agreed. this needs to be tested out in a user space lib first. On Thu, Jul 27, 2017 at 7:35 PM, Eric Mertens wrote:

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-elem-for-binary-search-trees-in-haskell#45362110 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

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On 28 July 2017 at 09:37, Niklas Hambüchen wrote:

On 28/07/17 00:45, David Feuer wrote:

...

Unfortunately, this is really awful for sets, hash maps, etc. If this is done at some point, I suppose we could fix `nub` in the same way?

I'd prefer we have a "needs only Eq" function like nub, as I've used it in the past. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com