Data.Functor.{Product,Sum} functions

[Mario Blažević]

On 2016-12-23 07:02 PM, Baldur Blöndal wrote:
> Let's see if any of these are useful (a lot more at 
> https://ghc.haskell.org/trac/ghc/ticket/13026)
>
> (||||) :: (f a -> b) -> (g a -> b) -> ((Sum f g) a -> b)
> f |||| g = \case
>   InL fa -> f fa
>   InR ga -> g ga

The corresponding destructor function in Data.Either is called either:

    either :: (a -> c) -> (b -> c) -> Either 
<http://hackage.haskell.org/package/base-4.9.0.0/docs/Data-Either.html#t:Either> 
a b -> c

Thus the most logical name for the Sum destructor would be sum. You can 
always import Data.Functor.Product qualified as Product to avoid 
clashes. In the same vein, I propose adding

    uncurry :: (f a -> g a -> b) -> Product f g a -> b
    uncurry f (Pair a b) = f a b


> (&&&&) :: (a -> f b) -> (a -> g b) -> (a -> (Product f g) b)
> (f &&&& g) a = f a `Pair` g a

I'm not fond of that name. Also, f &&&& g == uncurry Pair . (f &&& g)