Stephane Bortzmeyer wrote:
Parsec provides "count n p" to run the parser p exactly n times. I'm looking for a combinator "countBetween m n p" which will run the parser between m and n times. It does not exist in Parsec.
Much to my surprise, it seems quite difficult to write it myself and, until now, I failed (the best result I had was with the "option" combinator, which unfortunately requires a dummy value, returned when the parser fails).
Does anyone has a solution? Preferrably one I can understand, which means not yet with liftM :-
The problem with the other solutions posted so far is for (countBetween 0 100 p) they will always try to run p 100 times, regardless of when it starts to fail. This can be made more efficient by stopping when p first fails. Also, you probably have to use "try p" for the (n-m) cases, so if p consumes some input it does not cause the whole operation to fail. import Text.ParserCombinators.Parsec -- This stops when p first fails, using option to hide <|> countBetween m n p | m<=n = do xs <- count m p let loop 0 acc = return (acc []) loop i acc = do -- mx <- option Nothing (liftM Just p) mx <- option Nothing (do x <- try p return (Just x)) case mx of Nothing -> return (acc []) Just x -> loop (pred i) (acc . (x:)) ys <- loop (n-m) id return (xs++ys) -- This also works and uses <|> directly instead of via option countBetween' m n p | m<=n = do xs <- count m try p let loop 0 acc = return (acc []) loop i acc = (do x<-p loop (pred i) (acc . (x:))) <|> (return (acc [])) ys <- loop (n-m) id return (xs++ys)