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

In article <41F62049.4010403@imperial.ac.uk>, Keean Schupke wrote:

I think I see, but if the objects are types, arn't the morphisms functions on types not values?

Every morphism in any category has a "from" object and a "to" object: it is a morphism from object to object. In the "Haskell category", a function of type 'A -> B' is a morphism from object (type) A to object B. But in category theory, just because two morphisms are both from object A to object B does not mean that they are the same morphism. And so it is for the Haskell category: two functions may both have type 'A -> B' without being the same function. -- Ashley Yakeley, Seattle WA