isn't this moving directly into the territory of impredicative types?  

Ahem, maybe you are right.  Impredicativity means that you can

instantiate a type variable with a polytype

So if we allow, say (Eq (forall a.a->a)) then we’ve instantiated Eq’s type variable with a polytype.  Ditto Maybe (forall a. a->a).

But this is only bad from an inference point of view, especially for implicit instantiation.  Eg if we had

          class C a where

            op :: Int -> a

then if we have

          f :: C (forall a. a->a) =>...

          f = ...op...

do we expect op to be polymorphic??

For type families maybe things are easier because there is no implicit instantiation.  

But I’m not sure

Simon

From: Iavor Diatchki [mailto:iavor.diatchki@gmail.com]
Sent: 03 April 2013 18:48
To: Simon Peyton-Jones
Cc: Richard Eisenberg; ghc-devs
Subject: Re: Restrictions on polytypes with type families

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 <simonpj@microsoft.com> wrote:

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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