[Haskell-cafe] type-level programming support library

[Claus Reinke]

>I suppose having a good description of what I'd like to do might help: I'd
> like to be able to make an N-Tuple an instance of a type class.
> 
> class Foo a where
> instance Foo (,) where
> instance Foo (,,) where
>    ....
> The different kindedness of (,) and (,,) prevent this from working.

Not that this is going to help you much, but perhaps you might
want to refine the problem specification:-) 

Claus

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE EmptyDataDecls #-}
import Data.Char

data Ap0 (f :: *) 
data Ap1 (f :: * -> *) 
data Ap2 (f :: * -> * -> *)
data Ap3 (f :: * -> * -> * -> *)

type family F a b
type instance F (Ap0 b) b = b
type instance F (Ap1 f) b = f b
type instance F (Ap2 f) b = f b b
type instance F (Ap3 f) b = f b b b

class Foo a b where 
  foo   :: a -> b -> F a b
  unfoo :: a -> F a b -> b

instance Foo (Ap0 Bool) Bool where
  foo   _ b = b
  unfoo _ b = b

instance Foo (Ap2 (,)) Bool where 
  foo   _ b     = (b,not b)
  unfoo _ (a,b) = a&&b

instance Foo (Ap2 (,)) Char where 
  foo   _ c     = (chr (ord c+1), chr (ord c+2))
  unfoo _ (a,b) = maximum [a,b]

instance Foo (Ap3 (,,)) Char where 
  foo   _ c       = (c, chr (ord c+1), chr (ord c+2))
  unfoo _ (a,b,c) = maximum [a,b,c]

f bs | unfoo (undefined::Ap2 (,)) bs = foo (undefined::Ap3 (,,)) 'a'
     | otherwise                     = foo (undefined::Ap3 (,,)) 'b' 

g what1 what2 bs | unfoo what1 bs = foo what2 'a'
                 | otherwise      = foo what2 '0' 

main = do
  print (f (True,False)::(Char,Char,Char))
  print (g (undefined::Ap0 Bool) (undefined::Ap3 (,,)) False       ::(Char,Char,Char))
  print (g (undefined::Ap2 (,))  (undefined::Ap2 (,))  (True,False)::(Char,Char))