Problem with Monoid instance of Data.Map

[ahey@…]

See..

 http://www.haskell.org/pipermail/libraries/2007-May/007491.html

Data.Map Monoid instance is currently..

instance (Ord k) => Monoid (Map k v) where
    mempty  = empty
    mappend = union
    mconcat = unions

..but probably should be..

instance (Ord k, Monoid v) => Monoid (Map k v) where
 mempty            = empty
 mappend map0 map1 = unionWith mappend map0 map1
 mconcat maps      = foldr (unionWith mappend) empty maps

Adrian Hey

[SamB]

I'm inclined to agree, though I recognize that the other way may well be useful too. There ought to be a way to warn people when you've changed instances like this... release notes, maybe?

[igloo]

This proposal seems to be abandoned