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Avoid exponential unification paths

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🤦 when we check whether lambdas can be unified or must be
treated as disjoint we previously had a code path that is actually
exponential in its runtime, regardless of whether it succeeds or fails.
Now, it is linear, though degraded (with a clone) if it fails.
This commit is contained in:
Ayaz Hafiz 2022-12-12 15:46:05 -06:00
parent 1e120653ff
commit 23932137ef
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GPG key ID: 0E2A37416A25EF58
1 changed files with 8 additions and 12 deletions
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20
crates/compiler/unify/src/unify.rs
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@ -1490,29 +1490,25 @@ fn separate_union_lambdas<M: MetaCollector>(
// v1 ~ v2
// => {} -[ foo ({} -[ bar Str ]-> {}),
// foo ({} -[ bar U64 ]-> {}) ] -> {}
let variables_are_unifiable =
env.with_outcome_only_for_lambda_set(|env| {
unify_pool::<NoCollector>(env, pool, var1, var2, mode)
.mismatches
.is_empty()
});
let snapshot = env.subs.snapshot();
let outcome = unify_pool(env, pool, var1, var2, mode);
if !variables_are_unifiable {
if !outcome.mismatches.is_empty() {
env.subs.rollback_to(snapshot);
if env.is_inside_lambda_set {
// If the lambdas being compared are nested, mismatches mean that the
// transitively-outer lambdas should be treated as disjoint in
// their ultimate lambda set.
// As such, do not unify, even disjointly, at this level;
// force the transient parent to be treated disjointly.
whole_outcome.mismatches.push(Mismatch::DisjointLambdaSets);
return Err(whole_outcome);
return Err(outcome);
} else {
continue 'try_next_right;
}
} else {
let outcome = unify_pool(env, pool, var1, var2, mode);
whole_outcome.union(outcome);
}
let outcome = unify_pool(env, pool, var1, var2, mode);
whole_outcome.union(outcome);
}
// All the variables unified, so we can join the left + right.