[Haskell-cafe] Question about implementing an off-side rule in Parsec 2

[Bas van Gijzel]

Hey,

Thanks for the help thusfar. These are interesting suggestions, and I think
the occam-pi compiler would help a bit as example. I'll force myself to
learn some more about the state monad, but I haven't found really good
examples except in Real World Haskell until now so I hope I'll manage. I'll
keep you posted about my further progress.

Cheers,

Bas

On Tue, Apr 28, 2009 at 2:04 PM, Neil Brown <nccb2 at kent.ac.uk> wrote:

> Bas van Gijzel wrote:
>
>> Hello everyone,
>>
>> I'm doing a bachelor project focused on comparing parsers. One of the
>> parser libraries I'm using is Parsec (2) and I'm going to implement a very
>> small subset of haskell with it, with as most important feature the off-side
>> rule (indentation based parsing) used in function definitions and possibly
>> in where clauses.
>>
>> But I'm still a bit stuck on how to implement this cleanly. I tried to
>> search for some examples on blogs but I haven't found anything yet. As far
>> as I can see the way to go would be using getState and updateState methods
>> defined in Parsec.Prim and to use the methods in Parsec.Pos to compare the
>> difference in indendation for tokens.
>>
>> But I haven't completely wrapped my head around any state monad yet and I
>> don't understand Parsec enough yet to see how to use the methods Parsec.Pos
>> and state easily. Some examples or pointers to something to read would
>> really be helpful.
>>
> Hi,
>
> I work on a compiler for occam-pi, which has indentation-based syntax.
>  It's regular (two-spaces per indent) rather than
> different-number-of-spaces, and line continuations can only follow certain
> tokens, but perhaps our code might help you.
>
> We use alex for tokenising and parsec for parsing.  We tokenise the file,
> and then use the source positions to create indent/outdent tokens in the
> token stream, and after that the parser parses things like a PAR block as:
> do {reserved "PAR"; indent; many1 subItem; outdent}.  Our code can be found
> at:
>
> http://offog.org/darcs/tock/
>
> Look in the frontends subdirectory, particularly at StructureOccam.hs, but
> also LexOccam.x and ParseOccam.hs.  It may not be the most elegant way to do
> things (occam has all sorts of oddities that make parsing a pain), but it
> does work :-)
>
> Thanks,
>
> Neil.
>
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