Hello,
isn't this moving directly into the territory of impredicative types? Not
that there is anything wrong with that, but there seemed to be a number of
different ways to implement these sorts of things, and my impression was
that neither is particularly simple. I am thinking of examples like this:
type family F a
type instance F Int = Int
type instance F Char = forall a. a -> a
It is somewhat unclear in what contexts it is OK to use `F`. For
example, `Maybe
(F Int)` is OK, but `Maybe (F Char)` is not (or rather, it is
"impredicative").
-Iavor
On Wed, Apr 3, 2013 at 10:30 AM, 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@haskell.org [mailto:ghc-devs-bounces@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 | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://www.haskell.org/mailman/listinfo/ghc-devs
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