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Add some more comments on the behavior of compaction-mode unification

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Ayaz Hafiz 2022-07-26 18:33:52 -04:00
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27
crates/compiler/unify/src/unify.rs
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@ -1352,8 +1352,31 @@ fn unify_unspecialized_lambdas<M: MetaCollector>(
// If we're in the process of running the ambient lambda set
// specialization procedure, disjoint type variables being merged from
// the left and right lists are treated specially!
// To avoid introducing superfluous variables, we unify disjoint variables
// once, and then progress on both sides.
//
// In particular, we are unifying a local list of lambda sets, for
// which the specialization is for (on the left), with specialization
// lambda sets, which have just been freshened (on the right).
//
// [ .. a:lam:1 ] (local, undergoing specialization)
// [ .. a':lam:1 ] (specialization lambda sets, just freshened)
//
// Because the specialization lambdas are freshened, they certainly are
// disjoint from the local lambdas - but they may be equivalent in
// principle, from the perspective of a human looking at the
// unification!
//
// Running with the example above, the specialization lambda set has an
// unspecialized lambda `a':lam:1`. Now, this is disjoint from
// `a:lam:1` in the local lambda set, from the purely technical
// perspective that `a' != a`.
//
// But, in expected function, they **should not** be treated as disjoint!
// In this case, the specialization lambda is not introducing any new
// information, and is targetting exactly the local lambda `a:lam:1`.
//
// So, to avoid introducing superfluous variables, we unify these disjoint
// variables once, and then progress on both sides. We progress on both
// sides to avoid unifying more than what we should in our principle.
//
// It doesn't matter which side we choose to progress on, since after
// unification of flex vars roots are equivalent. So, choose the left