lifting functions to tuples?

[oleg at pobox.com]

Abraham Egnor wrote:

> The classic way to write a lift function for tuples is, of course:

> liftTup f (a, b) = (f a, f b)

> which has a type of (a -> b) -> (a, a) -> (b, b).  I've been wondering if
> it would be possible to write a function that doesn't require the types in
> the tuple to be the same, just that the types in the second tuple are the
> result of applying the type transformation implied in the function to be
> lifted to the types in the first tuple.

Well, it is possible in Haskell. It works even in Hugs!

{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}


class Funnable a b | a->b where
    f:: a -> b

instance Funnable Bool Int where
    f = fromEnum
instance Funnable Char Float where
    f = fromRational . toRational . fromEnum
     
class LP a b c d where
    liftf:: (a, b) -> (c, d)
    
instance (Funnable a c, Funnable b d) => LP a b c d where
    liftf (a,b) = (f a, f b)

Main> liftf (True,'z')
(1,122.0)