Re: [Boost-users] [proto] problem with templated recursive transform

Subject: Re: [Boost-users] [proto] problem with templated recursive transform
From: Eric Niebler (eric_at_[hidden])
Date: 2009-02-07 13:06:03

Daniel Oberhoff wrote:
> Eric Niebler wrote:
>> I've experimented with a variadic Proto. It's not without its problems.
<snip>
> ah, ok, bummer. A friend of mine recently mused if it wasn't time for a 
> new language. not like python, more like D. Not that I particularly like 
> D (well, I don't really know it either), just that it seems people have 
> taken c++ to it's limits, and c++0x can't really help totally, because 
> it is an incremental change (even though auto will be great I think, as 
> will variadic function templates and move semantics). I opposed him at 
> the time, but seeing how much black magic goes into ETs (or DSELs in 
> general), he might be right after all.

If programming language evolution stopped at C++, I would be shocked and 
disappointed. That said, I don't think C++ has been taken to its limits. 
I'm curious to see what the next generation of C++ programmers will do 
with new features like concepts, variadics and rvalue references.

If you're sweating the black magic that goes into building a DSELs, then 
I suggest you don't think so hard about how the sausage is made. ;-)

>> I've filed bugs against msvc for its mishandling of nested function
>> types. In the mean time, please continue using callable transforms if
>> you like them. You can always wrap them in proto::call<> (or, for object
>> transforms, proto::make<>) to keep msvc happy, if that's a compiler you
>> care about.
> 
> Ok, that isn't the worst of solutions, if I end up needing msvc (which 
> may actually be the case soon, as we may be required to write plugins 
> for windows programs with this stuff...).

You can get pretty far with preprocessor hacks to hide the msvc 
workarounds. Here's something to get your gears turning:

#if BOOST_WORKAROUND(BOOST_MSVC, BOOST_TESTED_AT(1500))
#define _value(x)     call<proto::_value(x)>
#endif

Now, if proto::_value(proto::_left) shows up in your grammar, it gets 
turned into proto::call<proto::_value(proto::_left)>. I use this trick a 
lot.

> Thanks again for taking the time and giving something like proto to the 
> community.

My pleasure. I'm glad to see folks starting to use it.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com