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.henry@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