[boost] [fusion] Re: more help with unfused_typed (previously off-list)

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-07-17 08:37:14

Hello Christian,

Christian Holmquist wrote:
> Hello,
> I decided to mail you directly not to spam the boost dev-list with 
> minor questions, hope it's ok.

Sure, but I think we're not spamming ;-). To the contrary, it's valuable 
to discuss this stuff on-list (forwarding to boost.devel -- hoping it's 
OK), because ...

> My question is how to disable the call operators with fewer arguments 
> than specified by Sequence, generated by unfused_typed<F, Sequence>. 
> Before the switch to boost::result_of one could specialize the result 
> struct to only handle the expected Sequence type, thus disabling 
> unwanted operators. I'm not familiar with boost::result_of to describe 
> this however.

... I removed that kind of control over the overload sets of the Fusion 
functional adapters (you can't pass emptiness through result_of), but 
I'm not really happy about it.

<snip code>
> Any ideas?
Currently I think it might be best to re-add that mechanism for the 
'unfused_*' adapters, either by checking nested 'result' of the target 
function for emptiness (exploiting SFINAE where applicable) or by 
introducing a special type (e.g. 'fusion::undefined_function'), that 
when returned from 'result_of' disables the overload...

Thoughts anyone?

Regards,
Tobias