I am reading Chapter 8 of Programming Haskell by Graham Hutton and trying to run the code in the book.

It seems that there are already defined library implementations of Parser so I used Parser' and generally ' on the end of each identifier to attempt to eliminate this problem.

So the code I copied from the book is:

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

return' :: a -> Parser' a

return' v = \x -> [(v, x)]

failure' :: Parser' a

failure' = \inp -> []

-- item doesn't seem to conflict with anything so no '

item :: Parser' Char

item = \inp -> case inp of

                    [] -> []

                    (x:xs) -> [(x,xs)]

parse' :: Parser' a -> String -> [(a, String)]

parse' p inp = p inp

p :: Parser' (Char, Char)

p = do  x <- item

        item

        y <- item

        return' (x,y)

When run from WinGHCi I get error:

prog_haskell.hs:458:9:

   Couldn't match type `[(Char, String)]' with `Char'

    Expected type: String -> [((Char, Char), String)]

      Actual type: Parser' ([(Char, String)], [(Char, String)])

    In the return type of a call of return'

    In a stmt of a 'do' block: return' (x, y)

    In the expression:

      do { x <- item;

           item;

           y <- item;

           return' (x, y) }

458.9 is line at end with return' (x,y)

Also in the chapter was the definition of >>= sequencing.  as in:

(>>=) :: Parser a -> (a -> Parser b) -> Parser b

p >>= f  =  \inp -> case parse p inp of

                      [] -> []

                      [(v,out)] -> parse (f v) out

But I assumed this would already be in standard Haskell library and so I didn't need to define.  But even when I did add, still get same error.

How do I fix this?