[Haskell-cafe] Re: Re: A parsec question
Antoine Latter wrote: > On Sun, Oct 3, 2010 at 11:55 AM, Ben Franksen > wrote: >> Stephen Tetley wrote: >>> Does this one give the "expected" error message for Parsec3.1 - >>> unfortunately I can't test as I'm still using Parsec 2.1.0.1. >>> parser = block (many digit "digit") >> >> Unfortunately, no. >> > Hey folks, sorry about this one - my changes to parsec in 3.1 made > these error messages worse. I've sent a patch off to the maintainer > which fixes the examples in this thread. Thanks! I hope we get a new minor release with these fixes soon. I love parsec-3 very much, especially since you fixed the speed problems. Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Re: A parsec question
Daniel Fischer wrote:
> On Wednesday 29 September 2010 19:10:22, Ben Franksen wrote:
>> >
>> > Note the last line mentions only '}'. I would rather like to see
>> >
>> > expecting "}" or digit
>> >
>> > since the parser could very well accept another digit here.
>
> parsec2 did that, I don't know whether that change is intentional or
> accidental.
This looks more like a bug than a feature to me. I checked parsec-3.0.1 and
it behaves like parsec-2, i.e. behaves as I expected.
>> > (1) What is the reason for this behaviour?
>> > (2) Is there another combinator that behaves as I would like?
>> > (3) Otherwise, how do I write one myself?
>>
>> I just saw that Christian Maeder answered a similar question recently. I
>>
>> tried his suggestion of using manyTill and bingo:
>> > {-# LANGUAGE NoMonomorphismRestriction #-}
>> > import Control.Applicative ((*>),(<*))
>> > import Text.Parsec
>> > block p = char '{' *> p <* char '}'
>> > parser = block (manyTill digit (char '}'))
>> > main = parseTest parser "{123a}"
> You would need
>
> block (manyTill digit (lookAhead (char '}'))
>
> to replicate the behaviour of block (many digit).
Right, so it gets even more complicated.
>> Is there a non-greedy variant of 'many' so
>> that modularity gets restored and efficiency is not lost?
So many questions...
Cheers
Ben
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