I was trying out the example given at http://www.haskell.org/haskellwiki/State_Monad#Complete_and_Concrete_Example_1

How this makes the solution composible is beyond my understanding. Here is what I tried but I get compile errors as follows:

Couldn't match expected type `GameValue -> StateT GameState Data.Functor.Identity.Identity b0'
            with actual type `State GameState GameValue'
In the second argument of `(>>=)', namely `g2'
In the expression: g1 >>= g2
In an equation for `g3': g3 = g1 >>= g2
Failed, modules loaded: none.

Here is the code: See the end lines

module StateGame where
import Control.Monad.State

type GameValue = Int
type GameState = (Bool, Int)

-- suppose I want to play one game after the other
g1 = playGame "abcaaacbbcabbab"
g2 = playGame "abcaaacbbcabb"
g3 = g1 >>= g2

m2 = print $ evalState g3 startState

playGame :: String -> State GameState GameValue
playGame []     = do
    (_, score) <- get
    return score

playGame (x:xs) = do
    (on, score) <- get
    case x of
         'a' | on -> put (on, score + 1)
         'b' | on -> put (on, score - 1)
         'c'      -> put (not on, score)
         _        -> put (on, score)
    playGame xs

startState = (False, 0)

main str = print $ evalState (playGame str) startState