How to read exact N chars with Parsec?

Question

I'm new to Haskell and Parsec. I wish to parse php-serialize format of string 's:numb:"string";' like

s:12:"123";6789012";

where number is count of chars. So, function looks like:

newtype PhpString = PhpString String

pString :: GenParser Char st PhpString
pString = do { string "s:"
        ; value1 <- many1 digit
        ; string ":\""
        ; value2 <- takeExactNChars (read value1) 
        ; string "\";"      
        ; return $ PhpString value2
    }
    where 
        takeExactNChars n = ???????

Answer 1

I would write it using replicateM from Control.Monad:

import Text.ParserCombinators.Parsec
import Control.Monad (replicateM)

pString :: Parser String
pString = do string "s:"
             n <- fmap read (many1 digit)
             string ":\""         -- Bug fix; you weren't picking up the colon
             s <- replicateM n anyChar
             string "\";"
             return s

Testing it in ghci:

*Main> parse pString "" "s:12:\"123\";6789012\";"
Right "123\";6789012"