Hi everyone, I've been playing with the parsers decribed in "Monadic Parser Combinators" (http://www.cs.nott.ac.uk/~gmh/bib.html#monparsing) and I've gotten stumped. I'm trying to get comfortable working monadically, so please excuse my ignorance. Here's the relevant portions of my code: data Parser a = Parser { parse :: (String -> [(a, String)]) } instance Monad Parser where return v = Parser (\inp -> [(v, inp)]) par@(Parser p) >>= f = Parser (\inp -> concat [parse (f v) out | (v, out) <- p inp]) instance MonadPlus Parser where mzero = Parser (\inp -> []) p `mplus` q = Parser (\inp -> (parse p inp ++ parse q inp)) item :: Parser Char item = Parser (\inp -> case inp of [] -> [] (x:xs) -> [(x, xs)]) sat :: (Char -> Bool) -> Parser Char sat p = Parser (\inp -> [ (v, out) | (v, out) <- parse item inp, p v]) lower :: Parser Char lower = Parser (\inp -> parse (sat (\x -> 'a' <= x && x <= 'z')) inp) upper :: Parser Char upper = Parser (\inp -> parse (sat (\x -> 'A' <= x && x <= 'Z')) inp) letter :: Parser Char letter = lower `mplus` upper -- word parses everything as [] word :: Parser String word = mzero `mplus` do x <- letter; xs <- word; return (x:xs) As I noted in the code, no matter what inputs I give it,
parse word "blah blah" always returns []. Any ideas where where my misunderstanding is?
David