Hi Martijn and Wouter, Based on the parser combinators paper, I put a monad together,
data Parser s t = Parser ([s] -> [(t, [s])])
pFail = Parser (const []) pReturn a = Parser (\inp -> [(a, inp)]) pSymbol s = Parser (\inp -> case inp of x:xs | x == s -> [(s,xs)] ; _ -> []) pChoice (Parser m) (Parser n) = Parser (\inp -> m inp ++ n inp) pBind :: Parser s a -> (a -> Parser s b) -> Parser s b pBind (Parser m) f = Parser (x m f) where x :: ([s] -> [(a, [s])]) -> (a -> Parser s b) -> [s] -> [(b, [s])] x m f inp = concatMap (y f) (m inp) y f (a,s) = case f a of Parser g -> g s
instance Monad (Parser s) where return = pReturn ; (>>=) = pBind instance MonadPlus (Parser s) where mzero = pFail ; mplus = pChoice
runParser (Parser f) inp = f inp
and wrote the parser using that:
left = pSymbol '(' right = pSymbol ')'
parens = wrappend `mplus` empty where empty = return Empty wrappend = do left ; p <- parens ; right ; q <- parens ; return (Wrap p & q) where m & Empty = m m & n = Append m n
I really like the approach Wouter used for the State monad and I hope to follow that idea for this, I am not sure how to do it yet but I will scribble and see where it goes. Thanks very much for the advice! -- View this message in context: http://www.nabble.com/Parsing-with-Proof-tp22814576p22835016.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.