问题
The documentation for algebra/2.1.1.2/doc/html shows a colossal number of type classes.
How do I declare that a structure in question must be equipped with a commutative associative operation and a unit/identity element, but without anything else (inverses, distributivity etc)?
I'm thinking of
reduce :: Monoid m => (a -> m) -> [a] -> m
but instances of Data.Monoid are not supposed to be commutative and I want users of my function to see that they need commutativity for the function to work by looking at the type.
回答1:
(Abelian m, Monoidal m)
It might seem that Monoidal is much more than you want, but it is all based on Natural being a Semiring.
回答2:
It looks like that package provides a Commutative class, so correct me if I'm wrong, but it looks like it's just a matter of specifying a second typeclass:
reduce :: (Monoid m, Commutative m) => (a -> m) -> [a] -> m
来源：https://stackoverflow.com/questions/12289746/commutative-monoid-from-algebra-package-on-hackage