These are present in Control.Arrow as (***), first and second respectively. They are easy to overlook because they work for *all* arrows, not just functions. So the type signatures look like: first :: Arrow a => a b c -> a (b, d) (c, d) If you replace a with (->), you'll see that this is exactly like your mapFst. On Tue, May 28, 2013 at 1:54 AM, Dominique Devriese < dominique.devriese@cs.kuleuven.be> wrote:
Hi all,
I often find myself needing the following definitions:
mapPair :: (a -> b) -> (c -> d) -> (a,c) -> (b,d) mapPair f g (x,y) = (f x, g y)
mapFst :: (a -> b) -> (a,c) -> (b,c) mapFst f = mapPair f id
mapSnd :: (b -> c) -> (a,b) -> (a,c) mapSnd = mapPair id
But they seem missing from the prelude and Hoogle or Hayoo only turn up versions of them in packages like scion or fgl. Has anyone else felt the need for these functions? Am I missing some generalisation of them perhaps?
Regards, Dominique
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe