[Haskell-cafe] Parsec lookahead and |
Goedemiddag café, Consider the following function, using parsec-3.0.0: la :: Parsec String () (Maybe Char) la = lookAhead (optionMaybe anyChar) *Lookahead parseTest (char 'a' | char 'b') a 'a' *Lookahead parseTest (char 'a' | char 'b') b 'b' *Lookahead parseTest (la * char 'a' | char 'b') a 'a' *Lookahead parseTest (la * char 'a' | char 'b') b parse error at (line 1, column 2): unexpected b expecting a The first three work fine and as expected, but the fourth example fails where I would expect success. I know | won't try the rhs if the lhs consumed input, but lookAhead's documentation promises not to consume any input. Is this a bug in Parsec or am I missing something? Thanks, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec lookahead and |
Yeah, that's weird. I played around with la and it seems to only cause problems when the parser passed into lookAhead succeeds, which seem to go directly against it's stated purpose. lookAhead isn't consuming, (hence the unexpected b) but still prevents | from doing it's thing. Seems like a bug to me... My off the hip fix is a modified form of the ugly try: lookAhead (ParsecT p) = ParsecT $ \s@(State _ pos _) - do res - p s case res of Consumed rep - do r - rep case r of Error err - return $ Empty $ return $ Error (setErrorPos pos err) Ok a state err - return $ Empty $ return $ Ok a s err empty- return $ empty The only potential annoyance with this fix that I can see, is that the error messages can be confusing if you are doing dumb things with your lookAhead parsers. For example: la :: Parsec String () (Char) la = lookAhead' (char 'r') *Main parseTest ((la char 'a') | char 'b') a parse error at (line 1, column 1): unexpected a expecting r or b *Main parseTest ((la char 'a') | char 'b') r parse error at (line 1, column 2): unexpected r expecting a or b But for the most part it behaves as expected. - Job (sorry for the double post Martijn, forgot to reply to all) On Thu, Aug 20, 2009 at 7:44 AM, Martijn van Steenbergen mart...@van.steenbergen.nl wrote: Goedemiddag café, Consider the following function, using parsec-3.0.0: la :: Parsec String () (Maybe Char) la = lookAhead (optionMaybe anyChar) *Lookahead parseTest (char 'a' | char 'b') a 'a' *Lookahead parseTest (char 'a' | char 'b') b 'b' *Lookahead parseTest (la * char 'a' | char 'b') a 'a' *Lookahead parseTest (la * char 'a' | char 'b') b parse error at (line 1, column 2): unexpected b expecting a The first three work fine and as expected, but the fourth example fails where I would expect success. I know | won't try the rhs if the lhs consumed input, but lookAhead's documentation promises not to consume any input. Is this a bug in Parsec or am I missing something? Thanks, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec lookahead and |
Am Donnerstag 20 August 2009 13:44:15 schrieb Martijn van Steenbergen: Goedemiddag café, Consider the following function, using parsec-3.0.0: la :: Parsec String () (Maybe Char) la = lookAhead (optionMaybe anyChar) *Lookahead parseTest (char 'a' | char 'b') a 'a' *Lookahead parseTest (char 'a' | char 'b') b 'b' *Lookahead parseTest (la * char 'a' | char 'b') a 'a' *Lookahead parseTest (la * char 'a' | char 'b') b parse error at (line 1, column 2): unexpected b expecting a The first three work fine and as expected, but the fourth example fails where I would expect success. I know | won't try the rhs if the lhs consumed input, but lookAhead's documentation promises not to consume any input. Is this a bug in Parsec or am I missing something? Bad bug in Parsec (from the beginning, the same happens in parsec-2), I'd say. Desugared, we have lookAhead p = getParserState = \st - p = \r - setParserState st = \_ - return r Due to the (=), whenever p consumes input, lookAhead will return (Consumed _) and there's no way to get rid of it, so (la * char 'a') returns Consumed (Error something) on the input b and (|) doesn't try char 'b'. The code for lookAhead should look something like (parsec-2, to avoid 'returns' cluttering the code): lookAhead p = Parser $ \st - case parserReply $ runP p st of Ok x s err - Empty (Ok x st err) Error err - Empty (Error err) Since exporting an 'unconsume' function wouldn't be desirable, lookAhead would have to move to Text.Parse(rCombinators.Parse)c.Prim. (not necessary in parsec-3 yet, since that exports all top level definitions from all modules so far). Thanks, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
