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
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
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