Re: [Haskell] Parsec question: attempted 'notMatching' combinator

18 Feb 2004


      On Tue, Feb 17, 2004 at 04:57:34PM -0500, Andrew Pimlott wrote:
...
What about a more prosaic implementation:
notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st ()
    notFollowedBy' p    = do  res <-  do a <- try p; return $ Just a
                                      <|>
                                      return Nothing
                              case res of Just a  -> unexpected (show a)
                                          Nothing -> return ()
After some pondering and fiddling, a version I like:

    notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st ()
    notFollowedBy' p    = join $  do a <- try p; return (unexpected (show a))
                                  <|>
                                  return (return ())

Andrew