Re: [boost] boost::proto, improving error handling

From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-26 12:57:40

Maurizio Vitale wrote:
>    - now in reality my number has 6 template arguments. 6 is larger than 5, and
>      with this I've given out what happened, at least to Eric. For others, 5 is the 
>      default maximum number of arguments in proto expressions.
> 
> So far, so good. The problem is that my grammar had a pattern
> terminal<number<_,_,_,_,_,_> >.  I don't know if it is possible to
> make proto complain hard and loud about building a pattern (or other expressions)
> with more arguments than the maximum allowed by configuration, but in my case
> the pattern was not matching causing my expressions to be invalid.

I don't think there is anything I can do. To detect a template with a 
too-high arity, I'd need specializations with at least that arity. And 
if I have an arity-6 specialization, why not make it do the right thing 
instead of reporting an error? There needs to be a cut-off somewhere, 
beyond which things will break mysteriously. I don't know if template 
varargs will improve this situation, but I hope so. (I'd need a 
template-template vararg.)

> And I think that 6 is a better default value for the number of allowed arguments.
> Just kidding, as long as defining it before including proto headers is officially
> blessed and documented, I'm ok.

It is. #define BOOST_PROTO_MAX_ARITY 6 should do the trick.

> Another nice thing to have, although I would only put it in if very easy to code:
> especially in presence of default parameters it might be nice to have a pattern matching
> "the rest of the argument list". Something like a proto::___.

Well, double-underscores are off-limits to user code, according to the 
standard. But maybe you'd like something like:

   std::basic_string< vararg<_> >

to be the equivalent of

   std::basic_string< _, _, _ >

It doesn't seem like a big win to me, and it wouldn't solve the template 
arity problem you ran into.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com