[Haskell-beginners] about parser question < Programming in haskell _ chapter 8 >

[anyzhen]

well,i am reading the book :<Programming in haskell >
chapter 8 functional parsers 
this chapter try teaching an parser technology,but i have some problem about do keyword 


--some  brief info of types :
type Parser a = String -> [(a,String)]

item :: Parser Char                           --item "abc"  = [('a',"bc")]

return' :: a -> Parser a                     --return' 'a' "foo" = [('a',"foo")]


--below code complier notice... sorry format
p2 :: Parser (Char,Char)
p2 = do x <- item
            item
            y <- item
            return' (x,y)

"No instance for (Monad ((-)) String))                                                        
    arising from a execute statement                                                             
 Possible fix : add an instance declaration for (Monad ((-)) String))           
In a stmt of a 'do' expression : y <- item                                                     
In the expression:                                                                                        
  do { x <- item;                                                                                                     item;                                                                                                   
          y <- item;                                                                                           
          return' (x,y)}                                                                                    
 In an equation for 'p2':                                                                               
        p2                                                                                                         
           = do {  x <- item;                                                                              
                       item;                                                                                       
                       y <- item;                                                                               
                       ....}"                                                                                      



thanks for any help 
jiangzhen <mail:jiangzhen3s at qq.com>
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