Am Sonntag, 12. Februar 2006 04:23 schrieb Juan Carlos Arevalo Baeza:
So... I see no reason why someone can't just do it themselves, but... I was playing around with Parsec (as included in GHC 6.4.1), and I found two functions that are... not quite what I believe they should be.
optional :: GenParser tok st a -> GenParser tok st () optional p = do{ p; return ()} <|> return ()
Now, this completely loses the result of the optional parser. Better would be:
optional :: GenParser tok st a -> GenParser tok st (Maybe a) optional p = do{ x <- p; return (Just x) } <|> return Nothing
Hmm, I haven't used Parsec very much yet, but so far, whenever the result of an optional parser (mostly a '-'-sign) mattered, I use e.g. option ' ' (char '-'). Your above parser would be option Nothing (fmap Just p) -- or you might use liftM. Both are easy enough. If you think the naming is unfortunate, I wouldn't flatly contradict, but it's too late now, I believe.
Same thing with manyTill:
manyTill :: GenParser tok st a -> GenParser tok st end -> GenParser tok st [a] manyTill p end = scan where scan = do{ end; return [] } <|> do{ x <- p; xs <- scan; return (x:xs) }
Better:
manyTill :: GenParser tok st a -> GenParser tok st end -> GenParser tok st ([a], end) manyTill p end = scan where scan = do{ endr <- end; return ([], endr) } <|> do{ x <- p; (xs, endr) <- scan; return (x:xs, endr) }
Is there any reason I can't see why they are the way they are?
JCAB
With manyTill, I tend to agree with you; sometimes (not often) I would need the result of end, too. So far, this meant that I didn't use Parsec then, but wrote my own library, including a 'test' combinator where 'test p' would succeed or fail as p would, but not consume any input in either case, so I would use 'manyTill p (test end)' and afterwards endr <- end. Since changing manyTill now would be unfeasible, I wonder if including such a 'test' would be a good idea (danger: somebody might use 'many (test p)') or if providing your parser (under the name manyTillPair or something) would be better. Cheers, Daniel -- "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton