8 Jul
2008
8 Jul
'08
9:59 a.m.
On Tue, 2008-07-08 at 06:39 +0100, Dominic Steinitz wrote:
I have a solution so this is for interest only.
It is not normally the case that two monads compose to give another monad. Monad transformers capture when this is possible. However, when there is a "swap" function satisfying some commutative diagrams then it can be proved that the monads compose to produce a monad.
Is there such a swap function in a library? I had a look in the comprehensive category-extra package but couldn't find anything.
You didn't look hard enough. What you want is called a distributive law in category theory. A class for exactly what you want is in Control.Functor.Extras. http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html/...