Hello Henry, In such cases, it is often worth thinking about how you would implement such a scheme manually, without using pre-existing monads. You will quickly see that the two candidate types: s -> ([a], s) [s -> (a, s)] both will not work (exercise: what semantics do they give?) In fact, you must use continuation passing style, and you must "resume" the computation with the latest state value you would extracted from the last run. See the LogicT monad for how to implement list-like monads in continuation passing style. Cheers, Edward Excerpts from Henry Laxen's message of Sat Aug 25 00:35:37 -0400 2012:
Dear Cafe,
It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following:
countCalls = do a <- [1..2] b <- [1..2] modify (+1) return (a,b)
where with some combination of ListT, StateT, List, State, or who knows what would result in:
([(1,1),(1,2),(2,1),(2,2)],4)
assuming we initialize the state to 0
Is there any way to make this happen? Thanks in advance.
Henry Laxen