See Agda.Utils.Tuple :-) -- | Bifunctoriality for pairs. (-*-) :: (a -> c) -> (b -> d) -> (a,b) -> (c,d) (f -*- g) ~(x,y) = (f x, g y) -- | @mapFst f = f -*- id@ mapFst :: (a -> c) -> (a,b) -> (c,b) mapFst f ~(x,y) = (f x, y) -- | @mapSnd g = id -*- g@ mapSnd :: (b -> d) -> (a,b) -> (a,d) mapSnd g ~(x,y) = (x, g y) I think mapPair, mapFst, and mapSnd are canonical names that could be added to Data.Tuple. But if you suggest this on libraries@haskell.org, you get probably turned down, see e.g. http://comments.gmane.org/gmane.comp.lang.haskell.libraries/17411 Cheers, Andreas On 28.05.2013 15:34, Petr Pudlák wrote:
Dne 28.5.2013 10:54, Dominique Devriese napsal(a):
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? Apart from Arrows, there is also package "bifunctors" that defines this functionality for (,), Either and a few others: http://hackage.haskell.org/packages/archive/bifunctors/3.2.0.1/doc/html/Data...
Petr Pudlak
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