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':

= do { x <- item;

item;

y <- item;

....}"

thanks for any help

jiangzhen <mail:jiangzhen3s@qq.com>