On Mon, 2010-11-29 at 18:36 +0000, Ross Paterson wrote:
On Mon, Nov 29, 2010 at 12:46:28PM -0500, David Menendez wrote:
On Mon, Nov 29, 2010 at 11:26 AM, Isaac Dupree
Is it useful to have Pointed non-Functors?
Is Pointed useful at all? The last time this discussion came up, I asked for algorithms which were generic over pointed functors (in the same way that traverse is generic over applicative functors) and no one could think of any.
fromFoldable :: (Foldable f, Pointed g, Monoid (g a)) => f a -> g a fromFoldable = foldMap pure
One instance `g` of `Pointed` where this is useful is the type constructor for difference lists (aka functional lists) which does provide singleton and monoid operations but no map function without converting to ordinary lists. With regard to laws one could still 'require' laws for the interaction of `Functor` and `Pointed` if both instances are defined. Sebastian