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 librar...@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-Bifunctor.html
Petr Pudlak
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Andreas Abel <>< Du bist der geliebte Mensch.
Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY
andreas.a...@ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/
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