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))
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
