Getting closer. Checking in for a break.

ast/src/solve_type.rs

@@ -1408,7 +1408,10 @@ fn adjust_rank_content(
             }
         }
 
-        LambdaSet(subs::LambdaSet { solved }) => {
+        LambdaSet(subs::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
             let mut rank = group_rank;
 
             for (_, index) in solved.iter_all() {
@@ -1419,6 +1422,15 @@ fn adjust_rank_content(
                 }
             }
 
+            if let Some(rec_var) = recursion_var.into_variable() {
+                // THEORY: the recursion var has the same rank as the tag union itself
+                // all types it uses are also in the tags already, so it cannot influence the
+                // rank
+                debug_assert!(
+                    rank >= adjust_rank(subs, young_mark, visit_mark, group_rank, rec_var)
+                );
+            }
+
             rank
         }
 
@@ -1596,7 +1608,14 @@ fn instantiate_rigids_help(
             instantiate_rigids_help(subs, max_rank, pools, real_type_var);
         }
 
-        LambdaSet(subs::LambdaSet { solved }) => {
+        LambdaSet(subs::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
+            if let Some(rec_var) = recursion_var.into_variable() {
+                instantiate_rigids_help(subs, max_rank, pools, rec_var);
+            }
+
             for (_, index) in solved.iter_all() {
                 let slice = subs[index];
                 for var_index in slice {
@@ -1872,7 +1891,10 @@ fn deep_copy_var_help(
             copy
         }
 
-        LambdaSet(subs::LambdaSet { solved }) => {
+        LambdaSet(subs::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
             let mut new_variable_slices = Vec::with_capacity(solved.len());
 
             let mut new_variables = Vec::new();
@@ -1898,7 +1920,13 @@ fn deep_copy_var_help(
             };
 
             let new_solved = UnionTags::from_slices(solved.tag_names(), new_variables);
-            let new_content = LambdaSet(subs::LambdaSet { solved: new_solved });
+            let new_rec_var =
+                recursion_var.map(|rec_var| deep_copy_var_help(subs, max_rank, pools, rec_var));
+
+            let new_content = LambdaSet(subs::LambdaSet {
+                solved: new_solved,
+                recursion_var: new_rec_var,
+            });
 
             subs.set(copy, make_descriptor(new_content));
 

bindgen/src/bindgen.rs

@@ -118,9 +118,10 @@ fn add_type_help<'a>(
                 fields,
             })
         }
-        Content::LambdaSet(LambdaSet { solved }) => {
+        Content::LambdaSet(LambdaSet {
-            add_tag_union(env, opt_name, solved, var, types)
+            solved,
-        }
+            recursion_var: _,
+        }) => add_tag_union(env, opt_name, solved, var, types),
         Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
             debug_assert!(ext_var_is_empty_tag_union(subs, *ext_var));
 

compiler/mono/src/copy.rs

@@ -607,7 +607,11 @@ fn deep_copy_type_vars<'a>(
                 })
             }
 
-            LambdaSet(subs::LambdaSet { solved }) => {
+            LambdaSet(subs::LambdaSet {
+                solved,
+                recursion_var,
+            }) => {
+                let new_rec_var = recursion_var.map(|var| descend_var!(var));
                 for variables_slice_index in solved.variables() {
                     let variables_slice = subs[variables_slice_index];
                     descend_slice!(variables_slice);
@@ -626,7 +630,10 @@ fn deep_copy_type_vars<'a>(
                     let new_solved =
                         UnionTags::from_slices(solved.tag_names(), new_variable_slices);
 
-                    LambdaSet(subs::LambdaSet { solved: new_solved })
+                    LambdaSet(subs::LambdaSet {
+                        solved: new_solved,
+                        recursion_var: new_rec_var,
+                    })
                 })
             }
 

compiler/mono/src/ir.rs

@@ -2739,7 +2739,10 @@ fn generate_runtime_error_function<'a>(
     )
     .unwrap();
 
-    eprintln!("emitted runtime error function {:?}", &msg);
+    eprintln!(
+        "emitted runtime error function {:?} for layout {:?}",
+        &msg, layout
+    );
 
     let runtime_error = Stmt::RuntimeError(msg.into_bump_str());
 
@@ -2911,6 +2914,7 @@ fn specialize_external<'a>(
         fn_var,
         roc_unify::unify::Mode::EQ,
     );
+    dbg!(&_unified);
 
     // This will not hold for programs with type errors
     // let is_valid = matches!(unified, roc_unify::unify::Unified::Success(_));
@@ -2928,8 +2932,13 @@ fn specialize_external<'a>(
         }
     };
 
-    let specialized =
+    let specialized = dbg!(build_specialized_proc_from_var(
-        build_specialized_proc_from_var(env, layout_cache, proc_name, pattern_symbols, fn_var)?;
+        env,
+        layout_cache,
+        proc_name,
+        pattern_symbols,
+        fn_var
+    ))?;
 
     // determine the layout of aliases/rigids exposed to the host
     let host_exposed_layouts = if host_exposed_variables.is_empty() {
@@ -3537,6 +3546,7 @@ where
             Ok((proc, raw))
         }
         Err(error) => {
+            dbg!(&error);
             env.subs.rollback_to(snapshot);
             layout_cache.rollback_to(cache_snapshot);
 

compiler/mono/src/layout.rs

@@ -72,7 +72,7 @@ impl<'a> RawFunctionLayout<'a> {
         content: Content,
     ) -> Result<Self, LayoutProblem> {
         use roc_types::subs::Content::*;
-        match content {
+        match dbg!(content) {
             FlexVar(_) | RigidVar(_) => Err(LayoutProblem::UnresolvedTypeVar(var)),
             FlexAbleVar(_, _) | RigidAbleVar(_, _) => todo_abilities!("Not reachable yet"),
             RecursionVar { structure, .. } => {
@@ -157,8 +157,9 @@ impl<'a> RawFunctionLayout<'a> {
         lset: subs::LambdaSet,
     ) -> Result<Self, LayoutProblem> {
         // Lambda set is just a tag union from the layout's perspective.
-        let subs::LambdaSet { solved } = lset;
+        dbg!(
-        Self::layout_from_flat_type(env, FlatType::TagUnion(solved, Variable::EMPTY_TAG_UNION))
+            Self::layout_from_flat_type(env, dbg!(lset.as_tag_union()))
+        )
     }
 
     fn layout_from_flat_type(
@@ -1696,8 +1697,7 @@ fn layout_from_lambda_set<'a>(
     lset: subs::LambdaSet,
 ) -> Result<Layout<'a>, LayoutProblem> {
     // Lambda set is just a tag union from the layout's perspective.
-    let subs::LambdaSet { solved } = lset;
+    layout_from_flat_type(env, lset.as_tag_union())
-    layout_from_flat_type(env, FlatType::TagUnion(solved, Variable::EMPTY_TAG_UNION))
 }
 
 fn layout_from_flat_type<'a>(

compiler/mono/src/layout_soa.rs

@@ -156,12 +156,7 @@ impl FunctionLayout {
         lset: subs::LambdaSet,
     ) -> Result<Self, LayoutError> {
         // Lambda set is just a tag union from the layout's perspective.
-        let subs::LambdaSet { solved } = lset;
+        Self::from_flat_type(layouts, subs, &lset.as_tag_union())
-        Self::from_flat_type(
-            layouts,
-            subs,
-            &FlatType::TagUnion(solved, Variable::EMPTY_TAG_UNION),
-        )
     }
 
     fn from_flat_type(
@@ -281,12 +276,7 @@ impl LambdaSet {
         lset: subs::LambdaSet,
     ) -> Result<Self, LayoutError> {
         // Lambda set is just a tag union from the layout's perspective.
-        let subs::LambdaSet { solved } = lset;
+        Self::from_flat_type(layouts, subs, &lset.as_tag_union())
-        Self::from_flat_type(
-            layouts,
-            subs,
-            &FlatType::TagUnion(solved, Variable::EMPTY_TAG_UNION),
-        )
     }
 
     fn from_flat_type(
@@ -730,12 +720,7 @@ impl Layout {
         lset: subs::LambdaSet,
     ) -> Result<Layout, LayoutError> {
         // Lambda set is just a tag union from the layout's perspective.
-        let subs::LambdaSet { solved } = lset;
+        Self::from_flat_type(layouts, subs, &lset.as_tag_union())
-        Self::from_flat_type(
-            layouts,
-            subs,
-            &FlatType::TagUnion(solved, Variable::EMPTY_TAG_UNION),
-        )
     }
 
     fn from_flat_type(

compiler/solve/src/solve.rs

@@ -1893,7 +1893,12 @@ fn type_to_variable<'a>(
                     Content::Structure(FlatType::EmptyTagUnion),
                 ));
 
-                let content = Content::LambdaSet(subs::LambdaSet { solved });
+                let content = Content::LambdaSet(subs::LambdaSet {
+                    solved,
+                    // We may figure out the lambda set is recursive during solving, but it never
+                    // is to begin with.
+                    recursion_var: OptVariable::NONE,
+                });
 
                 register_with_known_var(subs, destination, rank, pools, content)
             }
@@ -2528,13 +2533,24 @@ fn check_for_infinite_type(
     let var = loc_var.value;
 
     while let Err((recursive, _chain)) = subs.occurs(var) {
-        // try to make a tag union recursive, see if that helps
+        // try to make a union recursive, see if that helps
         match subs.get_content_without_compacting(recursive) {
             &Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
+                dbg!(1);
                 subs.mark_tag_union_recursive(recursive, tags, ext_var);
             }
+            &Content::LambdaSet(subs::LambdaSet {
+                solved,
+                recursion_var: _,
+            }) => {
+                dbg!(2);
+                subs.mark_lambda_set_recursive(recursive, solved);
+            }
 
-            _other => circular_error(subs, problems, symbol, &loc_var),
+            _other => {
+                dbg!(3);
+                circular_error(subs, problems, symbol, &loc_var)
+            }
         }
     }
 }
@@ -2875,7 +2891,10 @@ fn adjust_rank_content(
             rank
         }
 
-        LambdaSet(subs::LambdaSet { solved }) => {
+        LambdaSet(subs::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
             let mut rank = group_rank;
 
             for (_, index) in solved.iter_all() {
@@ -2886,6 +2905,26 @@ fn adjust_rank_content(
                 }
             }
 
+            if let (true, Some(rec_var)) = (cfg!(debug_assertions), recursion_var.into_variable()) {
+                // THEORY: unlike the situation for recursion vars under recursive tag unions,
+                // recursive vars inside lambda sets can't escape into higher let-generalized regions
+                // because lambda sets aren't user-facing.
+                //
+                // So the recursion var should be fully accounted by everything else in the lambda set
+                // (since it appears in the lambda set), and if the rank is higher, it's either a
+                // bug or our theory is wrong and indeed they can escape into higher regions.
+                let rec_var_rank = adjust_rank(subs, young_mark, visit_mark, group_rank, rec_var);
+
+                debug_assert!(
+                    rank >= rec_var_rank,
+                    "rank was {:?} but recursion var <{:?}>{:?} has higher rank {:?}",
+                    rank,
+                    rec_var,
+                    subs.get_content_without_compacting(rec_var),
+                    rec_var_rank
+                );
+            }
+
             rank
         }
 
@@ -3041,11 +3080,18 @@ fn instantiate_rigids_help(subs: &mut Subs, max_rank: Rank, initial: Variable) {
 
                 stack.push(var);
             }
-            LambdaSet(subs::LambdaSet { solved }) => {
+            LambdaSet(subs::LambdaSet {
+                solved,
+                recursion_var,
+            }) => {
                 for slice_index in solved.variables() {
                     let slice = subs.variable_slices[slice_index.index as usize];
                     stack.extend(var_slice!(slice));
                 }
+
+                if let Some(rec_var) = recursion_var.into_variable() {
+                    stack.push(rec_var);
+                }
             }
             &RangedNumber(typ, _) => {
                 stack.push(typ);
@@ -3306,10 +3352,20 @@ fn deep_copy_var_help(
                 subs.set_content_unchecked(copy, new_content);
             }
 
-            LambdaSet(subs::LambdaSet { solved }) => {
+            LambdaSet(subs::LambdaSet {
+                solved,
+                recursion_var,
+            }) => {
                 let new_solved = copy_union_tags!(solved);
+                let new_rec_var = recursion_var.map(|v| work!(v));
 
-                subs.set_content_unchecked(copy, LambdaSet(subs::LambdaSet { solved: new_solved }));
+                subs.set_content_unchecked(
+                    copy,
+                    LambdaSet(subs::LambdaSet {
+                        solved: new_solved,
+                        recursion_var: new_rec_var,
+                    }),
+                );
             }
 
             RangedNumber(typ, range) => {

compiler/solve/tests/solve_expr.rs

@@ -6500,7 +6500,37 @@ mod solve_expr {
                 r#"
                 app "test" provides [main] to "./platform"
 
-                main = Result.mapErr
+                greeting =
+                    hi = "Hello"
+                    name = "World"
+
+                    "\(hi), \(name)!"
+
+                main =
+                    when nestHelp 4 is
+                        _ -> greeting
+
+                nestHelp : I64 -> XEffect {}
+                nestHelp = \m ->
+                    when m is
+                        0 ->
+                            always {}
+
+                        _ ->
+                            always {} |> after \_ -> nestHelp (m - 1)
+
+
+                XEffect a := {} -> a
+
+                always : a -> XEffect a
+                always = \x -> @XEffect (\{} -> x)
+
+                after : XEffect a, (a -> XEffect b) -> XEffect b
+                after = \(@XEffect e), toB ->
+                    @XEffect \{} ->
+                        when toB (e {}) is
+                            @XEffect e2 ->
+                                e2 {}
                 "#
             ),
             "",

compiler/test_gen/src/gen_primitives.rs

@@ -3195,7 +3195,6 @@ fn alias_defined_out_of_order() {
 }
 
 #[test]
-#[ignore = "recursive lambda set"]
 #[cfg(any(feature = "gen-llvm"))]
 fn recursively_build_effect() {
     assert_evals_to!(

compiler/types/src/pretty_print.rs

@@ -221,7 +221,22 @@ fn find_names_needed(
             // TODO should we also look in the actual variable?
             // find_names_needed(_actual, subs, roots, root_appearances, names_taken);
         }
-        LambdaSet(subs::LambdaSet { solved: _ }) => {}
+        LambdaSet(subs::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
+            for slice_index in solved.variables() {
+                let slice = subs[slice_index];
+                for var_index in slice {
+                    let var = subs[var_index];
+                    find_names_needed(var, subs, roots, root_appearances, names_taken);
+                }
+            }
+
+            if let Some(rec_var) = recursion_var.into_variable() {
+                find_names_needed(rec_var, subs, roots, root_appearances, names_taken);
+            }
+        }
         &RangedNumber(typ, _) => {
             find_names_needed(typ, subs, roots, root_appearances, names_taken);
         }
@@ -944,7 +959,10 @@ pub fn resolve_lambda_set<'a>(
     fields: &mut Vec<(TagName, Vec<Variable>)>,
 ) {
     match subs.get_content_without_compacting(var) {
-        Content::LambdaSet(subs::LambdaSet { solved }) => {
+        Content::LambdaSet(subs::LambdaSet {
+            solved,
+            recursion_var: _,
+        }) => {
             push_union_tags(subs, solved, fields);
         }
         c => internal_error!("called with a non-lambda set {:?}", c),

compiler/types/src/subs.rs

@@ -718,11 +718,15 @@ fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
                 SubsFmtContent(subs.get_content_without_compacting(*actual), subs)
             )
         }
-        Content::LambdaSet(LambdaSet { solved }) => {
+        Content::LambdaSet(LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
             write!(
                 f,
-                "LambdaSet({:?})",
+                "LambdaSet({:?}, <{:?}>)",
-                SubsFmtUnionTags(solved, Variable::EMPTY_TAG_UNION, subs)
+                SubsFmtUnionTags(solved, Variable::EMPTY_TAG_UNION, subs),
+                recursion_var
             )
         }
         Content::RangedNumber(typ, range) => {
@@ -784,7 +788,9 @@ fn subs_fmt_flat_type(this: &FlatType, subs: &Subs, f: &mut fmt::Formatter) -> f
             let lambda_content = subs.get_content_without_compacting(*lambda_set);
             write!(
                 f,
-                "], {:?}, <{:?}>{:?})",
+                "], <{:?}={:?}>{:?}, <{:?}>{:?})",
+                lambda_set,
+                subs.get_root_key_without_compacting(*lambda_set),
                 SubsFmtContent(lambda_content, subs),
                 *result,
                 SubsFmtContent(result_content, subs)
@@ -908,6 +914,16 @@ impl OptVariable {
             Variable(self.0)
         }
     }
+
+    pub fn map<F>(self, f: F) -> OptVariable
+    where
+        F: FnOnce(Variable) -> Variable,
+    {
+        self.into_variable()
+            .map(f)
+            .map(OptVariable::from)
+            .unwrap_or(OptVariable::NONE)
+    }
 }
 
 impl fmt::Debug for OptVariable {
@@ -1759,6 +1775,30 @@ impl Subs {
         tags: UnionTags,
         ext_var: Variable,
     ) {
+        let (rec_var, new_tags) = self.mark_union_recursive_help(recursive, tags);
+
+        let new_ext_var = self.explicit_substitute(recursive, rec_var, ext_var);
+        let flat_type = FlatType::RecursiveTagUnion(rec_var, new_tags, new_ext_var);
+
+        self.set_content(recursive, Content::Structure(flat_type));
+    }
+
+    pub fn mark_lambda_set_recursive(&mut self, recursive: Variable, solved_lambdas: UnionTags) {
+        let (rec_var, new_tags) = self.mark_union_recursive_help(recursive, solved_lambdas);
+
+        let new_lambda_set = Content::LambdaSet(LambdaSet {
+            solved: new_tags,
+            recursion_var: OptVariable::from(rec_var),
+        });
+
+        self.set_content(recursive, new_lambda_set);
+    }
+
+    fn mark_union_recursive_help(
+        &mut self,
+        recursive: Variable,
+        tags: UnionTags,
+    ) -> (Variable, UnionTags) {
         let description = self.get(recursive);
 
         let rec_var = self.fresh_unnamed_flex_var();
@@ -1786,13 +1826,9 @@ impl Subs {
             self.variable_slices[variable_slice_index] = new_variables;
         }
 
-        let new_ext_var = self.explicit_substitute(recursive, rec_var, ext_var);
-
         let new_tags = UnionTags::from_slices(tags.tag_names(), new_variable_slices);
 
-        let flat_type = FlatType::RecursiveTagUnion(rec_var, new_tags, new_ext_var);
+        (rec_var, new_tags)
-
-        self.set_content(recursive, Content::Structure(flat_type));
     }
 
     pub fn explicit_substitute(
@@ -2020,6 +2056,45 @@ pub enum Content {
 pub struct LambdaSet {
     /// The resolved lambda symbols we know.
     pub solved: UnionTags,
+    /// Lambda sets may be recursive. For example, consider the annotated program
+    ///
+    /// ```text
+    /// XEffect : A -> B
+    ///                                                   
+    /// after : ({} -> XEffect) -> XEffect
+    /// after =
+    ///     \cont ->
+    ///         f = \A -[`f (typeof cont)]-> when cont {} is A -> B
+    ///         f
+    ///                                                   
+    /// nestForever : {} -> XEffect
+    /// nestForever = \{} -[`nestForever]-> after nestForever
+    /// ^^^^^^^^^^^ {} -[`nestForever]-> A -[`f ({} -[`nestForever]-> A -[`f ...]-> B)]-> B
+    /// ```
+    ///
+    /// where [`nestForever] and [`f ...] refer to the lambda sets of their respective arrows. `f`
+    /// captures `cont`. The usage of `after` in `nestForever` means that `nestForever` has type
+    /// ``nestForever : {} -[`nestForever]-> A -[`f (typeof cont)]-> B``. But also, `after` is called
+    /// with ``nestForever`, which means in this case `typeof cont = typeof nestForever``. So we see
+    /// that ``nestForever : {} -[`nestForever]-> A -[`f (typeof nestForever)]-> B``, and the lambda
+    /// set ``[`f (typeof nestForever)]`` is recursive.
+    ///
+    /// However, we don't know if a lambda set is recursive or not until type inference.
+    pub recursion_var: OptVariable,
+}
+
+impl LambdaSet {
+    /// For things like layout generation, we don't care about differentiating betweent lambda sets
+    /// and tag unions - this function normalizes lambda sets appropriately as a possibly-recursive
+    /// tag union.
+    pub fn as_tag_union(&self) -> FlatType {
+        match self.recursion_var.into_variable() {
+            Some(rec_var) => {
+                FlatType::RecursiveTagUnion(rec_var, self.solved, Variable::EMPTY_TAG_UNION)
+            }
+            None => FlatType::TagUnion(self.solved, Variable::EMPTY_TAG_UNION),
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, Default)]
@@ -2803,10 +2878,19 @@ fn occurs(
 
                 Ok(())
             }
-            LambdaSet(self::LambdaSet { solved }) => {
+            LambdaSet(self::LambdaSet {
+                solved,
+                recursion_var,
+            }) => {
                 let mut new_seen = seen.to_owned();
                 new_seen.push(root_var);
 
+                if include_recursion_var {
+                    if let Some(v) = recursion_var.into_variable() {
+                        new_seen.push(subs.get_root_key_without_compacting(v));
+                    }
+                }
+
                 occurs_union_tags(subs, root_var, &new_seen, include_recursion_var, solved)
             }
             RangedNumber(typ, _range_vars) => {
@@ -2884,14 +2968,13 @@ fn explicit_substitute(
     use self::Content::*;
     use self::FlatType::*;
     let in_root = subs.get_root_key(in_var);
-    if seen.contains(&in_root) {
+    if subs.get_root_key(from) == in_root {
+        to
+    } else if seen.contains(&in_root) {
         in_var
     } else {
         seen.insert(in_root);
 
-        if subs.get_root_key(from) == subs.get_root_key(in_var) {
-            to
-        } else {
         match subs.get(in_var).content {
             FlexVar(_)
             | RigidVar(_)
@@ -2930,8 +3013,7 @@ fn explicit_substitute(
                     TagUnion(tags, ext_var) => {
                         let new_ext_var = explicit_substitute(subs, from, to, ext_var, seen);
 
-                            let union_tags =
+                        let union_tags = explicit_substitute_union_tags(subs, from, to, tags, seen);
-                                explicit_substitute_union_tags(subs, from, to, tags, seen);
 
                         subs.set_content(in_var, Structure(TagUnion(union_tags, new_ext_var)));
                     }
@@ -2946,8 +3028,7 @@ fn explicit_substitute(
                         // NOTE rec_var is not substituted, verify that this is correct!
                         let new_ext_var = explicit_substitute(subs, from, to, ext_var, seen);
 
-                            let union_tags =
+                        let union_tags = explicit_substitute_union_tags(subs, from, to, tags, seen);
-                                explicit_substitute_union_tags(subs, from, to, tags, seen);
 
                         subs.set_content(
                             in_var,
@@ -2984,10 +3065,20 @@ fn explicit_substitute(
 
                 in_var
             }
-                LambdaSet(self::LambdaSet { solved }) => {
+            LambdaSet(self::LambdaSet {
+                solved,
+                recursion_var,
+            }) => {
+                // NOTE recursion_var is not substituted, verify that this is correct!
                 let new_solved = explicit_substitute_union_tags(subs, from, to, solved, seen);
 
-                    subs.set_content(in_var, LambdaSet(self::LambdaSet { solved: new_solved }));
+                subs.set_content(
+                    in_var,
+                    LambdaSet(self::LambdaSet {
+                        solved: new_solved,
+                        recursion_var,
+                    }),
+                );
 
                 in_var
             }
@@ -3001,7 +3092,6 @@ fn explicit_substitute(
         }
     }
 }
-}
 
 #[inline(always)]
 fn explicit_substitute_union_tags(
@@ -3092,8 +3182,15 @@ fn get_var_names(
                 get_var_names(subs, subs[arg_var], answer)
             }),
 
-            LambdaSet(self::LambdaSet { solved }) => {
+            LambdaSet(self::LambdaSet {
-                get_var_names_union_tags(subs, solved, taken_names)
+                solved,
+                recursion_var,
+            }) => {
+                let taken_names = get_var_names_union_tags(subs, solved, taken_names);
+                match recursion_var.into_variable() {
+                    Some(v) => get_var_names(subs, v, taken_names),
+                    None => taken_names,
+                }
             }
 
             RangedNumber(typ, _) => get_var_names(subs, typ, taken_names),
@@ -3332,9 +3429,19 @@ fn content_to_err_type(
             ErrorType::Alias(symbol, err_args, Box::new(err_type), kind)
         }
 
-        LambdaSet(self::LambdaSet { solved }) => {
+        LambdaSet(self::LambdaSet {
+            solved,
+            recursion_var,
+        }) => match recursion_var.into_variable() {
+            None => {
                 ErrorType::TagUnion(union_tags_to_err_tags(subs, state, solved), TypeExt::Closed)
             }
+            Some(v) => ErrorType::RecursiveTagUnion(
+                Box::new(var_to_err_type(subs, state, v)),
+                union_tags_to_err_tags(subs, state, solved),
+                TypeExt::Closed,
+            ),
+        },
 
         RangedNumber(typ, range) => {
             let err_type = var_to_err_type(subs, state, typ);
@@ -3640,11 +3747,16 @@ fn restore_help(subs: &mut Subs, initial: Variable) {
                 stack.push(*var);
             }
 
-            LambdaSet(self::LambdaSet { solved }) => {
+            LambdaSet(self::LambdaSet {
+                solved,
+                recursion_var,
+            }) => {
                 for slice_index in solved.variables() {
                     let slice = variable_slices[slice_index.index as usize];
                     stack.extend(var_slice(slice));
                 }
+
+                recursion_var.into_variable().map(|v| stack.push(v));
             }
 
             RangedNumber(typ, _vars) => {
@@ -3834,8 +3946,12 @@ impl StorageSubs {
                 Self::offset_variable(offsets, *actual),
                 *kind,
             ),
-            LambdaSet(self::LambdaSet { solved }) => LambdaSet(self::LambdaSet {
+            LambdaSet(self::LambdaSet {
+                solved,
+                recursion_var,
+            }) => LambdaSet(self::LambdaSet {
                 solved: Self::offset_union_tags(offsets, *solved),
+                recursion_var: recursion_var.map(|v| Self::offset_variable(offsets, v)),
             }),
             RangedNumber(typ, range) => RangedNumber(Self::offset_variable(offsets, *typ), *range),
             Error => Content::Error,
@@ -4236,10 +4352,17 @@ fn deep_copy_var_to_help(env: &mut DeepCopyVarToEnv<'_>, var: Variable) -> Varia
             copy
         }
 
-        LambdaSet(self::LambdaSet { solved }) => {
+        LambdaSet(self::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
             let new_solved = deep_copy_union_tags(env, solved);
+            let new_rec_var = recursion_var.map(|v| deep_copy_var_to_help(env, v));
 
-            let new_content = LambdaSet(self::LambdaSet { solved: new_solved });
+            let new_content = LambdaSet(self::LambdaSet {
+                solved: new_solved,
+                recursion_var: new_rec_var,
+            });
             env.target.set(copy, make_descriptor(new_content));
             copy
         }
@@ -4680,10 +4803,18 @@ fn copy_import_to_help(env: &mut CopyImportEnv<'_>, max_rank: Rank, var: Variabl
             copy
         }
 
-        LambdaSet(self::LambdaSet { solved }) => {
+        LambdaSet(self::LambdaSet {
+            solved,
+            recursion_var,
+        }) => {
             let new_solved = copy_union_tags(env, max_rank, solved);
+            let new_rec_var =
+                recursion_var.map(|rec_var| copy_import_to_help(env, max_rank, rec_var));
 
-            let new_content = LambdaSet(self::LambdaSet { solved: new_solved });
+            let new_content = LambdaSet(self::LambdaSet {
+                solved: new_solved,
+                recursion_var: new_rec_var,
+            });
 
             env.target.set(copy, make_descriptor(new_content));
 

compiler/unify/src/unify.rs

@@ -343,8 +343,8 @@ fn debug_print_unified_types(subs: &mut Subs, ctx: &Context, opt_outcome: Option
             "{}{}({:?}-{:?}): {:?} {:?} {} {:?} {:?}",
             " ".repeat(use_depth),
             prefix,
-            ctx.first,
+            subs.get_root_key_without_compacting(ctx.first),
-            ctx.second,
+            subs.get_root_key_without_compacting(ctx.second),
             ctx.first,
             SubsFmtContent(&content_1, subs),
             mode,
@@ -801,11 +801,14 @@ fn unify_lambda_set(
         Content::LambdaSet(other_lambda_set) => {
             unify_lambda_set_help(subs, pool, ctx, lambda_set, *other_lambda_set)
         }
+        RecursionVar { structure, .. } => {
+            // suppose that the recursion var is a lambda set
+            unify_pool(subs, pool, ctx.first, *structure, ctx.mode)
+        }
         RigidVar(..) | RigidAbleVar(..) => mismatch!("Lambda sets never unify with rigid"),
         FlexAbleVar(..) => mismatch!("Lambda sets should never have abilities attached to them"),
         Structure(..) => mismatch!("Lambda set cannot unify with non-lambda set structure"),
         RangedNumber(..) => mismatch!("Lambda sets are never numbers"),
-        RecursionVar { .. } => mismatch!("Lambda set not expected to be recursive!"),
         Alias(..) => mismatch!("Lambda set can never be directly under an alias!"),
         Error => merge(subs, ctx, Error),
     }
@@ -822,8 +825,21 @@ fn unify_lambda_set_help(
     // LambdaSets unify like TagUnions, but can grow unbounded regardless of the extension
     // variable.
 
-    let LambdaSet { solved: solved1 } = lset1;
+    let LambdaSet {
-    let LambdaSet { solved: solved2 } = lset2;
+        solved: solved1,
+        recursion_var: rec1,
+    } = lset1;
+    let LambdaSet {
+        solved: solved2,
+        recursion_var: rec2,
+    } = lset2;
+
+    debug_assert!(
+        (rec1.into_variable().into_iter())
+            .chain(rec2.into_variable().into_iter())
+            .all(|v| is_recursion_var(subs, v)),
+        "Recursion var is present, but it doesn't have a recursive content!"
+    );
 
     let (separate_solved, _, _) = separate_union_tags(
         subs,
@@ -843,7 +859,7 @@ fn unify_lambda_set_help(
 
     let mut joined_lambdas = vec![];
     for (tag_name, (vars1, vars2)) in in_both {
-        let mut joined_vars = vec![];
+        let mut matching_vars = vec![];
 
         if vars1.len() != vars2.len() {
             continue; // this is a type mismatch; not adding the tag will trigger it below.
@@ -853,16 +869,32 @@ fn unify_lambda_set_help(
         for (var1, var2) in (vars1.into_iter()).zip(vars2.into_iter()) {
             let (var1, var2) = (subs[var1], subs[var2]);
 
+            // Lambda sets are effectively tags under another name, and their usage can also result
+            // in the arguments of a lambda name being recursive. It very well may happen that
+            // during unification, a lambda set previously marked as not recursive becomes
+            // recursive. See the docs of [LambdaSet] for one example, or https://github.com/rtfeldman/roc/pull/2307.
+            //
+            // Like with tag unions, if it has, we'll always pass through this branch. So, take
+            // this opportunity to promote the lambda set to recursive if need be.
+            maybe_mark_union_recursive(subs, var1);
+            maybe_mark_union_recursive(subs, var2);
+
             let outcome = unify_pool(subs, pool, var1, var2, ctx.mode);
 
+            // TODO: i think we can get rid of this
+            // clearly, this is very suspicious: these variables have just been unified. And yet,
+            // not doing this leads to stack overflows
+            if rec2.is_some() {
                 if outcome.mismatches.is_empty() {
-                // otherwise this is a type mismatch; not adding the variable will trigger it below.
+                    matching_vars.push(var2);
-                joined_vars.push(var1);
+                }
+            } else if outcome.mismatches.is_empty() {
+                matching_vars.push(var1);
             }
         }
 
-        if joined_vars.len() == num_vars {
+        if matching_vars.len() == num_vars {
-            joined_lambdas.push((tag_name, joined_vars));
+            joined_lambdas.push((tag_name, matching_vars));
         }
     }
 
@@ -885,8 +917,17 @@ fn unify_lambda_set_help(
             }),
         );
 
+        let recursion_var = match (rec1.into_variable(), rec2.into_variable()) {
+            // Prefer left when it's available.
+            (Some(rec), _) | (_, Some(rec)) => OptVariable::from(rec),
+            (None, None) => OptVariable::NONE,
+        };
+
         let new_solved = UnionTags::insert_into_subs(subs, all_lambdas);
-        let new_lambda_set = Content::LambdaSet(LambdaSet { solved: new_solved });
+        let new_lambda_set = Content::LambdaSet(LambdaSet {
+            solved: new_solved,
+            recursion_var,
+        });
 
         merge(subs, ctx, new_lambda_set)
     } else {
@@ -1483,19 +1524,39 @@ enum OtherTags2 {
     ),
 }
 
-fn maybe_mark_tag_union_recursive(subs: &mut Subs, tag_union_var: Variable) {
+/// Promotes a non-recursive tag union or lambda set to its recursive variant, if it is found to be
-    'outer: while let Err((recursive, chain)) = subs.occurs(tag_union_var) {
+/// recursive.
+fn maybe_mark_union_recursive(subs: &mut Subs, union_var: Variable) {
+    'outer: while let Err((recursive, chain)) = subs.occurs(union_var) {
         let description = subs.get(recursive);
-        if let Content::Structure(FlatType::TagUnion(tags, ext_var)) = description.content {
+        match description.content {
+            Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
                 subs.mark_tag_union_recursive(recursive, tags, ext_var);
-        } else {
+            }
-            // walk the chain till we find a tag union
+            LambdaSet(self::LambdaSet {
+                solved,
+                recursion_var: OptVariable::NONE,
+            }) => {
+                subs.mark_lambda_set_recursive(recursive, solved);
+            }
+            _ => {
+                // walk the chain till we find a tag union or lambda set
                 for v in &chain[..chain.len() - 1] {
                     let description = subs.get(*v);
-                if let Content::Structure(FlatType::TagUnion(tags, ext_var)) = description.content {
+                    match description.content {
+                        Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
                             subs.mark_tag_union_recursive(*v, tags, ext_var);
                             continue 'outer;
                         }
+                        LambdaSet(self::LambdaSet {
+                            solved,
+                            recursion_var: OptVariable::NONE,
+                        }) => {
+                            subs.mark_lambda_set_recursive(recursive, solved);
+                            continue 'outer;
+                        }
+                        _ => { /* fall through */ }
+                    }
                 }
 
                 // Might not be any tag union if we only pass through `Apply`s. Otherwise, we have a bug!
@@ -1512,6 +1573,7 @@ fn maybe_mark_tag_union_recursive(subs: &mut Subs, tag_union_var: Variable) {
             }
         }
     }
+}
 
 fn unify_shared_tags_new(
     subs: &mut Subs,
@@ -1563,8 +1625,8 @@ fn unify_shared_tags_new(
             // since we're expanding tag unions to equal depths as described above,
             // we'll always pass through this branch. So, we promote tag unions to recursive
             // ones here if it turns out they are that.
-            maybe_mark_tag_union_recursive(subs, actual);
+            maybe_mark_union_recursive(subs, actual);
-            maybe_mark_tag_union_recursive(subs, expected);
+            maybe_mark_union_recursive(subs, expected);
 
             let mut outcome = Outcome::default();
 
@@ -2094,7 +2156,8 @@ fn unify_recursion(
         ),
 
         LambdaSet(..) => {
-            mismatch!("RecursionVar {:?} with LambdaSet {:?}", ctx.first, &other)
+            // suppose that the recursion var is a lambda set
+            unify_pool(subs, pool, structure, ctx.second, ctx.mode)
         }
 
         Error => merge(subs, ctx, Error),
@@ -2172,7 +2235,10 @@ fn unify_function_or_tag_union_and_func(
     {
         let tag_name = TagName::Closure(tag_symbol);
         let union_tags = UnionTags::tag_without_arguments(subs, tag_name);
-        let lambda_set_content = LambdaSet(self::LambdaSet { solved: union_tags });
+        let lambda_set_content = LambdaSet(self::LambdaSet {
+            solved: union_tags,
+            recursion_var: OptVariable::NONE,
+        });
 
         let tag_lambda_set = register(
             subs,