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Subject: Re: [boost] phoenix::bind
From: Joel de Guzman (joel_at_[hidden])
Date: 20081002 22:26:09
Peter Dimov wrote:
> Joel de Guzman:
> ...
>> This:
>>
>> lambda( _x, _y, _z = 123 )[ ... ]
>>
>> will be transformed to this:
>>
>> lambda( _x = _1, _y = _2, _z = 123 )[ ... ]
>>
>> I wonder though if it's a good idea to separate the signature
>> from the locals:
>>
>> lambda( _x, _y )[ let( _z = 123 )[ ... ] ]
>>
>> seems to be more "idiomatic"? No?
>
> Under my mental model of the "new lambda", these two are not the same.
> _z = 123 in the lambda initializes _z to 123 once when the lambda is
> defined; let _z = 123 declares _z to be 123 each time the lambda is
> called. That is, the outer _z is a member variable, the inner _z is a
> local temporary.
>
> So, if we return to my generator example that was to yield 1, 2, 3 on
> successive calls, it would be spelled
>
> lambda( _a = 0 )[ ++_a ]
>
> as I first expected.
Hrm. I'm not so sure about this one. I think I prefer
Steven's suggestion to follow C++ lambda in that _a = 0
mimicking default arguments.
> Incidentally, you might consider supporting a scopeless let (if it
> doesn't already work):
>
> lambda(_x, _y)[ let( _z = _x + _y ), _z * _z ]
Nice one!
> analogous to a C++ local variable:
>
> auto _z = _x + _y;
> return _z * _z;
>
> and maybe even dispense with the let at all:
>
> lambda(_x, _y)[ _z = _x + _y, _z * _z ]
Hmm. Might be tough.
> although this has the usual drawback with being ambiguous with an
> assignment to an outerscope _z:
>
> lambda( _x, _y, _z = 0 )[ _z = _x + _y, _z * _z ]
Ok, all good points. Let me start and I'll see what can be done.
I think all the ingredients are already present. It's just a
matter of having the right behavior.
Regards,
 Joel de Guzman http://www.boostpro.com http://spirit.sf.net
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