Re: A more useful Monoid instance for Data.Map

* Herbert Valerio Riedel [2013-01-06 11:15:46+0100]

Roman Cheplyaka writes:

...

But there's another problem... The "right" instance should really be based on Semigroup, not Monoid, but Semigroup is not currently in the base. And adding a dependency on semigroups to containers is hardly an option.

just to be clear, do you propose the instance to read

instance (Ord k, Semigroup v) => Monoid (Map k v)

or rather

instance (Ord k, Semigroup v) => Semigroup (Map k v)

The former, where mempty = Map.empty. (But the latter would also be present, since every monoid is (mathematically) a semigroup.) Roman