Re: [Haskell-beginners] parse block comments

18 Mar 2017


      Monadic and applicative.

blockCmnt :: Parser ()
blockCmnt = do
  string "{-"
  manyTill item (string "-}")
  return ()

manyTill p endp = scan where
  scan = do 
           endp
           return []
           <|>
           do
            x <- p
            xs <- scan
            return (x:xs)


blockCmnt' :: Parser () 
blockCmnt' = string "{-" >> manyTill item (string "-}") >> return ()

manyTill' p endp = scan' where
  scan' = endp *> return [] <|> pure (:) <*> p <*> scan’

Thanks.
M
...
On 18 Mar 2017, at 21:47, Francesco Ariis  wrote:
On Sat, Mar 18, 2017 at 09:11:20PM +0000, mike h wrote:
...
Hi,
Below is code I’m building up  for simple monadic and applicative parsers
from first principles.
Hello Mike,
   You might want to check `manyTill` from Parsec to get an idea:
manyTill :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m end -> ParsecT s u m [a]
   manyTill p end      = scan
       where
             scan  = do { end; return [] } <|>
                     do { x <- p; xs <- scan; return (x:xs) }
The 'trick' lies in <|> (alternative). Does that help?
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