tuple component functions

[Ketil Z. Malde]

<trb@eastpac.com.au> writes:

> S.D.Mechveliani writes:

>> As Haskell has the standard functions  fst, snd  to decompose  (a,b),
>> maybe, it worths to provide also [...]

> I've found some of these useful, except I named them differently:

>> fst3 :: (a,b,c) -> a
>> snd3 :: (a,b,c) -> b
>> thd3 :: (a,b,c) -> c

> .... never got around to quadruples etc.

I'd like a general 'nth', but of course that would restrict us to
monotyped tuples (e.g.,

        nth :: Int -> (a,a,...,a,a) -> a
) 

This isn't possible to do more generally with some language extension,
is it?

A better way might be to define classes:

class TwoTuple t a b | t -> a b where
        fst :: t -> a
        snd :: t -> b

instance TwoTuple (a,b) where ...

class (TwoTuple t) => ThreeTuple t c | t -> c where
        thd :: t -> c

instance TwoTuple (a,b,c) where ...
instance ThreeTuple (a,b,c) where ...

 --and so on.

Quite verbose, but avoids the need to tag the functions with the tuple
size.

-kzm
-- 
If I haven't seen further, it is by standing in the footprints of giants