[Haskell-cafe] mapFst and mapSnd

[Tikhon Jelvis]

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 at 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
>
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