Foldable.Mapped

[Oliver Charles]

Could you provide some examples where you have found it useful?

On Sun, Jan 3, 2016 at 9:11 PM Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> I found the following data type useful when working with the Foldable
> class:
>
>
> {- |
> It holds:
>
> > foldMap f . Mapped g = foldMap f . fmap g
>
> but use of 'Mapped' avoids 'Functor' constraint.
> -}
> data Mapped f a b = Mapped (a -> b) (f a)
>
>
> instance (Foldable f) => Foldable (Mapped f a) where
>     foldMap g (Mapped f xs) = Fold.foldMap (g . f) xs
>     foldr g x (Mapped f xs) = Fold.foldr (g . f) x xs
>     foldl g x (Mapped f xs) = Fold.foldl (\acc -> g acc . f) x xs
>
>
> Should it be added to Data.Foldable?
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