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@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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I have them defined for my stuff. Generally I find it much quicker to "roll my own" than to (1) ask on this list if someone else has done it... (2) look at arrows or MyFavouriteCategoryTheoryBitOfFPBecauseICantGetAbstractEnough and the try to figure out what is going on. The joy of Haskell is both that rolling your own is often so easy, and that there are lots of approaches out there that allow you to get abstract as well... Regards, Andrew (Engineering graduate, feet still firmly embedded in concrete :-) On 28 May 2013, at 09:54, Dominique Devriese 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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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On Tue, May 28, 2013 at 1:54 AM, Dominique Devriese 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?

One generalization of them is to lenses. For example `lens` has "both", "_1", "_2", such that "mapPair = over both", "mapFst = over _1", etc., but you can also get "fst = view _1", "set _2 = \y' (x,_) -> (x,y')", and so on. (Since "both" refers to two elements, you end up with "view both = \(x,y) -> mappend x y".) The types you end up with are simple generalizations of mapFoo, with just an extra Functor or Applicative (think mapMFoo): both :: Applicative f => (a -> f b) -> (a,a) -> f (b,b) both f (x,y) = (,) <$> f x <*> g y _2 :: Functor f => (a -> f b) -> (e,a) -> f (e,b) _2 f (x,y) = (,) x <$> f y With an appropriate choice of f you can get many useful functions. Shachaf