Re: [Boost-users] [Phoenix][v3] Argument/State dependent return type of actors?

Subject: Re: [Boost-users] [Phoenix][v3] Argument/State dependent return type of actors?
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2012-01-04 10:51:08

On 01/03/2012 09:17 AM, Josef Weinbub wrote:
> On 01/-10/-28163 08:59 PM, Josef Weinbub wrote:
>> Hi
>>
>> Recently I made myself familiar with Phoenix v3, more specifically
>> with actors.
>> I did a few test implementations, and I wondered if there is a
>> way to access the argument types (nested in the context's environment)
>> and/or the type of the state(s) (nested in the proto expression) for
>> the actor's result type declaration.
>>
> ....
>
> Hi again!
>
> Indeed I have overlooked something, being 'result_of' ... as provided by
> the Boost Utility library and adopted
> by the C++11 standard. Sorry for that, should have known ..
>
> In case someone else wonders how to approach my problem, checkout the
> Boost.Utility::result_of documentation:
> http://www.boost.org/doc/libs/1_48_0/libs/utility/utility.htm#result_of
>
> One has to nest a result struct with the functors parameter types in the
> Boost.Phoenix actor's evaluation functor.
> For the sake of completeness, in the following my previous evaluation
> functor is depicted again, but instead
> of the static "typedef int result_type; " we now use the nested result
> struct:
>
> struct myactor_eval {
>
> template<class> struct result;
>
> template<class F, typename StateExpr, typename Context>
> struct result<F(StateExpr, Context)>
> {
> // here we can relate the return type both to the state type
> // and to the functor's argument types ...
> typedef ..... type;
> };
>
> template <typename StateExpr, typename Context>
> typename myactor_eval::result< myactor_eval(StateExpr, Context) >::type
> operator()(StateExpr const& state_expr, Context & ctx) const {
> return ...;
> }
> };
>
>
> Best, Josef
>
> PS: Thank you, Joel, for noticing my post and forwarding it to Thomas -
> much appreciated!

Sorry I missed it, yes, the result_of protocol is used throughout the 
library. Additionally you can access the context with special meta 
functions. Looks like the documentation of those is currently incomplete ...
Have a look here: 
http://www.boost.org/doc/libs/1_48_0/libs/phoenix/doc/html/phoenix/inside/actions.html

At the bottom of this page is a list of functions and proto transforms 
to access the environment inside the context. For every function, there 
exists a boost::phoenix::result_of::*name* metafunction, where *name* 
stands for thing you want to extract.
Let's say you want to compute the type of the first passed parameter:

// Extract the Environment, which is a fusion vector ...
typedef
     typename boost::phoenix::result_of::env<Context>::type
     env_type;
// Or using result_of:
typedef
     typename boost::result_of<
         boost::phoenix::functional::env(Context)
     >::type
     env_type;

// ... and extract the first argument:
typedef
     typename boost::fusion::result_of::at_c<env_type, 1>::type
     arg_type;

Best regards,
Thomas