wouldsucceed.hs

import Control.Applicative
import Data.Attoparsec
import Data.IORef
import System.IO.Unsafe

------------------------------------------------------------------------------
wouldSucceed :: Parser a -> Parser Bool
wouldSucceed p = do
    result <- return $! unsafePerformIO (newIORef False)
    (do
        p
        return $! unsafePerformIO (writeIORef result True)
        empty) <|> (return $! unsafePerformIO (readIORef result))