On Thu, Mar 17, 2011 at 1:33 PM, Eric Niebler <eric@boostpro.com> wrote:
On 3/17/2011 6:17 PM, Joel de Guzman wrote:
On 3/17/11 5:24 PM, Daniel James wrote:
On 17 March 2011 08:52, Thomas Heller<thom.heller@googlemail.com> wrote:
There are still some unresolved issues. Mainly how to deal with the migration of Boost.Bind, Boost.Lambda and Phoenix V2. There hasn't been any discussion regarding that.
I don't think it'd be a good idea to do anything to bind or lambda in this release. Was there a consensus on replacing them? Personally, I'd like to keep the existing Boost.Bind, even if in a different namespace. I appreciate its relative simplicity and portability.
Bind should not be touched. People rely on Bind esp. those using old compilers. Also, Bind is extremely lightweight with almost zero dependencies. Phoenix cannot get anywhere near that.
Joel is right. The best we can do is make phoenix::bind a drop-in replacement and let people choose.
OTOH, it would be very nice if Bind's placeholders were the same as Phoenix's. So it would be worth taking the time to see if we could refactor Proto such that terminals could be defined in a very light-weight way. But I also know that Bind jumps through hoops of fire with the placeholder definitions to support ancient compilers, so this might not be possible.
Right now, the placeholders defined in Boost.Bind can be used with just one little minor inconvience* inside Phoenix. *The problem is that Boost.Bind placeholders are not wrapped inside any proto terminal stuff or phoenix actor. This means that we can not except something like ::_1 + ::_2 to be seen as a phoenix expression. BUT: ::_1 + phoenix::placeholders::_2 or phoenix::bind(::_1, ::_2) or phoenix::bind(::_1, phoenix::placeholders::_2) will work just fine! The same is true for the placeholders defined in BLL ... though i haven't checked if they specialize boost::is_placeholder.
However, if it /were/ possible (and we navigate the tricky proto-domain waters), then I suggestion the placeholders be spun into their own mini-library that can be shared across all the EDSLs in Boost that require placeholders. (Except MPL, where the placeholders are types and not objects.)
Oh ... proto-domain waters *shiver* .... we need to investigate that topic very well ... This topic still needs very deep research .... Consider the above: ::_1 + phoenix::placeholders::_2 If ::_1 (magically, somehow) belongs to something like bind_domain and phoenix::placeholders::_2 to phoenix_domain. It is not clear in which domain the resulting expression will be, and not clear how to handle the resulting expression. Three cases: 1) phoenix_domain is the super domain of bind_domain: Ok, no problem here, the resulting expression will be in phoenix_domain and we can evaluate without problems in phoenix. However, what is the scenario for other libraries that feel the need to reuse ::_1 ... 2) The common domain of phoenix_domain and bind_domain is proto::default_domain: The resulting expression is in neither domain and we have lost the ability to evaluate the resulting expression in the bind_domain sense or in the phoenix_domain sense. 3) The common domain of phoenix_domain and bind_domain is proto::not_a_domain: The expression will be not valid. Problem solved, somehow ;)