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

Ashley Yakeley ashley at semantic.org
Mon Jan 24 06:24:54 EST 2005

In article <41F4CEB3.5060707 at imperial.ac.uk>,
 Keean Schupke <k.schupke at imperial.ac.uk> 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

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