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

Ashley Yakeley ashley at semantic.org
Tue Jan 25 05:24:30 EST 2005

In article <41F61A0B.90005 at imperial.ac.uk>,
 Keean Schupke <k.schupke at imperial.ac.uk> wrote:

> I am sure monads in Haskell (and other functional languages like ML) are 
> defined on types not values.

The objects of the category are types. The morphisms on the category are 
functions. Two functions are the same if they match each value to the 
same value. For the Functor laws and the Monad laws, the values 
certainly do matter: if they didn't, they wouldn't correspond to the 
category theory notions of functor and monad because the morphisms would 
be wrong.

Ashley Yakeley, Seattle WA

More information about the Haskell-Cafe mailing list