On Tue, May 28, 2013 at 10:54 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)

That's Control.Arrow.(***), e.g.:

ghci> (+3) *** (*5) $ (20,30)

(23,150)

mapFst :: (a -> b) -> (a,c) -> (b,c)
mapFst f = mapPair f id

mapSnd :: (b -> c) -> (a,b) -> (a,c)
mapSnd = mapPair id

That's Control.Arrow.{first, second}:

ghci> first (+10) (1,1)

(11,1)

ghci> second (+10) (1,1)

(1,11)