Restrictions on polytypes with type families

[Simon Peyton-Jones]

I don't think there's a fundamental reason.  At least, provided we stick to impredicative polymorphism, we can just treat forall as another type former. The unifier *already* deals properly with forall, yielding a suitable coercion, at least I think so.

Dimitrios may think of some gotchas, but mostly I think it'd be a question of pushing through the details.

Simon



| -----Original Message-----
| From: ghc-devs-bounces at haskell.org [mailto:ghc-devs-bounces at haskell.org]
| On Behalf Of Richard Eisenberg
| Sent: 03 April 2013 17:40
| To: ghc-devs
| Subject: Restrictions on polytypes with type families
| 
| Hi all,
| 
| GHC doesn't allow type families to be used with polytypes:
| 1) The right-hand side of a type family instance cannot have a "forall".
| 2) A type family cannot be applied to a type containing a "forall".
| 3) A pattern in a type family instance is (oddly) allowed to contain
| "forall", but this is silly because of (2).
| 
| Do these restrictions have known reasons for their existence? Or, are
| there any that are restricted because someone needs to think hard before
| lifting it, and no one has yet done that thinking? I know, for example,
| that the unify function in types/Unify.lhs will have to be completed to
| work with foralls, but this doesn't seem hard.
| 
| I've run into two separate cases where I've hit this restriction, so
| this isn't just idle thought.
| 
| Thanks,
| Richard
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