http://blog.sigfpe.com/2006/05/grok-haskell-monad-transformers.html 第3个例子的signature搞了两天才根据下面的问题弄正确 http://stackoverflow.com/questions/15625867/why-dont-you-need-to-use-lift-when-interacting-with-a-nested-statet-monadt-in?rq=1

monad_transformer.hs

import Control.Monad.State
import Control.Monad.Identity
import Data.Char(toUpper)
 

test1 :: State Int (Int,Int)
test1 = do
    a <- get
    modify (+1)
    b <- get
    return (a,b)


test2 :: State [Char] ([Char],[Char])
test2 = do
    a <- get
    modify (++"1")
    b <- get
    return (a,b)

--let go1 = evalState test1 0
--let go2 = evalState test2 "0"

 
test3 :: StateT Int ( StateT [Char] Identity ) (Int, [Char])
test3 = do
    modify (+1)
    lift $ modify (++ "1")
    a <- get
    b <- lift get
    return (a,b)

--let go3 = runIdentity $ evalStateT (evalStateT test3 0) "0"