lifting functions to tuples?

[sebc at macs.hw.ac.uk]

On Tue, Nov 18, 2003 at 05:56:19PM -0800, oleg at pobox.com wrote:
> 
> 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)

This seems different from what Abraham Egnor asked for, because it
allows one to provide many different implementations for f.
It corresponds more closely to

> liftTup' (f1, f2) (a, b) = (f1 a, f2 b)

which is of course typable with in Haskell.

Main> liftTup' (fromEnum, fromRational . toRational . fromEnum) (1, 122.0)
(1,122.0)

-- 
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