Proto for Boost mailing list page: Re: [proto] My own lambda for MSM / wish list

Subject: Re: [proto] My own lambda for MSM / wish list
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2011-03-14 17:26:54

Hi Eric,

>First let me say that I have limited bandwidth at the moment, and apologize in advance for my short reply...

No need to apologize, I can understand this ;-)

>And what you need is for the type of the Proto expression generated by
>"(g1 || g2) && !g3" to be default-constructable? Have you tried doing
>this with an expression adaptor and a domain in which child nodes are
>all held by value (because reference members cannot be
>default-constructed)? What errors did you run into?

I'm sorry to tell you this, but I'm afraid you think I know much more
than I really do ;-)
Sounds great, but how would I do this?

>I don't see how that could ever work. Any way you slice it, (_1 < _2) is
>an expression, not a type. decltype(_1 < _2) could be made to work, for
>some suitably defined _1, _2, and <. And yes, (_1 < _2) would need to be
>default-constructable.

Of course I don't think it could. I can still define an ideal and see
how close I can get.

>>     std::transform(vec.begin(),vec.end(),vec.begin(),
>>             if_(_1)[ ++_1,return_(++_1)].else_ [return_(++_1)]  );

>This just looks like Phoenix. Why can't you use it directly?

It sure does, but I'm wondering if I'll be cheaper with phoenix than
with my very simple grammar, which I at least about understand ;-)

>> I have at least 2 more:
>>
>> - MPL. Great great library, but the template syntax is keeping too
>> many away from it.
>> I want what the FTMPL guys are doing (hi Matt, hi Zach).
>> I'd really prefer to write _1 + _2 instead of plus<_1,_2>

>You're talking about a complete C++0x redesign of MPL. Doable, but a big job.

I'm not in a hurry and probably not alone with this idea ;-)

>> - proto. Yes, proto itself. Eric talked about it in his BoostCon10
>> talk. Why not write
>> when_() || when_()
>> instead of proto::or< proto::when<>, proto::when<> >
>> Now this is really perverse. A proto grammar to generate... proto. I love it!

>I love it, too! Are you volunteering? ;-) A whole-hog redesign of proto
>for C++0x, taking full advantage of decltype to allow syntax like that,
>is one of those things I wish I had time for.

Why not? ;-)
Though my knowledge of proto is too limited to do it alone. I'd be
happy to help though. This is the second reason I wrote my own
mini-phoenix, learning by doing.

>Exciting stuff! Truly Christophe, your ideas re decltype and EDSLs in
>C++ are revolutionary. But unfortunately, I fear it will require a
>revolution. This is all do-able, but the changes to MPL, Proto and even
>to Phoenix in the case of the lambda capture stuff would require
>breaking API changes.

French have some experience with revolutions ;-)
Seriously I bet quite a few would love to do it too and it could be
great fun. Joel, Matt? *wink* :)

>As for MPL and Proto, someone needs to sit down and do some hard
>thinking about what meta-programming will look like in C++0x. I suspect
>it'll look less like today's MPL and Proto, and much more like what you
>envision. It's a huge opportunity for someone to do some really
>ground-breaking work.

After you write this, it won't be too hard to find candidates :)

Christophe