Commit a0cea7ba authored by's avatar
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Don't use deriveUnique *twice* in flattenTys.

Previously, we used deriveUnique and then uniqAway. This worked
doubly hard to avoid clashes. Doing just uniqAway is enough.

This commit also includes clarifying comments.
parent 4e6bcc2c
......@@ -986,6 +986,24 @@ means replacing all top-level uses of type functions with fresh variables,
taking care to preserve sharing. That is, the type (Either (F a b) (F a b)) should
flatten to (Either c c), never (Either c d).
Here is a nice example of why it's all necessary:
type family F a b where
F Int Bool = Char
F a b = Double
type family G a -- open, no instances
How do we reduce (F (G Float) (G Float))? The first equation clearly doesn't match,
while the second equation does. But, before reducing, we must make sure that the
target can never become (F Int Bool). Well, no matter what G Float becomes, it
certainly won't become *both* Int and Bool, so indeed we're safe reducing
(F (G Float) (G Float)) to Double.
This is necessary not only to get more reductions, but for substitutivity. If
we have (F x x), we can see that (F x x) can reduce to Double. So, it had better
be the case that (F blah blah) can reduce to Double, no matter what (blah) is!
Flattening as done below ensures this.
Defined here because of module dependencies.
......@@ -1044,10 +1062,11 @@ coreFlattenTyFamApp in_scope m fam_tc fam_args
= case lookupTypeMap m fam_ty of
Just tv -> (m, tv)
-- we need fresh variables here, but this is called far from
-- any good source of uniques. So, we generate one from thin
-- air, using the arbitrary prime number 71 as a seed
Nothing -> let tyvar_unique = deriveUnique (getUnique fam_tc) 71
tyvar_name = mkSysTvName tyvar_unique (fsLit "fl")
-- any good source of uniques. So, we just use the fam_tc's unique
-- and trust uniqAway to avoid clashes. Recall that the in_scope set
-- contains *all* tyvars, even locally bound ones elsewhere in the
-- overall type, so this really is fresh.
Nothing -> let tyvar_name = mkSysTvName (getUnique fam_tc) (fsLit "fl")
tv = uniqAway in_scope $ mkTyVar tyvar_name (typeKind fam_ty)
m' = extendTypeMap m fam_ty tv in
(m', tv)
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