[boost] Re: Re: FC++ Formal review is now in progress (2/13-2/23)

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2004-02-19 12:06:15

"Brian McNamara" <lorgon_at_[hidden]> escribió en el mensaje
news:20040219023655.GA20333_at_lennon.cc.gatech.edu...
> On Wed, Feb 18, 2004 at 08:04:05PM -0500, Brian McNamara wrote:
> > It occurs to me today that there is some middle ground.  Specifically,
> > when you aren't using partial application (that is, "currying" in FC++,
> > or "bind" in boost::lambda), I think that most of the reference issues
> > go away.  Perhaps there's a way to implement things so that functoids
> > can have reference parameters, but trying to curry reference parameters
> > is a compile-time error.  I'll have to think about that more deeply.
>
> Oh yeah, the other "hard problem" dealing with reference parameters is
> the "forwarding problem" (as described in
>    http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm
> ).  Somehow I always forget about this issue.  It's the real killer.
>
> [A good description of the problems snipped]
>
> In general, without a context-sensitive type system (like Haskell),
> I think there is no good solution to the problem of multiple parameter
> modes (value and reference) in the context of higher-order functions.
> That is, you cannot write "app" so that both calls here:
>
>    int f(int);
>    int g(int&);
>    int x;
>    ...
>    app(_,3)(f);
>    app(_,x)(g);
>
> work properly.  It's impossible.
>

Though the problems you're addressing are real I think the solution is not
to ban reference paramaters altoghether along the entire functoid taxonomy
but to prescribe the following rule:

first-order functoids can take parameters by value or by reference.
high-order functoids should take parameteres only by value (but ref() can be
used by the caller to denote a reference)

Assuming app follows the rule above:

   app(_,3)(f);
   app(_,x)(g);  // compile-time error, cannot bound a reference to a
temporary.
   app(_,ref(x))(g);  // ok

Fernando Cacciola
SciSoft