Re: [Haskell-cafe] Parsing R5RS Scheme with Parsec

I don't know if this applies to Scheme parsing, but I find it's often helpful to introduce a tokenizer into the parser to centralize the use of "try" to one place:: type Token = String tokRaw :: Parser Token tokRaw = {- implement this yourself depending on language spec -} tok :: Parser Token tok = do t <- tokRaw many spaces return t -- wrap your outside parser with this; it gets the tokenizer -- started because we only handle eating spaces after tokens, -- not before startParser :: Parser a -> Parser a startParser a = many spaces >> a sat :: (Token -> Maybe a) -> Parser a sat f = try $ do t <- tok case f t of Nothing -> mzero Just a -> return a lit :: Token -> Parser Token lit s = sat (test s) s where test s t = if (s == t) then Just s else Nothing Now if you replace uses of "string" and "char" in your code with "lit", you avoid the problem of parses failing because they consumed some input from the "wrong" token type before failing. On 10/2/07, Alex Queiroz wrote:

Hallo,

On 10/2/07, Brandon S. Allbery KF8NH wrote:

...

Sorry, just woke up and still not quite tracking right, so I modified the wrong snippet of code. The trick is to wrap parseLeftList in a try, so the parser retries the next alternative when it fails.

Since "..." can only appear at the end of a list, I removed "..." from the possible symbols and added a new function:

parseThreeDottedList :: [SchDatum] -> Parser SchDatum parseThreeDottedList ls = do string "..." many parseAtmosphere char ')' return $ SchList $ ls ++ [SchSymbol "..."]

parseList :: Parser SchDatum parseList = do ls <- parseLeftList try (parseThreeDottedList ls) <|> (parseDottedList ls) <|> (parseProperList ls)

Thanks for the help.

Cheers, -- -alex http://www.ventonegro.org/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe