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

Hi,

I am learning haskell and wondering why my definition of firstAndThird does not work with the do operator, however when i try to use the same using bind (firstandThird2) it works as expected. I basically want to return a tuple of first and third character of a string. Can someone please correct me what i might be overseeing?

I have pasted the same code on codetidy in case if the email lose formatting.

http://codetidy.com/5726/

Many Thanks,

Rohit

zero :: Parser a

zero = \inp -> []

result :: a -> Parser a

result x = \inp -> [(x, inp)]

item :: Parser Char

item = \inp -> case inp of 

                   [] -> []

                   (x:xs) -> [(x,xs)]

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

p `bind` f = \inp -> concat [ ((f x) inp') | (x, inp') <- p inp]

sat :: (Char -> Bool) -> Parser Char

sat predicate = item `bind` (\x -> if predicate x then result x else zero )

lower :: Parser Char

lower = sat (\x -> 'a' <= x && x <= 'z')

firstAndThird :: Parser (Char, Char) 

firstAndThird = do x <- item 

                        item 

                   y <- item 

                   return (x,y)

firstAndThird2 :: Parser (Char, Char)

firstAndThird2 = item `bind` \x ->

      item `bind` \y ->

      item  `bind` \z ->

      result (x,z)

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