[Haskell-cafe] Another monad question...

[David LaPalomento]

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
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