fp101x parsers lecture working example

parsers_lecture.hs

module Parsers where
import Prelude hiding (fail, return, (>>=))

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

item :: Parser Char
item [] = []
item (c:cs) = [(c, cs)]

fail :: Parser a
fail _ = []

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

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

p :: Parser (Char, Char)
p = item >>= \c1 ->
    item >>= \_ ->
    item >>= \c2 ->
    return (c1, c2)