3 lines Parser in Haskell (Line 9,11,12)

Parser.hs

import Control.Monad.State.Lazy
import Control.Monad.Error
import Control.Monad.Identity
import Control.Applicative

import Data.Char

-- Begin parser
type Parser a = StateT String (ErrorT String Identity) a

parse:: Parser a -> String -> Either String (a, String)
parse p = runIdentity . runErrorT . runStateT p

-- End of parser.. That's 3 lines..

-- Example time!
satisfy :: String -> (Char -> Bool) -> Parser Char
satisfy desc f = get
              >>= \s -> case s of
                     []    -> throwError "Unexpected EOF"
                     (x : xs) -> if f x then put xs >> return x
                                        else throwError 
                                            ("Expected " ++ desc ++ " here")

char :: Parser Char
char = satisfy "Alphabet" isAlpha

digit :: Parser Int
digit = digitToInt <$> satisfy "Digit" isDigit

-- Combine'em
digitThenChar :: Parser String
digitThenChar = (\d c -> show d ++ [c])  <$> digit <*> char

-- Test'em

main :: IO ()
main = mapM_ print [parse digitThenChar "2DHehehehe", 
                    parse digitThenChar "WhoopsErrorHere"]