[Haskell-cafe] mapFst and mapSnd

[Emil Axelsson]

`mapPair` also exists as `tup2` in patch-combinators:

   http://hackage.haskell.org/package/patch-combinators

/ Emil

2013-05-28 16:01, Andreas Abel skrev:
> 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 at 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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>
>