Data.Functor.{Product,Sum} functions

[David Feuer]

Er ... that was silly. I guess bifoldMap isn't really (||||), exactly.
It's pretty much `either`, but needs to have a Monoid wrapper. The
generalization I was roughly imagining would be a sort of
higher-kinded Bifoldable1:

class Bifoldable1' p where
  bifoldMap1' :: Semigroup m => (f a -> m) -> (g a -> m) -> p f g a -> m

which would then get the job done with Data.Semigroup.First.

But that's getting pretty far from the point, so, we need `||||` or
`sum` or whatever.

On Sat, Dec 24, 2016 at 7:43 AM, David Feuer <david.feuer at gmail.com> wrote:
> This |||| is the sum eliminator. We can't call it `sum`, but maybe
> `sumE`? I do think it should be included with some name. Another
> option might be to drag Data.Bifoldable into base from bifunctors;
> (||||) = bifoldMap, but I think the case for a specialized name is
> still pretty good.
>
> &&&& is definitely a nice thing to have. However, (&&&&) = liftA2
> Pair, so I'm not convinced we need to give it its own name.
>
> On Fri, Dec 23, 2016 at 7:02 PM, Baldur Blöndal <baldurpet at gmail.com> 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
>>
>> (&&&&) :: (a -> f b) -> (a -> g b) -> (a -> (Product f g) b)
>> (f &&&& g) a = f a `Pair` g a
>>
>> I couldn't think of any for Data.Functor.Compose, names are up for
>> bikeshedding
>>
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