[Haskell-cafe] Parsing R5RS Scheme with Parsec

[Ryan Ingram]

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 <asandroq at gmail.com> wrote:
> Hallo,
>
> On 10/2/07, Brandon S. Allbery KF8NH <allbery at ece.cmu.edu> 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/
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