[Haskell-cafe] a little parsec enhancement

[Petr Pudlák]

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 at . 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

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1 patch for repository http://code.haskell.org/parsec3:

Thu Sep  5 11:12:47 CEST 2013  Petr Pudlak <petr at pudlak.name>
  * Add 'emptyIf' which allows a parser to optinally fail without consuming any input.

New patches:

[Add 'emptyIf' which allows a parser to optinally fail without consuming any input.
Petr Pudlak <petr at pudlak.name>**20130905091247
 Ignore-this: 59b93c660fe860acd9a5fff887f7678f
] {
hunk ./Text/Parsec/Prim.hs 38
     , parserPlus
     , (<?>)
     , (<|>)
+    , emptyIf
     , label
     , labels
     , lookAhead
hunk ./Text/Parsec/Prim.hs 455
     p' = ParsecT $ \s cok cerr eok eerr ->
          unParser p s eok cerr eok eerr
 
+-- | @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 at . 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
+          
+
 -- | The parser @token showTok posFromTok testTok@ accepts a token @t@
 -- with result @x@ when the function @testTok t@ returns @'Just' x at . The
 -- source position of the @t@ should be returned by @posFromTok t@ and
}

Context:

[Fix haddock module links.
Bjorn Buckwalter <bjorn at buckwalter.se>**20130821095713
 Ignore-this: 304217ec8b73f59edcd96dd13aca67af
] 
[TAG 3.1.3
Antoine Latter <aslatter at gmail.com>**20120612020909
 Ignore-this: 7100375a3e4853c09f1ea993ae32eed7
] 
Patch bundle hash:
173111b8bbc5a6eb69d41926053184c35ae9b3cc