Re: [boost] [bind][phoenix] unified placeholders, yea or nay?

Subject: Re: [boost] [bind][phoenix] unified placeholders, yea or nay?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-05-28 22:56:00

AMDG

On 05/27/2012 04:22 PM, Eric Niebler wrote:
> I'm considering taking on as a side project the unification of the bind
> and phoenix placeholders, a perennial source of confusion and annoyance.
> I hesitate before beginning because it necessarily introduces some
> complexity into boost.bind, a very small, simple, and light-weight
> library. In particular, the unification would:
> 
> 1) Cause boost.bind to depend on boost.proto. (I would do what I could
> to keep that dependency as slight as possible.)
> 2) Could not be supported on all platforms; e.g. not on borland or (gcc
> < 4) where the placeholders are actually static inline functions(!).
> (Peter, is this an ODR thing?)
> 3) Would introduce Phoenix behaviors into Bind, insofar as _1 is a
> lambda such that _1(42) evaluates to 42.
> 
> At this point, it's not obvious to me that the benefits outweigh the
> costs. Opinions? Peter, I'd especially like to hear your thoughts.
> 

I think this is hard to do without breaking user code.
Consider:

using ::_1;
boost::function<int(int)> f1;
bind(f1, _1); // boost::bind
...
using boost::phoenix::placeholders::_1;
bind(std::plus<int>(), _1, _1); // boost::phoenix::bind

Unifying ::_1 and boost::phoenix::placeholders::_1
has to break one of these unless boost::bind
is also the same as boost::phoenix::bind.

In Christ,
Steven Watanabe