Re: [boost] phoenix::bind

Subject: Re: [boost] phoenix::bind
From: Joel de Guzman (joel_at_[hidden])
Date: 2008-10-02 11:48:26

Corrado Zoccolo wrote:
> On Thu, Oct 2, 2008 at 3:41 PM, Joel de Guzman <joel_at_[hidden]>wrote:
> 
>> Giovanni Piero Deretta wrote:
>>
>>  As I said elsewere, the extra round is hidden when lambda is used to
>>> protect a nested expression, but it shows when used a top level:
>>>
>>> lambda[ for_each(arg1, lambda(_a = arg2)[push_back(arg1, _a)])] () (r, i);
>>>
>>> There is really really no need for the extra '()' there. There are no
>>> local variables in the outer lambda and even if there were, the user
>>> can initialize them with lambda(/* init here*/)[...]
>>>
>> Ok, this I agree.
>>
> 
> Hmm,
> I think this could complicate the understanding.
> It seems Giovanni's complaint is because he sees lambda[] as an introducer
> for a lambda expression, but this is in contradiction with the fact that
> expressions within the lambda itself could be eagerly evaluated if not
> properly wrapped (and introduction of optional lazy functions could even
> worse the problem).
> 
> I fear an user could think that
>  lambda[ std::cout<<"Hello world" ] is actually constructing a delayed
> invocation (as in C++0x), while it is not, and it cannot be.
> 
> On the other hand, if lambda[] is reserved as intended by Giovanni, what
> would be the syntax for creating a lazy function that returns a lazy
> function that returns a lazy function? We could handle differently the
> topmost lambda and the inner ones, but guessing the number of lambdas
> necessary to achieve what you want might be difficult.
> 
> For the other requests, e.g. having lazy functions  that capture by
> reference or const reference, I think a cast like syntax could be handy:
>  capture<by_ref>(arg1+1)
> 
> In this way, the way the arguments are captured can be changed not only at
> definition site, but also at call site.

Very good points. I somehow feel the same but couldn't articulate
my thoughts well enough. Let's see Giovanni's answer to this one.
I hope other folks would as well chime in on this crucial matter.

Cheers,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net