Re: [boost] phoenix::bind

Subject: Re: [boost] phoenix::bind
From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-10-02 20:14:14

Steven Watanabe:
> Joel de Guzman wrote:
>> Yep. Phoenix can do that. A local variable may hide an outer local
>> variable. Here, we just reuse the locals for arguments to the
>> lambda as well. So, in current terms, this:
>>
>>     lambda( _x, _y )[ _x + _y ]
>>
>> is just this:
>>
>>     lambda( _x = _1, _y = _2 )[ _x + _y ]
>
> Am I misunderstanding something?  I thought that this would give
> lambda( _x = _1, _y = _2 )[ _x + _y ] (1, 2)() == 3
> instead of
> lambda( _x = _1, _y = _2 )[ _x + _y ] ()(1, 2) == 3

Under the model I have in mind:

    lambda( _x, _y )[ _x + _y ] ( 1, 2 ) == 3

That is, lambda is not lazy, and it doesn't compose further.

    lambda( _x )[ _x ] + _1

doesn't compile; there's no operator+ taking a lambda.

> I don't think it is quite the same if you use an arbitrary expression
> instead of _x + _y.  For instance, what should this mean:
>
> lambda(_x, _y) [ _x + _y + _1 ]

If we're allowed to not think about backward compatibility, I'd say that _1 
would no longer exist. Not even at top level.