[Haskell-cafe] mapFst and mapSnd

[Tony Morris]

class BinaryFunctor f where
  bimap :: (a -> c) -> (b -> d) -> f a b -> f c d

mapFst = (`bimap id`)
mapSnd = bimap id
 On 31/05/2013 12:16 PM, "Shachaf Ben-Kiki" <shachaf at gmail.com> wrote:

> On Thu, May 30, 2013 at 7:12 PM, Shachaf Ben-Kiki <shachaf at gmail.com>
> wrote:
> > 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.
> >
>
> I spoke too quickly -- your mapPair is something different. Indeed
> bimap (or (***), if you prefer base) is the place to find it -- lenses
> don't really fit here. My "both" is for mapping one function over both
> elements.
>
>     Shachaf
>
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