Could you provide some examples where you have found it useful? On Sun, Jan 3, 2016 at 9:11 PM Henning Thielemann < lemming@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? _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries