Re: higher-order typing errors

Hi!

On Thu, Jan 23, 2003 at 10:28:13AM -0000, Simon Peyton-Jones wrote:
> 
> To get around it, define your own data type:
> 
>       data MyPr = MyPr (forall a.a->a) (forall a.a->a)
> 
>       swap1 :: MyPr -> MyPr

I played a bit with using highed order polymorphism for this problem,
and have encountered compile errors I don't understand.

The first version doesn't compile - the error is:
 Type synonym `F3' should have 1 argument, but has been given 0
 In the type: Pair F3 F2
 While checking the type signature for `p'

  {-# OPTIONS -fglasgow-exts #-}

  module T where

  type F3 a = a -> a -> a
  type F2 a = a -> a

  data Pair c1 c2 = Pair (forall a. c1 a) (forall b. c2 b)

  swap :: Pair c1 c2 -> Pair c2 c1
  swap (Pair f g) = Pair g f

  f2 :: F2 a
  f2 = id

  f3 :: F3 a
  f3 = const

  p :: Pair F3 F2
  p = Pair f3 f2

The other version uses newtype instead of type synonyms for F2 and F3.
This one compiles without problems.

  {-# OPTIONS -fglasgow-exts #-}

  module T2 where

  newtype F3 a = F3 (a -> a -> a)
  newtype F2 a = F2 (a -> a)

  data Pair c1 c2 = Pair (forall a. c1 a) (forall b. c2 b)

  swap :: Pair c1 c2 -> Pair c2 c1
  swap (Pair f g) = Pair g f

  f2 :: F2 a
  f2 = F2 id

  f3 :: F3 a
  f3 = F3 const

  p :: Pair F3 F2
  p = Pair f3 f2

There is probably some restriction I don't know about or haven't
recognized in this particular case. I hope you will enlighten me.

> Simon

Best regards,
Tom

-- 
.signature: Too many levels of symbolic links