Hello, the state should be inside, to count, so type is ListT (State Int) (Int,Int)
the runner is then runState (runListT countCalls) 0 but [] is not of type ListT m so you need to wrap it in ListT . return import Control.Monad.List import Control.Monad.State import Control.Monad.Instances countCalls :: ListT (State Int) (Int,Int) countCalls = do a <- ListT . return $ [1..2] b <- ListT . return $ [1..2] modify (+1) return (a,b) regards paolino 2012/8/25 Henry Laxen <nadine.and.he...@pobox.com> > 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 > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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