Udo Stenzel wrote:
Chris Kuklewicz wrote:
Again, Parsec requires you to put "try" where you need it
I'm pretty sure it does, although this
Udo Stenzel wrote:
countBetween 0 n p = p <:> countBetween 0 (n-1) p <|> return []
is a place where it's not needed in general. You should know what
you're doing, though. And I like ReadP better in general, exactly
because 'try' is a bit trippy.
Udo.
I just tried to mimic regular expression matching with ReadP and got what seems
like a non-terminating program. Is there another way to use ReadP to do this?
import Control.Monad
import Text.ParserCombinators.ReadP
type R = ReadP Int
-- Consume a specific character, return length 1
c :: Char -> R
c x = char x >> return 1
-- Consume like x? x+ x* and return the length
quest,plus,star :: R -> R
quest x = option 0 x
plus x = liftM sum (many1 x)
star x = liftM sum (many x)
-- Concatenate two with sum of lengths
infixr 5 +>
(+>) :: R -> R -> R
(+>) x y = liftM2 (+) x y
-- Concatenate list with running total of length
match xs = match' xs 0
where match' [] t = return t
match' (x:xs) t = do v <- x
match' xs $! t+v
-- Simulate "(a?|b+|c*)*d" regular expression
test = star (choice [quest (c 'a')
,plus (c 'b')
,star (c 'c')]) +> c 'd'
go foo = readP_to_S test foo
'go' works if I remove the leading 'star' operation from 'test'
But 'go' seems to not terminate with the leading 'star'
My regex-dfa package has a failure which seems similar, and I have been adding
the ability to internally rewrite the pattern to avoid the problem.
--
Chris
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