
Boost : 
Subject: Re: [boost] phoenix::bind
From: Peter Dimov (pdimov_at_[hidden])
Date: 20081002 20:25:57
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.
Incidentally, you might consider supporting a scopeless let (if it doesn't
already work):
lambda(_x, _y)[ let( _z = _x + _y ), _z * _z ]
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 ]
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 ]
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk