[Haskell-beginners] Functional Parses

[Robert Heumüller]

I am sorry, I must have hit the wrong hotkey.
My code looks like this:

type Parser a = String -> [(a, String)]

result :: a -> Parser a
result v = \inp -> [(v, inp)]

zero :: Parser a
zero = \inp -> []

item :: Parser Char
item = \inp -> case inp of
    [] -> []
    (x:xs) -> [(x, xs)]

parse :: Parser a -> String -> [(a, String)]
parse p inp = p inp

(>>=) :: Parser a -> (a -> Parser b) -> Parser b
(>>=) p f = \inp -> concat [f v inp' | (v, inp') <- p inp]

sat :: (Char -> Bool) -> Parser Char
sat p = do  x <- item
            if p x then result x else zero


I beleive I understand how this code is meant to work, but when I run
it in ghci I get the follwing error-message:

parser.hs:21:13:
    No instance for (Monad ((->) String))
      arising from a do statement
    Possible fix: add an instance declaration for (Monad ((->) String))
    In a stmt of a 'do' block: x <- item
    In the expression:
      do { x <- item;
           if p x then result x else zero }
    In an equation for `sat':
        sat p
          = do { x <- item;
                 if p x then result x else zero }

parser.hs:22:18:
    Couldn't match expected type `Char'
                with actual type `[(Char, String)]'
    In the first argument of `p', namely `x'
    In the expression: p x
    In a stmt of a 'do' block: if p x then result x else zero
Failed, modules loaded: none.


Sadly I have no idea how to fix this :( 

Thanks again :)