Re: [boost] [Proto][Phoenix] Use default transform with SFINAE for expressions?

Subject: Re: [boost] [Proto][Phoenix] Use default transform with SFINAE for expressions?
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2010-09-12 04:18:51

Mathias Gaunard wrote:

> I would quite like it if polymorphic function objects created by Phoenix
> could restrict overload resolution to types for which their function
> body is valid, using SFINAE for expressions.
> 
> Basically, this would allow something like this:
> 
> struct foo {};
> 
> void dummy_function(...)
> {
>      std::cout << "[not streamable]";
> }
> 
> make_overload(
>      std::cout << arg1,
>      dummy_function
> )
> (foo());
> 
> (with make_overload what you'd expect)
> 
> The dummy function would be called if the first one doesn't match,
> instead of resulting in a hard error.
> (surely it would also be nice to add a module to create "..." functions
> in Phoenix, but that should be easy)
> 
> Now I looked a bit at how Phoenix v3 and Proto work, and it seems the
> main problem to do that is in the default transform.
> 
> Albeit it uses decltype for its return type, this is not done in a
> template deduction context, which prevents it from doing its SFINAE magic.
> I tried to do that, but then a transform is assumed everywhere to be
> monomorphic, so SFINAE doesn't "propagate" out of default_TAG.
> 
> Before I go ahead and try to convert all result_type to result with
> templates, which could affect compile-time, I'd like to have some
> feedback on the whole idea, and whether this is the right way to do it.

Phoenix3 uses result_of protocol, and result_type wherever the return type 
is monomorphic.

My take would be to use boost::result_of<Expr(T1, T2, ...)>::type before 
calling the phoenix expression and use SFINAE here.