vec![Problem::TagTypo(
+            Some((f, _, _, _)) => Status::Different(vec![Problem::TagTypo(
                 f.clone(),
                 fields1.keys().cloned().collect(),
             )]),
@@ -2319,17 +2573,27 @@ fn diff_tag_union<'b>(
         left: vec![],
         right: vec![],
         status: Status::Similar,
+        left_able: vec![],
+        right_able: vec![],
     };
 
     for diff in both {
         fields_diff.left.push(diff.left);
         fields_diff.right.push(diff.right);
         fields_diff.status.merge(diff.status);
+        fields_diff.left_able.extend(diff.left_able);
+        fields_diff.right_able.extend(diff.right_able);
     }
 
     if !all_fields_shared {
-        fields_diff.left.extend(left);
+        for (tag, tag_doc, args, able) in left {
-        fields_diff.right.extend(right);
+            fields_diff.left.push((tag, tag_doc, args));
+            fields_diff.left_able.extend(able);
+        }
+        for (tag, tag_doc, args, able) in right {
+            fields_diff.right.push((tag, tag_doc, args));
+            fields_diff.right_able.extend(able);
+        }
         fields_diff.status.merge(Status::Different(vec![]));
     }
 
@@ -2356,6 +2620,8 @@ fn diff_tag_union<'b>(
         left: doc1,
         right: doc2,
         status: fields_diff.status,
+        left_able: fields_diff.left_able,
+        right_able: fields_diff.right_able,
     }
 }
 
@@ -2373,12 +2639,16 @@ fn ext_to_diff<'b>(
             left: ext_doc_1,
             right: ext_doc_2,
             status,
+            left_able: vec![],
+            right_able: vec![],
         },
         Status::Different(_) => Diff {
             // NOTE elm colors these differently at this point
             left: ext_doc_1,
             right: ext_doc_2,
             status,
+            left_able: vec![],
+            right_able: vec![],
         },
     }
 }
@@ -2518,7 +2788,7 @@ mod report_text {
         let entry_to_doc = |(name, tipe): (Lowercase, RecordField<ErrorType>)| {
             (
                 alloc.string(name.as_str().to_string()),
-                to_doc(alloc, Parens::Unnecessary, tipe.into_inner()),
+                to_doc(alloc, Parens::Unnecessary, tipe.into_inner()).0,
             )
         };
 
@@ -2863,7 +3133,14 @@ fn type_problem_to_pretty<'b>(
 
             match tipe {
                 Infinite | Error | FlexVar(_) => alloc.nil(),
-                RigidVar(y) => bad_double_rigid(x, y),
+                FlexAbleVar(_, ability) => bad_rigid_var(
+                    x,
+                    alloc.concat(vec![
+                        alloc.reflow("an instance of the ability "),
+                        alloc.symbol_unqualified(ability),
+                    ]),
+                ),
+                RigidVar(y) | RigidAbleVar(y, _) => bad_double_rigid(x, y),
                 Function(_, _, _) => bad_rigid_var(x, alloc.reflow("a function value")),
                 Record(_, _) => bad_rigid_var(x, alloc.reflow("a record value")),
                 TagUnion(_, _) | RecursiveTagUnion(_, _, _) => {

reporting/src/report.rs

@@ -72,6 +72,12 @@ pub enum Severity {
     Warning,
 }
 
+#[derive(Clone, Copy, Debug)]
+pub enum RenderTarget {
+    ColorTerminal,
+    Generic,
+}
+
 /// A textual report.
 pub struct Report<'b> {
     pub title: String,
@@ -81,6 +87,19 @@ pub struct Report<'b> {
 }
 
 impl<'b> Report<'b> {
+    pub fn render(
+        self,
+        target: RenderTarget,
+        buf: &'b mut String,
+        alloc: &'b RocDocAllocator<'b>,
+        palette: &'b Palette,
+    ) {
+        match target {
+            RenderTarget::Generic => self.render_ci(buf, alloc),
+            RenderTarget::ColorTerminal => self.render_color_terminal(buf, alloc, palette),
+        }
+    }
+
     /// Render to CI console output, where no colors are available.
     pub fn render_ci(self, buf: &'b mut String, alloc: &'b RocDocAllocator<'b>) {
         let err_msg = "<buffer is not a utf-8 encoded string>";

reporting/tests/helpers/mod.rs

@@ -1,6 +1,7 @@
 extern crate bumpalo;
 
 use self::bumpalo::Bump;
+use roc_can::abilities::AbilitiesStore;
 use roc_can::constraint::{Constraint, Constraints};
 use roc_can::env::Env;
 use roc_can::expected::Expected;
@@ -31,10 +32,19 @@ pub fn infer_expr(
     constraints: &Constraints,
     constraint: &Constraint,
     aliases: &mut Aliases,
+    abilities_store: &mut AbilitiesStore,
     expr_var: Variable,
 ) -> (Content, Subs) {
     let env = solve::Env::default();
-    let (solved, _) = solve::run(constraints, &env, problems, subs, aliases, constraint);
+    let (solved, _) = solve::run(
+        constraints,
+        &env,
+        problems,
+        subs,
+        aliases,
+        constraint,
+        abilities_store,
+    );
 
     let content = *solved.inner().get_content_without_compacting(expr_var);
 

reporting/tests/test_reporting.rs

@@ -8,17 +8,20 @@ mod helpers;
 
 #[cfg(test)]
 mod test_reporting {
-    use crate::helpers::test_home;
+    use crate::helpers::{can_expr, infer_expr, test_home, CanExprOut, ParseErrOut};
-    use crate::helpers::{can_expr, infer_expr, CanExprOut, ParseErrOut};
     use bumpalo::Bump;
     use indoc::indoc;
+    use roc_can::abilities::AbilitiesStore;
+    use roc_can::def::Declaration;
+    use roc_can::pattern::Pattern;
+    use roc_load::{self, LoadedModule, LoadingProblem};
     use roc_module::symbol::{Interns, ModuleId};
     use roc_mono::ir::{Procs, Stmt, UpdateModeIds};
     use roc_mono::layout::LayoutCache;
     use roc_region::all::LineInfo;
     use roc_reporting::report::{
-        can_problem, mono_problem, parse_problem, type_problem, Report, Severity, ANSI_STYLE_CODES,
+        can_problem, mono_problem, parse_problem, type_problem, RenderTarget, Report, Severity,
-        DEFAULT_PALETTE,
+        ANSI_STYLE_CODES, DEFAULT_PALETTE,
     };
     use roc_reporting::report::{RocDocAllocator, RocDocBuilder};
     use roc_solve::solve;
@@ -43,6 +46,214 @@ mod test_reporting {
         }
     }
 
+    fn promote_expr_to_module(src: &str) -> String {
+        let mut buffer =
+            String::from("app \"test\" provides [ main ] to \"./platform\"\n\nmain =\n");
+
+        for line in src.lines() {
+            // indent the body!
+            buffer.push_str("    ");
+            buffer.push_str(line);
+            buffer.push('\n');
+        }
+
+        buffer
+    }
+
+    fn run_load_and_infer<'a>(
+        arena: &'a Bump,
+        src: &'a str,
+    ) -> (String, Result<LoadedModule, LoadingProblem<'a>>) {
+        use std::fs::File;
+        use std::io::Write;
+        use tempfile::tempdir;
+
+        let module_src = if src.starts_with("app") {
+            // this is already a module
+            src.to_string()
+        } else {
+            // this is an expression, promote it to a module
+            promote_expr_to_module(src)
+        };
+
+        let exposed_types = Default::default();
+        let loaded = {
+            let dir = tempdir().unwrap();
+            let filename = PathBuf::from("Test.roc");
+            let file_path = dir.path().join(filename);
+            let full_file_path = file_path.clone();
+            let mut file = File::create(file_path).unwrap();
+            writeln!(file, "{}", module_src).unwrap();
+            let result = roc_load::load_and_typecheck(
+                arena,
+                full_file_path,
+                dir.path(),
+                exposed_types,
+                roc_target::TargetInfo::default_x86_64(),
+                RenderTarget::Generic,
+            );
+            drop(file);
+
+            dir.close().unwrap();
+
+            result
+        };
+
+        (module_src, loaded)
+    }
+
+    fn infer_expr_help_new<'a>(
+        arena: &'a Bump,
+        expr_src: &'a str,
+    ) -> Result<
+        (
+            String,
+            Vec<solve::TypeError>,
+            Vec<roc_problem::can::Problem>,
+            Vec<roc_mono::ir::MonoProblem>,
+            ModuleId,
+            Interns,
+        ),
+        LoadingProblem<'a>,
+    > {
+        let (module_src, result) = run_load_and_infer(arena, expr_src);
+        let LoadedModule {
+            module_id: home,
+            mut can_problems,
+            mut type_problems,
+            interns,
+            mut solved,
+            exposed_to_host,
+            mut declarations_by_id,
+            ..
+        } = result?;
+
+        let can_problems = can_problems.remove(&home).unwrap_or_default();
+        let type_problems = type_problems.remove(&home).unwrap_or_default();
+
+        let subs = solved.inner_mut();
+
+        for var in exposed_to_host.values() {
+            name_all_type_vars(*var, subs);
+        }
+
+        let mut mono_problems = Vec::new();
+
+        // MONO
+
+        if type_problems.is_empty() && can_problems.is_empty() {
+            let arena = Bump::new();
+
+            assert!(exposed_to_host.len() == 1);
+            let (sym, _var) = exposed_to_host.into_iter().next().unwrap();
+
+            let home_decls = declarations_by_id.remove(&home).unwrap();
+            let (loc_expr, var) = home_decls
+                .into_iter()
+                .find_map(|decl| match decl {
+                    Declaration::Declare(def) => match def.loc_pattern.value {
+                        Pattern::Identifier(s) if s == sym => Some((def.loc_expr, def.expr_var)),
+                        _ => None,
+                    },
+                    _ => None,
+                })
+                .expect("No expression to monomorphize found!");
+
+            // Compile and add all the Procs before adding main
+            let mut procs = Procs::new_in(&arena);
+            let mut ident_ids = interns.all_ident_ids.get(&home).unwrap().clone();
+            let mut update_mode_ids = UpdateModeIds::new();
+
+            // Populate Procs and Subs, and get the low-level Expr from the canonical Expr
+            let target_info = roc_target::TargetInfo::default_x86_64();
+            let mut layout_cache = LayoutCache::new(target_info);
+            let mut mono_env = roc_mono::ir::Env {
+                arena: &arena,
+                subs,
+                problems: &mut mono_problems,
+                home,
+                ident_ids: &mut ident_ids,
+                update_mode_ids: &mut update_mode_ids,
+                target_info,
+                // call_specialization_counter=0 is reserved
+                call_specialization_counter: 1,
+            };
+            let _mono_expr = Stmt::new(
+                &mut mono_env,
+                loc_expr.value,
+                var,
+                &mut procs,
+                &mut layout_cache,
+            );
+        }
+
+        Ok((
+            module_src,
+            type_problems,
+            can_problems,
+            mono_problems,
+            home,
+            interns,
+        ))
+    }
+
+    fn list_reports_new<F>(arena: &Bump, src: &str, finalize_render: F) -> String
+    where
+        F: FnOnce(RocDocBuilder<'_>, &mut String),
+    {
+        use ven_pretty::DocAllocator;
+
+        let filename = filename_from_string(r"\code\proj\Main.roc");
+
+        let mut buf = String::new();
+
+        match infer_expr_help_new(arena, src) {
+            Err(LoadingProblem::FormattedReport(fail)) => fail,
+            Ok((module_src, type_problems, can_problems, mono_problems, home, interns)) => {
+                let lines = LineInfo::new(&module_src);
+                let src_lines: Vec<&str> = module_src.split('\n').collect();
+                let mut reports = Vec::new();
+
+                let alloc = RocDocAllocator::new(&src_lines, home, &interns);
+
+                for problem in can_problems {
+                    let report = can_problem(&alloc, &lines, filename.clone(), problem.clone());
+                    reports.push(report);
+                }
+
+                for problem in type_problems {
+                    if let Some(report) =
+                        type_problem(&alloc, &lines, filename.clone(), problem.clone())
+                    {
+                        reports.push(report);
+                    }
+                }
+
+                for problem in mono_problems {
+                    let report = mono_problem(&alloc, &lines, filename.clone(), problem.clone());
+                    reports.push(report);
+                }
+
+                let has_reports = !reports.is_empty();
+
+                let doc = alloc
+                    .stack(reports.into_iter().map(|v| v.pretty(&alloc)))
+                    .append(if has_reports {
+                        alloc.line()
+                    } else {
+                        alloc.nil()
+                    });
+
+                finalize_render(doc, &mut buf);
+                buf
+            }
+            Err(other) => {
+                assert!(false, "failed to load: {:?}", other);
+                unreachable!()
+            }
+        }
+    }
+
     fn infer_expr_help<'a>(
         arena: &'a Bump,
         expr_src: &'a str,
@@ -85,12 +296,14 @@ mod test_reporting {
         }
 
         let mut unify_problems = Vec::new();
+        let mut abilities_store = AbilitiesStore::default();
         let (_content, mut subs) = infer_expr(
             subs,
             &mut unify_problems,
             &constraints,
             &constraint,
             &mut solve_aliases,
+            &mut abilities_store,
             var,
         );
 
@@ -298,6 +511,27 @@ mod test_reporting {
         assert_eq!(readable, expected_rendering);
     }
 
+    fn new_report_problem_as(src: &str, expected_rendering: &str) {
+        let arena = Bump::new();
+
+        let finalize_render = |doc: RocDocBuilder<'_>, buf: &mut String| {
+            doc.1
+                .render_raw(70, &mut roc_reporting::report::CiWrite::new(buf))
+                .expect("list_reports")
+        };
+
+        let buf = list_reports_new(&arena, src, finalize_render);
+
+        // convenient to copy-paste the generated message
+        if buf != expected_rendering {
+            for line in buf.split('\n') {
+                println!("                {}", line);
+            }
+        }
+
+        assert_multiline_str_eq!(expected_rendering, buf.as_str());
+    }
+
     fn human_readable(str: &str) -> String {
         str.replace(ANSI_STYLE_CODES.red, "<red>")
             .replace(ANSI_STYLE_CODES.white, "<white>")
@@ -8682,7 +8916,7 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn ability_demands_not_indented_with_first() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
                 Eq has
@@ -8699,19 +8933,18 @@ I need all branches in an `if` to have the same type!
                 I was partway through parsing an ability definition, but I got stuck
                 here:
 
-                2│      eq : a, a -> U64 | a has Eq
+                5│          eq : a, a -> U64 | a has Eq
-                3│          neq : a, a -> U64 | a has Eq
+                6│              neq : a, a -> U64 | a has Eq
-                            ^
+                                ^
 
-                I suspect this line is indented too much (by 4 spaces)
+                I suspect this line is indented too much (by 4 spaces)"#
-                "#
             ),
         )
     }
 
     #[test]
     fn ability_demand_value_has_args() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
                 Eq has
@@ -8727,12 +8960,11 @@ I need all branches in an `if` to have the same type!
                 I was partway through parsing an ability definition, but I got stuck
                 here:
 
-                2│      eq b c : a, a -> U64 | a has Eq
+                5│          eq b c : a, a -> U64 | a has Eq
-                           ^
+                               ^
 
                 I was expecting to see a : annotating the signature of this value
-                next.
+                next."#
-                "#
             ),
         )
     }
@@ -8898,8 +9130,8 @@ I need all branches in an `if` to have the same type!
     }
 
     #[test]
-    fn bad_type_parameter_in_ability() {
+    fn ability_bad_type_parameter() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
                 Hash a b c has
@@ -8914,8 +9146,8 @@ I need all branches in an `if` to have the same type!
 
                 The definition of the `Hash` ability includes type variables:
 
-                1│  Hash a b c has
+                4│      Hash a b c has
-                         ^^^^^
+                             ^^^^^
 
                 Abilities cannot depend on type variables, but their member values
                 can!
@@ -8924,8 +9156,8 @@ I need all branches in an `if` to have the same type!
 
                 `Hash` is not used anywhere in your code.
 
-                1│  Hash a b c has
+                4│      Hash a b c has
-                    ^^^^
+                        ^^^^
 
                 If you didn't intend on using `Hash` then remove it so future readers of
                 your code don't wonder why it is there.
@@ -8936,12 +9168,12 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn alias_in_has_clause() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
-                Hash has hash : a, b -> U64 | a has Hash, b has Bool
+                app "test" provides [ hash ] to "./platform"
 
-                1
+                Hash has hash : a, b -> U64 | a has Hash, b has Bool
                 "#
             ),
             indoc!(
@@ -8950,18 +9182,8 @@ I need all branches in an `if` to have the same type!
 
                 The type referenced in this "has" clause is not an ability:
 
-                1│  Hash has hash : a, b -> U64 | a has Hash, b has Bool
+                3│  Hash has hash : a, b -> U64 | a has Hash, b has Bool
                                                                     ^^^^
-
-                ── UNUSED DEFINITION ───────────────────────────────────────────────────────────
-
-                `hash` is not used anywhere in your code.
-
-                1│  Hash has hash : a, b -> U64 | a has Hash, b has Bool
-                             ^^^^
-
-                If you didn't intend on using `hash` then remove it so future readers of
-                your code don't wonder why it is there.
                 "#
             ),
         )
@@ -8969,12 +9191,12 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn shadowed_type_variable_in_has_clause() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
-                Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
+                app "test" provides [ ab1 ] to "./platform"
 
-                1
+                Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
                 "#
             ),
             indoc!(
@@ -8983,26 +9205,16 @@ I need all branches in an `if` to have the same type!
 
                 The `a` name is first defined here:
 
-                1│  Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
+                3│  Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
                                             ^^^^^^^^^
 
                 But then it's defined a second time here:
 
-                1│  Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
+                3│  Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
                                                        ^^^^^^^^^
 
                 Since these variables have the same name, it's easy to use the wrong
                 one on accident. Give one of them a new name.
-
-                ── UNUSED DEFINITION ───────────────────────────────────────────────────────────
-
-                `ab1` is not used anywhere in your code.
-
-                1│  Ab1 has ab1 : a -> {} | a has Ab1, a has Ab1
-                            ^^^
-
-                If you didn't intend on using `ab1` then remove it so future readers of
-                your code don't wonder why it is there.
                 "#
             ),
         )
@@ -9010,7 +9222,7 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn alias_using_ability() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
                 Ability has ab : a -> {} | a has Ability
@@ -9027,8 +9239,8 @@ I need all branches in an `if` to have the same type!
 
                 The definition of the `Alias` aliases references the ability `Ability`:
 
-                3│  Alias : Ability
+                6│      Alias : Ability
-                    ^^^^^
+                        ^^^^^
 
                 Abilities are not types, but you can add an ability constraint to a
                 type variable `a` by writing
@@ -9041,8 +9253,8 @@ I need all branches in an `if` to have the same type!
 
                 `ab` is not used anywhere in your code.
 
-                1│  Ability has ab : a -> {} | a has Ability
+                4│      Ability has ab : a -> {} | a has Ability
-                                ^^
+                                    ^^
 
                 If you didn't intend on using `ab` then remove it so future readers of
                 your code don't wonder why it is there.
@@ -9053,7 +9265,7 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn ability_shadows_ability() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
                 Ability has ab : a -> U64 | a has Ability
@@ -9069,13 +9281,13 @@ I need all branches in an `if` to have the same type!
 
                 The `Ability` name is first defined here:
 
-                1│  Ability has ab : a -> U64 | a has Ability
+                4│      Ability has ab : a -> U64 | a has Ability
-                    ^^^^^^^
+                        ^^^^^^^
 
                 But then it's defined a second time here:
 
-                3│  Ability has ab1 : a -> U64 | a has Ability
+                6│      Ability has ab1 : a -> U64 | a has Ability
-                    ^^^^^^^
+                        ^^^^^^^
 
                 Since these abilities have the same name, it's easy to use the wrong
                 one on accident. Give one of them a new name.
@@ -9084,8 +9296,8 @@ I need all branches in an `if` to have the same type!
 
                 `ab` is not used anywhere in your code.
 
-                1│  Ability has ab : a -> U64 | a has Ability
+                4│      Ability has ab : a -> U64 | a has Ability
-                                ^^
+                                    ^^
 
                 If you didn't intend on using `ab` then remove it so future readers of
                 your code don't wonder why it is there.
@@ -9096,12 +9308,12 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn ability_member_does_not_bind_ability() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
-                Ability has ab : {} -> {}
+                app "test" provides [ ] to "./platform"
 
-                1
+                Ability has ab : {} -> {}
                 "#
             ),
             indoc!(
@@ -9111,7 +9323,7 @@ I need all branches in an `if` to have the same type!
                 The definition of the ability member `ab` does not include a `has` clause
                 binding a type variable to the ability `Ability`:
 
-                1│  Ability has ab : {} -> {}
+                3│  Ability has ab : {} -> {}
                                 ^^
 
                 Ability members must include a `has` clause binding a type variable to
@@ -9125,7 +9337,7 @@ I need all branches in an `if` to have the same type!
 
                 `Ability` is not used anywhere in your code.
 
-                1│  Ability has ab : {} -> {}
+                3│  Ability has ab : {} -> {}
                     ^^^^^^^
 
                 If you didn't intend on using `Ability` then remove it so future readers
@@ -9135,7 +9347,7 @@ I need all branches in an `if` to have the same type!
 
                 `ab` is not used anywhere in your code.
 
-                1│  Ability has ab : {} -> {}
+                3│  Ability has ab : {} -> {}
                                 ^^
 
                 If you didn't intend on using `ab` then remove it so future readers of
@@ -9147,13 +9359,13 @@ I need all branches in an `if` to have the same type!
 
     #[test]
     fn ability_member_binds_extra_ability() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
+                app "test" provides [ eq ] to "./platform"
+
                 Eq has eq : a, a -> Bool | a has Eq
                 Hash has hash : a, b -> U64 | a has Eq, b has Hash
-
-                1
                 "#
             ),
             indoc!(
@@ -9163,7 +9375,7 @@ I need all branches in an `if` to have the same type!
                 The definition of the ability member `hash` includes a has clause
                 binding an ability it is not a part of:
 
-                2│  Hash has hash : a, b -> U64 | a has Eq, b has Hash
+                4│  Hash has hash : a, b -> U64 | a has Eq, b has Hash
                                                   ^^^^^^^^
 
                 Currently, ability members can only bind variables to the ability they
@@ -9173,19 +9385,9 @@ I need all branches in an `if` to have the same type!
 
                 ── UNUSED DEFINITION ───────────────────────────────────────────────────────────
 
-                `eq` is not used anywhere in your code.
-
-                1│  Eq has eq : a, a -> Bool | a has Eq
-                           ^^
-
-                If you didn't intend on using `eq` then remove it so future readers of
-                your code don't wonder why it is there.
-
-                ── UNUSED DEFINITION ───────────────────────────────────────────────────────────
-
                 `hash` is not used anywhere in your code.
 
-                2│  Hash has hash : a, b -> U64 | a has Eq, b has Hash
+                4│  Hash has hash : a, b -> U64 | a has Eq, b has Hash
                              ^^^^
 
                 If you didn't intend on using `hash` then remove it so future readers of
@@ -9196,15 +9398,55 @@ I need all branches in an `if` to have the same type!
     }
 
     #[test]
-    fn has_clause_outside_of_ability() {
+    fn ability_member_binds_parent_twice() {
-        report_problem_as(
+        new_report_problem_as(
             indoc!(
                 r#"
+                app "test" provides [ ] to "./platform"
+
+                Eq has eq : a, b -> Bool | a has Eq, b has Eq
+                "#
+            ),
+            indoc!(
+                r#"
+                ── ABILITY MEMBER BINDS MULTIPLE VARIABLES ─────────────────────────────────────
+
+                The definition of the ability member `eq` includes multiple variables
+                bound to the `Eq`` ability:`
+
+                3│  Eq has eq : a, b -> Bool | a has Eq, b has Eq
+                                               ^^^^^^^^^^^^^^^^^^
+
+                Ability members can only bind one type variable to their parent
+                ability. Otherwise, I wouldn't know what type implements an ability by
+                looking at specializations!
+
+                Hint: Did you mean to only bind `a` to `Eq`?
+
+                ── UNUSED DEFINITION ───────────────────────────────────────────────────────────
+
+                `eq` is not used anywhere in your code.
+
+                3│  Eq has eq : a, b -> Bool | a has Eq, b has Eq
+                           ^^
+
+                If you didn't intend on using `eq` then remove it so future readers of
+                your code don't wonder why it is there.
+                "#
+            ),
+        )
+    }
+
+    #[test]
+    fn has_clause_outside_of_ability() {
+        new_report_problem_as(
+            indoc!(
+                r#"
+                app "test" provides [ hash, f ] to "./platform"
+
                 Hash has hash : a -> U64 | a has Hash
 
                 f : a -> U64 | a has Hash
-
-                f
                 "#
             ),
             indoc!(
@@ -9213,21 +9455,206 @@ I need all branches in an `if` to have the same type!
 
                 A `has` clause is not allowed here:
 
-                3│  f : a -> U64 | a has Hash
+                5│  f : a -> U64 | a has Hash
                                    ^^^^^^^^^^
 
                 `has` clauses can only be specified on the top-level type annotation of
                 an ability member.
+                "#
+            ),
+        )
+    }
 
-                ── UNUSED DEFINITION ───────────────────────────────────────────────────────────
+    #[test]
+    fn ability_specialization_with_non_implementing_type() {
+        new_report_problem_as(
+            indoc!(
+                r#"
+                app "test" provides [ hash ] to "./platform"
 
-                `hash` is not used anywhere in your code.
+                Hash has hash : a -> U64 | a has Hash
 
-                1│  Hash has hash : a -> U64 | a has Hash
+                hash = \{} -> 0u64
-                             ^^^^
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
 
-                If you didn't intend on using `hash` then remove it so future readers of
+                Something is off with this specialization of `hash`:
-                your code don't wonder why it is there.
+
+                5│  hash = \{} -> 0u64
+                    ^^^^
+
+                This value is a declared specialization of type:
+
+                    {}a -> U64
+
+                But the type annotation on `hash` says it must match:
+
+                    a -> U64 | a has Hash
+
+                Note: Some types in this specialization don't implement the abilities
+                they are expected to. I found the following missing implementations:
+
+                    {}a does not implement Hash
+                "#
+            ),
+        )
+    }
+
+    #[test]
+    fn ability_specialization_does_not_match_type() {
+        new_report_problem_as(
+            indoc!(
+                r#"
+                app "test" provides [ hash ] to "./platform"
+
+                Hash has hash : a -> U64 | a has Hash
+
+                Id := U32
+
+                hash = \$Id n -> n
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+                Something is off with this specialization of `hash`:
+
+                7│  hash = \$Id n -> n
+                    ^^^^
+
+                This value is a declared specialization of type:
+
+                    Id -> U32
+
+                But the type annotation on `hash` says it must match:
+
+                    Id -> U64
+                "#
+            ),
+        )
+    }
+
+    #[test]
+    fn ability_specialization_is_incomplete() {
+        new_report_problem_as(
+            indoc!(
+                r#"
+                app "test" provides [ eq, le ] to "./platform"
+
+                Eq has
+                    eq : a, a -> Bool | a has Eq
+                    le : a, a -> Bool | a has Eq
+
+                Id := U64
+
+                eq = \$Id m, $Id n -> m == n
+                "#
+            ),
+            indoc!(
+                r#"
+                ── INCOMPLETE ABILITY IMPLEMENTATION ───────────────────────────────────────────
+
+                The type `Id` does not fully implement the ability `Eq`. The following
+                specializations are missing:
+
+                A specialization for `le`, which is defined here:
+
+                5│      le : a, a -> Bool | a has Eq
+                        ^^
+
+                Note: `Id` specializes the following members of `Eq`:
+
+                `eq`, specialized here:
+
+                9│  eq = \$Id m, $Id n -> m == n
+                    ^^
+                "#
+            ),
+        )
+    }
+
+    #[test]
+    fn ability_specialization_overly_generalized() {
+        new_report_problem_as(
+            indoc!(
+                r#"
+                app "test" provides [ hash ] to "./platform"
+
+                Hash has
+                    hash : a -> U64 | a has Hash
+
+                hash = \_ -> 0u64
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+                This specialization of `hash` is overly general:
+
+                6│  hash = \_ -> 0u64
+                    ^^^^
+
+                This value is a declared specialization of type:
+
+                    a -> U64
+
+                But the type annotation on `hash` says it must match:
+
+                    a -> U64 | a has Hash
+
+                Note: The specialized type is too general, and does not provide a
+                concrete type where a type variable is bound to an ability.
+
+                Specializations can only be made for concrete types. If you have a
+                generic implementation for this value, perhaps you don't need an
+                ability?
+                "#
+            ),
+        )
+    }
+
+    #[test]
+    fn ability_specialization_conflicting_specialization_types() {
+        new_report_problem_as(
+            indoc!(
+                r#"
+                app "test" provides [ eq ] to "./platform"
+
+                Eq has
+                    eq : a, a -> Bool | a has Eq
+
+                You := {}
+                AndI := {}
+
+                eq = \$You {}, $AndI {} -> False
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+                Something is off with this specialization of `eq`:
+
+                9│  eq = \$You {}, $AndI {} -> False
+                    ^^
+
+                This value is a declared specialization of type:
+
+                    You, AndI -> [ False, True ]
+
+                But the type annotation on `eq` says it must match:
+
+                    You, You -> Bool
+
+                Tip: Type comparisons between an opaque type are only ever equal if
+                both types are the same opaque type. Did you mean to create an opaque
+                type by wrapping it? If I have an opaque type Age := U32 I can create
+                an instance of this opaque type by doing @Age 23.
                 "#
             ),
         )