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Mark tag unions to recursive when they become so during unification

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See the comment on line 811 of unify.rs in this commit for motivation
and justification.

Closes #2217
This commit is contained in:
ayazhafiz 2021-12-22 17:51:07 -06:00
parent 78a247e6f2
commit 88888b0854
4 changed files with 101 additions and 36 deletions
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26
compiler/unify/src/unify.rs
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@ -749,6 +749,20 @@ enum OtherTags2 {
),
}
fn maybe_mark_tag_union_recursive(subs: &mut Subs, tag_union_var: Variable) {
while let Err((recursive, _chain)) = subs.occurs(tag_union_var) {
let description = subs.get(recursive);
match description.content {
Content::Structure(FlatType::TagUnion(tags, ext_var)) => {
subs.mark_tag_union_recursive(recursive, tags, ext_var);
}
// We'll pick this up as an error elsewhere
_ => {}
}
}
}
fn unify_shared_tags_new(
subs: &mut Subs,
pool: &mut Pool,
@ -773,6 +787,8 @@ fn unify_shared_tags_new(
let expected = subs[expected_index];
// NOTE the arguments of a tag can be recursive. For instance in the expression
//
// ConsList a : [ Nil, Cons a (ConsList a) ]
//
// Cons 1 (Cons "foo" Nil)
//
// We need to not just check the outer layer (inferring ConsList Int)
@ -791,6 +807,15 @@ fn unify_shared_tags_new(
// > RecursiveTagUnion(rvar, [ Cons a [ Cons a rvar, Nil ], Nil ], ext)
//
// and so on until the whole non-recursive tag union can be unified with it.
//
// One thing we have to watch out for is that a tag union we're hoping to
// match a recursive tag union with didn't itself become recursive. If it has,
// 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_tag_union_recursive(subs, expected);
let mut problems = Vec::new();
problems.extend(unify_pool(subs, pool, actual, expected));
@ -1062,7 +1087,6 @@ fn unify_flat_type(
let tags1 = UnionTags::from_tag_name_index(*tag_name);
let rec = Rec::Right(*recursion_var);
// NOTE arguments are flipped by design
unify_tag_union_new(subs, pool, ctx, tags1, *ext1, *tags2, *ext2, rec)
}