[Haskell-cafe] Re: what is inverse of mzero and return?

In article <41F4CEB3.5060707@imperial.ac.uk>, Keean Schupke wrote:

Right, but we are dealing with the type system here. Remember Haskell monoids are functors on types, not on values ... (ie the base objects the 'category theory' is applied to are the types not the values)...

Therefore we only consider the types when considering Monads.

If you remember your category theory, you'll recall that two morphisms are not necessarily the same just because they're between the same two objects. For instance, the objects may be sets, and the morphisms may be functions between sets: morphisms from A to B are the same only if they map each element in A to the same element in B. -- Ashley Yakeley, Seattle WA