[boost] [geometry][metaparse] UDL-based compile-time DSL parser (was: Re: [offtopic] user-defined raw string literals)

Subject: [boost] [geometry][metaparse] UDL-based compile-time DSL parser (was: Re: [offtopic] user-defined raw string literals)
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-02-01 07:31:11

Hi Abel,

Abel Sinkovics wrote:
> On 2014-01-30 17:25, Adam Wulkiewicz wrote:
>> Ok so now the tricky part. It should be parsed in compile-time, the 
>> type of the geometry retrieved from the string literal and proper 
>> type returned (bg::polygon<> in this case).
>> Unfortunately string literals can't be handled as raw literals, e.g. 
>> using the variadic-template version of operator"":
>>
>> template <char...>
>> void operator "" ()
>>
>> Why? Is there a reason for this?
>> If we had the ability to use the compile-time version with string 
>> literals it would be possible to implement various compile-time 
>> parsers, not only for the mentioned WKT.
>>
>> So what do you think about it?
>
> I don't know the reason for the string literal limitation,

AFAIU it would be difficult to implement consistent behavior because 
string literals are concatenated by the preprocessor automatically.

> but I have a workaround for it. You can use the MPLLIBS_STRING macro 
> from Metaparse - not in Boost yet. (see 
> http://abel.web.elte.hu/mpllibs/metaparse/manual.html#the-input-of-the-parsers 
> and http://abel.web.elte.hu/mpllibs/metaparse/MPLLIBS_STRING.html). 
> Using that you can parse string literals at compile-time.
>
> Metaparse itself is a library supporting the creation of parsers for 
> DSLs that are parsed at compile time. You might be able to create an 
> interface like this using that:
>
> auto polygon = WKT("POLYGON((30 10,40 40,20 40,10 20,30 10))");
> auto point = WKT("POINT(30 10)");
> // ...
>
> where WKT is a macro expanding to a template metaprogram-based parser 
> invocation. If you are interested in trying it out, there is a 
> tutorial which helps you getting started: 
> https://github.com/sabel83/metaparse_tutorial#metaparse-tutorial.

Thanks for the info. However using macros isn't exactly the thing I 
wanted to do because it would be less readable:

auto polygon = BOOST_GEOMETRY_WKT("POLYGON((30 10,40 40,20 40,10 20,30 
10))");

vs

auto polygon = "POLYGON((30 10,40 40,20 40,10 20,30 10))"_wkt;

but I'll for sure check Metaparse out in a free time. I may let you know 
when it's done. Even if it wasn't released with Geometry you'd be able 
to put it as an example in the docs.

Regards,
Adam