a little parsec enhancement

5 Sep 2013

      Hi,

when thinking about this SO question 
http://stackoverflow.com/q/18583416/1333025, I couldn't find a 
combinator that allows a parser to /optionally/ fail without consuming 
input, or consume input and return its value. So I'm suggesting such a 
function:

|-- | @emptyIf p@ parses @p@ and if its return value is @Nothing@, pretends
-- that an error has occured with no input consumed.
--
-- If @p@ fails and consumes some input, so does @emptyIf p@. Combine with
-- 'try' if this is undesirable.

emptyIf  :: (Stream  s m t) =>ParsecT  s u m (Maybe  a) ->ParsecT  s u m a
emptyIf  p =ParsecT  $ \s cok  cerr eok eerr ->
                         let  cok' (Just  x) s e = cok x s e
                             cok'Nothing   _ e = eerr e
                             eok' (Just  x) s e = eok x s e
                             eok'Nothing   _ e = eerr e
                         in  unParser p s cok' cerr eok' eerr|

With this function, the answer to the SO question becomes really easy:

|rcomb  :: (Stream  s m t) =>ParsecT  s u m a ->ParsecT  s u m b ->ParsecT  s u m b
rcomb  p q = emptyIf $ runMaybeT (opt p *> opt q)
   where
     opt =MaybeT  . optional-- optional from Control.Applicative!|

Whenever |p| or |q| fails without consuming input, then |rcomb p q| 
fails without consuming input.

Unfortunately |ParsecT| constructor isn't exported so I'm not able to 
implement it outside /parsec/. (Perhaps it would make sense to export 
|ParsecT| in some module such as |Text.Parsec.Internal|?) Therefore I'm 
suggesting to add such a function to /parsec/ (darcs patch included). 
Perhaps change the name, I couldn't think of anything better.

Best regards,
Petr

Petr Pudlák

Roman Cheplyaka

Petr Pudlák

Antoine Latter

Petr Pudlák

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