I'm pretty sure we *could*, but special case hacks in the type checker are
horrible. So we shouldn't.
On Jun 10, 2016 11:13 AM, "Edward Z. Yang"
Excerpts from Brendan Hay's message of 2016-06-10 01:59:51 -0700:
* How does the use of ($) vs (.) fix this particular issue? (I'd naively assume the usage here would be equivalent.) I recall reading about ($)'s magical type alias somewhere - is this related?
When you say f $ a, where a is a polymorphic variable, we need to instantiate the type variables in ($) :: (a -> b) -> a -> b with quantified types; i.e., do an impredicative instantiation. GHC has never been able to do this, so there's a hack for the typechecker to treat 'f $ a' as if it were just 'f a' (no more impredicative instantiation).
(.) is not special cased similarly, which is why it doesn't work. I don't know if we could special case it to solve this problem.
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