Re: Discussion: Allow custom constraint for elem in Foldable

I would prefer to see designs for something like this explored in a separate package before we worried about changing base’s Foldable class.

On Jul 27, 2017, at 3:45 PM, David Feuer wrote:

The Foldable class offers the method

elem :: Eq a => a -> t a -> Bool

Unfortunately, this is really awful for sets, hash maps, etc. See https://stackoverflow.com/questions/45361801/implement-an-olog-n-foldable-el... for an example. We could fix it, kinda:

class Foldable t where type ElemConstr t :: * -> Constraint type ElemConstr t = Eq

elem :: ElemConstr t a => a -> t a -> Bool default elem :: (ElemConstr t ~ Eq, Eq a) => a -> t a -> Bool

One might legitimately complain that such a wild type is ad hoc, but one might counter that complaint by saying that most of Foldable is already ad hoc.

David _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries