[Haskell-cafe] ReadP question

[Chris Kuklewicz]

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