[Haskell-cafe] Type families and polymorphism

[Brandon S. Allbery KF8NH]

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

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


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