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)
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