gistfile1.hs

module Test where

import Data.Maybe
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Maybe(MaybeT(..))
import Control.Monad.State(State, runState, forM, get, put)

positiveOnly :: Int -> Maybe Int
positiveOnly x = if x > 0 then Just x else Nothing

f :: MaybeT (State Int) ()
f = do
  state <- get
  -- how to get rid of the 'case' by taking advantage of monad transformers?
  case positiveOnly state of 
    Nothing -> mzero
    Just x' -> put $ x' - 1