Re: [Haskell-cafe] a little parsec enhancement
I'd appreciate exporting adding such aliases for creating and running ParsecT the 3.1.x way. Thanks, Petr Dne 09/06/2013 10:56 PM, Antoine Latter napsal(a): The exported `mkPT` is equivalent to the old 'ParsecT' data constructor from parsec 3.0.x. I wouldn't mind exporting a similar alias for the new 'ParsecT' constructor from 3.1.x. On Fri, Sep 6, 2013 at 2:21 PM, Petr Pudlák petr@gmail.com mailto:petr@gmail.com wrote: Dne 09/05/2013 01:38 PM, Roman Cheplyaka napsal(a): * Petr Pudlák petr@gmail.com mailto:petr@gmail.com [2013-09-05 11:18:25+0200] Unfortunately |ParsecT| constructor isn't exported so I'm not able to implement it outside /parsec/. No, but there's an 'mkPT' function which is equivalent to the ParsecT constructor. (Although I, too, wish the ParsecT constructor were exposed.) Roman Yes, I tried to use `mkPT`, but the result looked very complicated and I wasn't quite sure if it'll be working correctly in all cases. Implementing the same thing with the `ParsecT` constructor is simple and comprehensible. Best regards, Petr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] a little parsec enhancement
Dne 09/05/2013 01:38 PM, Roman Cheplyaka napsal(a): * Petr Pudlák petr@gmail.com [2013-09-05 11:18:25+0200] Unfortunately |ParsecT| constructor isn't exported so I'm not able to implement it outside /parsec/. No, but there's an 'mkPT' function which is equivalent to the ParsecT constructor. (Although I, too, wish the ParsecT constructor were exposed.) Roman Yes, I tried to use `mkPT`, but the result looked very complicated and I wasn't quite sure if it'll be working correctly in all cases. Implementing the same thing with the `ParsecT` constructor is simple and comprehensible. Best regards, Petr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] a little parsec enhancement
The exported `mkPT` is equivalent to the old 'ParsecT' data constructor from parsec 3.0.x. I wouldn't mind exporting a similar alias for the new 'ParsecT' constructor from 3.1.x. On Fri, Sep 6, 2013 at 2:21 PM, Petr Pudlák petr@gmail.com wrote: Dne 09/05/2013 01:38 PM, Roman Cheplyaka napsal(a): * Petr Pudlák petr@gmail.com [2013-09-05 11:18:25+0200] Unfortunately |ParsecT| constructor isn't exported so I'm not able to implement it outside /parsec/. No, but there's an 'mkPT' function which is equivalent to the ParsecT constructor. (Although I, too, wish the ParsecT constructor were exposed.) Roman Yes, I tried to use `mkPT`, but the result looked very complicated and I wasn't quite sure if it'll be working correctly in all cases. Implementing the same thing with the `ParsecT` constructor is simple and comprehensible. Best regards, Petr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] a little parsec enhancement
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
1 patch for repository http://code.haskell.org/parsec3:
Thu Sep 5 11:12:47 CEST 2013 Petr Pudlak p...@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 p...@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@. 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@. The
-- source position of the @t@ should be returned by @posFromTok t@ and
}
Context:
[Fix haddock module links.
Bjorn Buckwalter bj...@buckwalter.se**20130821095713
Ignore-this: 304217ec8b73f59edcd96dd13aca67af
]
[TAG 3.1.3
Antoine Latter aslat...@gmail.com**20120612020909
Ignore-this: 7100375a3e4853c09f1ea993ae32eed7
]
Patch bundle hash:
173111b8bbc5a6eb69d41926053184c35ae9b3cc
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Re: [Haskell-cafe] a little parsec enhancement
* Petr Pudlák petr@gmail.com [2013-09-05 11:18:25+0200] Unfortunately |ParsecT| constructor isn't exported so I'm not able to implement it outside /parsec/. No, but there's an 'mkPT' function which is equivalent to the ParsecT constructor. (Although I, too, wish the ParsecT constructor were exposed.) Roman signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
