Move solve and unify into their own crates

compiler/unify/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "roc_unify"
+version = "0.1.0"
+authors = ["Richard Feldman <oss@rtfeldman.com>"]
+edition = "2018"
+
+[dependencies]
+roc_collections = { path = "../collections" }
+roc_module = { path = "../module" }
+roc_types = { path = "../types" }
+
+[dev-dependencies]
+pretty_assertions = "0.5.1 "
+maplit = "1.0.1"
+indoc = "0.3.3"
+quickcheck = "0.8"
+quickcheck_macros = "0.8"

compiler/unify/src/lib.rs

@@ -0,0 +1,14 @@
+#![warn(clippy::all, clippy::dbg_macro)]
+// I'm skeptical that clippy:large_enum_variant is a good lint to have globally enabled.
+//
+// It warns about a performance problem where the only quick remediation is
+// to allocate more on the heap, which has lots of tradeoffs - including making it
+// long-term unclear which allocations *need* to happen for compilation's sake
+// (e.g. recursive structures) versus those which were only added to appease clippy.
+//
+// Effectively optimizing data struture memory layout isn't a quick fix,
+// and encouraging shortcuts here creates bad incentives. I would rather temporarily
+// re-enable this when working on performance optimizations than have it block PRs.
+#![allow(clippy::large_enum_variant)]
+
+pub mod unify;

compiler/unify/src/unify.rs

@@ -0,0 +1,741 @@
+use roc_collections::all::{relative_complement, union, MutMap, SendSet};
+use roc_module::ident::{Lowercase, TagName};
+use roc_module::symbol::Symbol;
+use roc_types::boolean_algebra::{Atom, Bool};
+use roc_types::subs::Content::{self, *};
+use roc_types::subs::{Descriptor, FlatType, Mark, OptVariable, Subs, Variable};
+use roc_types::types::{Mismatch, Problem};
+use std::hash::Hash;
+
+macro_rules! mismatch {
+    () => {{
+        if cfg!(debug_assertions) {
+            println!(
+                "Mismatch in {} Line {} Column {}",
+                file!(),
+                line!(),
+                column!()
+            );
+        }
+        vec![Mismatch::TypeMismatch]
+    }};
+}
+
+type Pool = Vec<Variable>;
+
+struct Context {
+    first: Variable,
+    first_desc: Descriptor,
+    second: Variable,
+    second_desc: Descriptor,
+}
+
+pub struct RecordStructure {
+    pub fields: MutMap<Lowercase, Variable>,
+    pub ext: Variable,
+}
+
+#[derive(Debug)]
+struct TagUnionStructure {
+    tags: MutMap<TagName, Vec<Variable>>,
+    ext: Variable,
+}
+
+pub struct Unified {
+    pub vars: Pool,
+    pub mismatches: Vec<Problem>,
+}
+
+type Outcome = Vec<Mismatch>;
+
+#[inline(always)]
+pub fn unify(subs: &mut Subs, var1: Variable, var2: Variable) -> Unified {
+    let mut vars = Vec::new();
+    let mismatches = unify_pool(subs, &mut vars, var1, var2)
+        .into_iter()
+        .map(|problem| {
+            let type1 = subs.var_to_error_type(var1);
+            let type2 = subs.var_to_error_type(var2);
+
+            subs.union(var1, var2, Content::Error.into());
+
+            Problem::Mismatch(problem, type1, type2)
+        })
+        .collect();
+
+    Unified { vars, mismatches }
+}
+
+#[inline(always)]
+pub fn unify_pool(subs: &mut Subs, pool: &mut Pool, var1: Variable, var2: Variable) -> Outcome {
+    if subs.equivalent(var1, var2) {
+        Vec::new()
+    } else {
+        let ctx = Context {
+            first: var1,
+            first_desc: subs.get(var1),
+            second: var2,
+            second_desc: subs.get(var2),
+        };
+
+        unify_context(subs, pool, ctx)
+    }
+}
+
+fn unify_context(subs: &mut Subs, pool: &mut Pool, ctx: Context) -> Outcome {
+    // println!( "{:?} {:?} ~ {:?} {:?}", ctx.first, ctx.first_desc.content, ctx.second, ctx.second_desc.content);
+    match &ctx.first_desc.content {
+        FlexVar(opt_name) => unify_flex(subs, pool, &ctx, opt_name, &ctx.second_desc.content),
+        RigidVar(name) => unify_rigid(subs, &ctx, name, &ctx.second_desc.content),
+        Structure(flat_type) => {
+            unify_structure(subs, pool, &ctx, flat_type, &ctx.second_desc.content)
+        }
+        Alias(symbol, args, real_var) => unify_alias(subs, pool, &ctx, *symbol, args, *real_var),
+        Error => {
+            // Error propagates. Whatever we're comparing it to doesn't matter!
+            merge(subs, &ctx, Error)
+        }
+    }
+}
+
+#[inline(always)]
+fn unify_alias(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    symbol: Symbol,
+    args: &[(Lowercase, Variable)],
+    real_var: Variable,
+) -> Outcome {
+    let other_content = &ctx.second_desc.content;
+
+    match other_content {
+        FlexVar(_) => {
+            // Alias wins
+            merge(subs, &ctx, Alias(symbol, args.to_owned(), real_var))
+        }
+        RigidVar(_) => unify_pool(subs, pool, real_var, ctx.second),
+        Alias(other_symbol, other_args, other_real_var) => {
+            if symbol == *other_symbol {
+                if args.len() == other_args.len() {
+                    let mut problems = Vec::new();
+                    for ((_, l_var), (_, r_var)) in args.iter().zip(other_args.iter()) {
+                        problems.extend(unify_pool(subs, pool, *l_var, *r_var));
+                    }
+
+                    problems.extend(merge(subs, &ctx, other_content.clone()));
+
+                    problems
+                } else {
+                    mismatch!()
+                }
+            } else {
+                unify_pool(subs, pool, real_var, *other_real_var)
+            }
+        }
+        Structure(_) => unify_pool(subs, pool, real_var, ctx.second),
+        Error => merge(subs, ctx, Error),
+    }
+}
+
+#[inline(always)]
+fn unify_structure(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    flat_type: &FlatType,
+    other: &Content,
+) -> Outcome {
+    match other {
+        FlexVar(_) => {
+            // TODO special-case boolean here
+            match flat_type {
+                FlatType::Boolean(Bool(Atom::Variable(var), _rest)) => {
+                    unify_pool(subs, pool, *var, ctx.second)
+                }
+                _ => {
+                    // If the other is flex, Structure wins!
+                    merge(subs, ctx, Structure(flat_type.clone()))
+                }
+            }
+        }
+        RigidVar(_) => {
+            // Type mismatch! Rigid can only unify with flex.
+            mismatch!()
+        }
+
+        Structure(ref other_flat_type) => {
+            // Unify the two flat types
+            unify_flat_type(subs, pool, ctx, flat_type, other_flat_type)
+        }
+        Alias(_, _, real_var) => unify_pool(subs, pool, ctx.first, *real_var),
+        Error => merge(subs, ctx, Error),
+    }
+}
+
+/// Like intersection_with, except for MutMap and specialized to return
+/// a tuple. Also, only clones the values that will be actually returned,
+/// rather than cloning everything.
+fn get_shared<K, V>(map1: &MutMap<K, V>, map2: &MutMap<K, V>) -> MutMap<K, (V, V)>
+where
+    K: Clone + Eq + Hash,
+    V: Clone,
+{
+    let mut answer = MutMap::default();
+
+    for (key, right_value) in map2 {
+        match std::collections::HashMap::get(map1, &key) {
+            None => (),
+            Some(left_value) => {
+                answer.insert(key.clone(), (left_value.clone(), right_value.clone()));
+            }
+        }
+    }
+
+    answer
+}
+
+fn unify_record(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    rec1: RecordStructure,
+    rec2: RecordStructure,
+) -> Outcome {
+    let fields1 = rec1.fields;
+    let fields2 = rec2.fields;
+    let shared_fields = get_shared(&fields1, &fields2);
+    // NOTE: don't use `difference` here. In contrast to Haskell, im's `difference` is symmetric
+    let unique_fields1 = relative_complement(&fields1, &fields2);
+    let unique_fields2 = relative_complement(&fields2, &fields1);
+
+    if unique_fields1.is_empty() {
+        if unique_fields2.is_empty() {
+            let ext_problems = unify_pool(subs, pool, rec1.ext, rec2.ext);
+            let other_fields = MutMap::default();
+            let mut field_problems =
+                unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, rec1.ext);
+
+            field_problems.extend(ext_problems);
+
+            field_problems
+        } else {
+            let flat_type = FlatType::Record(unique_fields2, rec2.ext);
+            let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
+            let ext_problems = unify_pool(subs, pool, rec1.ext, sub_record);
+            let other_fields = MutMap::default();
+            let mut field_problems =
+                unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, sub_record);
+
+            field_problems.extend(ext_problems);
+
+            field_problems
+        }
+    } else if unique_fields2.is_empty() {
+        let flat_type = FlatType::Record(unique_fields1, rec1.ext);
+        let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
+        let ext_problems = unify_pool(subs, pool, sub_record, rec2.ext);
+        let other_fields = MutMap::default();
+        let mut field_problems =
+            unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, sub_record);
+
+        field_problems.extend(ext_problems);
+
+        field_problems
+    } else {
+        let other_fields = union(unique_fields1.clone(), &unique_fields2);
+
+        let ext = fresh(subs, pool, ctx, Content::FlexVar(None));
+        let flat_type1 = FlatType::Record(unique_fields1, ext);
+        let flat_type2 = FlatType::Record(unique_fields2, ext);
+
+        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, rec1.ext, sub2);
+        let rec2_problems = unify_pool(subs, pool, sub1, rec2.ext);
+
+        let mut field_problems =
+            unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, ext);
+
+        field_problems.reserve(rec1_problems.len() + rec2_problems.len());
+        field_problems.extend(rec1_problems);
+        field_problems.extend(rec2_problems);
+
+        field_problems
+    }
+}
+
+fn unify_shared_fields(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    shared_fields: MutMap<Lowercase, (Variable, Variable)>,
+    other_fields: MutMap<Lowercase, Variable>,
+    ext: Variable,
+) -> Outcome {
+    let mut matching_fields = MutMap::default();
+    let num_shared_fields = shared_fields.len();
+
+    for (name, (actual, expected)) in shared_fields {
+        let problems = unify_pool(subs, pool, actual, expected);
+
+        if problems.is_empty() {
+            matching_fields.insert(name, actual);
+        }
+    }
+
+    if num_shared_fields == matching_fields.len() {
+        let flat_type = FlatType::Record(union(matching_fields, &other_fields), ext);
+
+        merge(subs, ctx, Structure(flat_type))
+    } else {
+        mismatch!()
+    }
+}
+
+fn unify_tag_union(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    rec1: TagUnionStructure,
+    rec2: TagUnionStructure,
+    recursion: (Option<Variable>, Option<Variable>),
+) -> Outcome {
+    let tags1 = rec1.tags;
+    let tags2 = rec2.tags;
+    let shared_tags = get_shared(&tags1, &tags2);
+    // NOTE: don't use `difference` here. In contrast to Haskell, im's `difference` is symmetric
+    let unique_tags1 = relative_complement(&tags1, &tags2);
+    let unique_tags2 = relative_complement(&tags2, &tags1);
+
+    let recursion_var = match recursion {
+        (None, None) => None,
+        (Some(v), None) | (None, Some(v)) => Some(v),
+        (Some(v1), Some(v2)) => {
+            unify_pool(subs, pool, v1, v2);
+            Some(v1)
+        }
+    };
+
+    if unique_tags1.is_empty() {
+        if unique_tags2.is_empty() {
+            let ext_problems = unify_pool(subs, pool, rec1.ext, rec2.ext);
+            let mut tag_problems = unify_shared_tags(
+                subs,
+                pool,
+                ctx,
+                shared_tags,
+                MutMap::default(),
+                rec1.ext,
+                recursion_var,
+            );
+
+            tag_problems.extend(ext_problems);
+
+            tag_problems
+        } else {
+            let flat_type = FlatType::TagUnion(unique_tags2, rec2.ext);
+            let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
+            let ext_problems = unify_pool(subs, pool, rec1.ext, sub_record);
+            let mut tag_problems = unify_shared_tags(
+                subs,
+                pool,
+                ctx,
+                shared_tags,
+                MutMap::default(),
+                sub_record,
+                recursion_var,
+            );
+
+            tag_problems.extend(ext_problems);
+
+            tag_problems
+        }
+    } else if unique_tags2.is_empty() {
+        let flat_type = FlatType::TagUnion(unique_tags1, rec1.ext);
+        let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
+        let ext_problems = unify_pool(subs, pool, sub_record, rec2.ext);
+        let mut tag_problems = unify_shared_tags(
+            subs,
+            pool,
+            ctx,
+            shared_tags,
+            MutMap::default(),
+            sub_record,
+            recursion_var,
+        );
+
+        tag_problems.extend(ext_problems);
+
+        tag_problems
+    } else {
+        let other_tags = union(unique_tags1.clone(), &unique_tags2);
+
+        let ext = fresh(subs, pool, ctx, Content::FlexVar(None));
+        let flat_type1 = FlatType::TagUnion(unique_tags1, rec1.ext);
+        let flat_type2 = FlatType::TagUnion(unique_tags2, rec2.ext);
+
+        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, rec1.ext, sub2);
+        let rec2_problems = unify_pool(subs, pool, sub1, rec2.ext);
+
+        let mut tag_problems =
+            unify_shared_tags(subs, pool, ctx, shared_tags, other_tags, ext, recursion_var);
+
+        tag_problems.reserve(rec1_problems.len() + rec2_problems.len());
+        tag_problems.extend(rec1_problems);
+        tag_problems.extend(rec2_problems);
+
+        tag_problems
+    }
+}
+
+fn unify_shared_tags(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    shared_tags: MutMap<TagName, (Vec<Variable>, Vec<Variable>)>,
+    other_tags: MutMap<TagName, Vec<Variable>>,
+    ext: Variable,
+    recursion_var: Option<Variable>,
+) -> Outcome {
+    let mut matching_tags = MutMap::default();
+    let num_shared_tags = shared_tags.len();
+
+    for (name, (actual_vars, expected_vars)) in shared_tags {
+        let mut matching_vars = Vec::with_capacity(actual_vars.len());
+
+        let actual_len = actual_vars.len();
+        let expected_len = expected_vars.len();
+
+        for (actual, expected) in actual_vars.into_iter().zip(expected_vars.into_iter()) {
+            let problems = unify_pool(subs, pool, actual, expected);
+
+            if problems.is_empty() {
+                matching_vars.push(actual);
+            }
+        }
+
+        // only do this check after unification so the error message has more info
+        if actual_len == expected_len && actual_len == matching_vars.len() {
+            matching_tags.insert(name, matching_vars);
+        }
+    }
+
+    if num_shared_tags == matching_tags.len() {
+        let flat_type = if let Some(rec) = recursion_var {
+            FlatType::RecursiveTagUnion(rec, union(matching_tags, &other_tags), ext)
+        } else {
+            FlatType::TagUnion(union(matching_tags, &other_tags), ext)
+        };
+
+        merge(subs, ctx, Structure(flat_type))
+    } else {
+        mismatch!()
+    }
+}
+
+#[inline(always)]
+fn unify_flat_type(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    left: &FlatType,
+    right: &FlatType,
+) -> Outcome {
+    use roc_types::subs::FlatType::*;
+
+    match (left, right) {
+        (EmptyRecord, EmptyRecord) => merge(subs, ctx, Structure(left.clone())),
+
+        (Record(fields, ext), EmptyRecord) if fields.is_empty() => {
+            unify_pool(subs, pool, *ext, ctx.second)
+        }
+
+        (EmptyRecord, Record(fields, ext)) if fields.is_empty() => {
+            unify_pool(subs, pool, ctx.first, *ext)
+        }
+
+        (Record(fields1, ext1), Record(fields2, ext2)) => {
+            let rec1 = gather_fields(subs, fields1.clone(), *ext1);
+            let rec2 = gather_fields(subs, fields2.clone(), *ext2);
+
+            unify_record(subs, pool, ctx, rec1, rec2)
+        }
+
+        (EmptyTagUnion, EmptyTagUnion) => merge(subs, ctx, Structure(left.clone())),
+
+        (TagUnion(tags, ext), EmptyTagUnion) if tags.is_empty() => {
+            unify_pool(subs, pool, *ext, ctx.second)
+        }
+
+        (EmptyTagUnion, TagUnion(tags, ext)) if tags.is_empty() => {
+            unify_pool(subs, pool, ctx.first, *ext)
+        }
+
+        (TagUnion(tags1, ext1), TagUnion(tags2, ext2)) => {
+            let union1 = gather_tags(subs, tags1.clone(), *ext1);
+            let union2 = gather_tags(subs, tags2.clone(), *ext2);
+
+            unify_tag_union(subs, pool, ctx, union1, union2, (None, None))
+        }
+
+        (TagUnion(tags1, ext1), RecursiveTagUnion(recursion_var, tags2, ext2)) => {
+            let union1 = gather_tags(subs, tags1.clone(), *ext1);
+            let union2 = gather_tags(subs, tags2.clone(), *ext2);
+
+            unify_tag_union(
+                subs,
+                pool,
+                ctx,
+                union1,
+                union2,
+                (None, Some(*recursion_var)),
+            )
+        }
+
+        (RecursiveTagUnion(rec1, tags1, ext1), RecursiveTagUnion(rec2, tags2, ext2)) => {
+            let union1 = gather_tags(subs, tags1.clone(), *ext1);
+            let union2 = gather_tags(subs, tags2.clone(), *ext2);
+
+            unify_tag_union(subs, pool, ctx, union1, union2, (Some(*rec1), Some(*rec2)))
+        }
+
+        (Boolean(Bool(free1, rest1)), Boolean(Bool(free2, rest2))) => {
+            // unify the free variables
+            let (new_free, mut free_var_problems) = unify_free_atoms(subs, pool, *free1, *free2);
+
+            let combined_rest: SendSet<Atom> = rest1
+                .clone()
+                .into_iter()
+                .chain(rest2.clone().into_iter())
+                .collect::<SendSet<Atom>>();
+
+            let mut combined = if let Err(false) = chase_atom(subs, new_free) {
+                // if the container is shared, all elements must be shared too
+                for atom in combined_rest {
+                    let (_, atom_problems) = unify_free_atoms(subs, pool, atom, Atom::Zero);
+                    free_var_problems.extend(atom_problems);
+                }
+                Bool(Atom::Zero, SendSet::default())
+            } else {
+                Bool(new_free, combined_rest)
+            };
+
+            combined.apply_subs(subs);
+
+            // force first and second to equal this new variable
+            let content = Content::Structure(FlatType::Boolean(combined));
+            merge(subs, ctx, content);
+
+            free_var_problems
+        }
+
+        (Apply(l_symbol, l_args), Apply(r_symbol, r_args)) if l_symbol == r_symbol => {
+            let problems = unify_zip(subs, pool, l_args.iter(), r_args.iter());
+
+            if problems.is_empty() {
+                merge(subs, ctx, Structure(Apply(*r_symbol, (*r_args).clone())))
+            } else {
+                problems
+            }
+        }
+        (Func(l_args, l_ret), Func(r_args, r_ret)) if l_args.len() == r_args.len() => {
+            let arg_problems = unify_zip(subs, pool, l_args.iter(), r_args.iter());
+            let ret_problems = unify_pool(subs, pool, *l_ret, *r_ret);
+
+            if arg_problems.is_empty() && ret_problems.is_empty() {
+                merge(subs, ctx, Structure(Func((*r_args).clone(), *r_ret)))
+            } else {
+                let mut problems = ret_problems;
+
+                problems.extend(arg_problems);
+
+                problems
+            }
+        }
+        (_other1, _other2) => {
+            // Can't unify other1 and other2
+            // dbg!(&_other1, &_other2);
+            mismatch!()
+        }
+    }
+}
+
+fn chase_atom(subs: &mut Subs, atom: Atom) -> Result<Variable, bool> {
+    match atom {
+        Atom::Zero => Err(false),
+        Atom::One => Err(true),
+        Atom::Variable(var) => match subs.get(var).content {
+            Content::Structure(FlatType::Boolean(Bool(first, rest))) => {
+                debug_assert!(rest.is_empty());
+                chase_atom(subs, first)
+            }
+            _ => Ok(var),
+        },
+    }
+}
+
+fn unify_free_atoms(subs: &mut Subs, pool: &mut Pool, b1: Atom, b2: Atom) -> (Atom, Vec<Mismatch>) {
+    match (b1, b2) {
+        (Atom::Variable(v1), Atom::Variable(v2)) => {
+            (Atom::Variable(v1), unify_pool(subs, pool, v1, v2))
+        }
+        (Atom::Variable(var), other) | (other, Atom::Variable(var)) => {
+            subs.set_content(
+                var,
+                Content::Structure(FlatType::Boolean(Bool(other, SendSet::default()))),
+            );
+
+            (other, vec![])
+        }
+        (Atom::Zero, Atom::Zero) => (Atom::Zero, vec![]),
+        (Atom::One, Atom::One) => (Atom::One, vec![]),
+        _ => unreachable!(
+            "invalid boolean unification. Because we never infer One, this should never happen!"
+        ),
+    }
+}
+
+fn unify_zip<'a, I>(subs: &mut Subs, pool: &mut Pool, left_iter: I, right_iter: I) -> Outcome
+where
+    I: Iterator<Item = &'a Variable>,
+{
+    let mut problems = Vec::new();
+
+    let it = left_iter.zip(right_iter);
+
+    for (&l_var, &r_var) in it {
+        problems.extend(unify_pool(subs, pool, l_var, r_var));
+    }
+
+    problems
+}
+
+#[inline(always)]
+fn unify_rigid(subs: &mut Subs, ctx: &Context, name: &Lowercase, other: &Content) -> Outcome {
+    match other {
+        FlexVar(_) => {
+            // If the other is flex, rigid wins!
+            merge(subs, ctx, RigidVar(name.clone()))
+        }
+        RigidVar(_) | Structure(_) | Alias(_, _, _) => {
+            // Type mismatch! Rigid can only unify with flex, even if the
+            // rigid names are the same.
+            mismatch!()
+        }
+        Error => {
+            // Error propagates.
+            merge(subs, ctx, Error)
+        }
+    }
+}
+
+#[inline(always)]
+fn unify_flex(
+    subs: &mut Subs,
+    pool: &mut Pool,
+    ctx: &Context,
+    opt_name: &Option<Lowercase>,
+    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.clone()))
+        }
+
+        Structure(FlatType::Boolean(Bool(Atom::Variable(var), _rest))) => {
+            unify_pool(subs, pool, ctx.first, *var)
+        }
+
+        FlexVar(Some(_)) | RigidVar(_) | Structure(_) | Alias(_, _, _) => {
+            // TODO special-case boolean here
+            // In all other cases, if left is flex, defer to right.
+            // (This includes using right's name if both are flex and named.)
+            merge(subs, ctx, other.clone())
+        }
+
+        Error => merge(subs, ctx, Error),
+    }
+}
+
+pub fn gather_fields(
+    subs: &mut Subs,
+    fields: MutMap<Lowercase, Variable>,
+    var: Variable,
+) -> RecordStructure {
+    use roc_types::subs::FlatType::*;
+
+    match subs.get(var).content {
+        Structure(Record(sub_fields, sub_ext)) => {
+            gather_fields(subs, union(fields, &sub_fields), sub_ext)
+        }
+
+        Alias(_, _, var) => {
+            // TODO according to elm/compiler: "TODO may be dropping useful alias info here"
+            gather_fields(subs, fields, var)
+        }
+
+        _ => RecordStructure { fields, ext: var },
+    }
+}
+
+fn gather_tags(
+    subs: &mut Subs,
+    tags: MutMap<TagName, Vec<Variable>>,
+    var: Variable,
+) -> TagUnionStructure {
+    use roc_types::subs::FlatType::*;
+
+    match subs.get(var).content {
+        Structure(TagUnion(sub_tags, sub_ext)) => {
+            gather_tags(subs, union(tags, &sub_tags), sub_ext)
+        }
+
+        Alias(_, _, var) => {
+            // TODO according to elm/compiler: "TODO may be dropping useful alias info here"
+            gather_tags(subs, tags, var)
+        }
+
+        _ => TagUnionStructure { tags, ext: var },
+    }
+}
+
+fn merge(subs: &mut Subs, ctx: &Context, content: Content) -> Outcome {
+    let rank = ctx.first_desc.rank.min(ctx.second_desc.rank);
+    let desc = Descriptor {
+        content,
+        rank,
+        mark: Mark::NONE,
+        copy: OptVariable::NONE,
+    };
+
+    subs.union(ctx.first, ctx.second, desc);
+
+    Vec::new()
+}
+
+fn register(subs: &mut Subs, desc: Descriptor, pool: &mut Pool) -> Variable {
+    let var = subs.fresh(desc);
+
+    pool.push(var);
+
+    var
+}
+
+fn fresh(subs: &mut Subs, pool: &mut Pool, ctx: &Context, content: Content) -> Variable {
+    register(
+        subs,
+        Descriptor {
+            content,
+            rank: ctx.first_desc.rank.min(ctx.second_desc.rank),
+            mark: Mark::NONE,
+            copy: OptVariable::NONE,
+        },
+        pool,
+    )
+}