[Haskell-cafe] Type families and polymorphism

Roman Cheplyaka roma at ro-che.info
Sat Jul 11 17:54:13 EDT 2009

* Brandon S. Allbery KF8NH <allbery at ece.cmu.edu> [2009-07-11 17:01:35-0400]
> On Jul 11, 2009, at 14:31 , Jeremy Yallop wrote:
>> Why does compiling the following program give an error?
>>> {-# LANGUAGE TypeFamilies, RankNTypes #-}
>>> type family TF a
>>> identity :: (forall a. TF a) -> (forall a. TF a)
>>> identity x = x
> The scope of each a is the surrounding parentheses, so the de facto type 
> is TF a -> TF b.  Or, put otherwise, you're saying that for *any* type (TF 
> a)

No, for any (forall a. TF a), which should make the difference.

> you can produce *any* type (TF a) (because of the delimited forall-s), 
> but then the code asserts that they are the same type.

Roman I. Cheplyaka :: http://ro-che.info/
"Don't let school get in the way of your education." - Mark Twain

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