[boost] Re: Review Request: FC++

From: David B. Held (dheld_at_[hidden])
Date: 2003-10-09 06:31:48

"David Abrahams" <dave_at_[hidden]> wrote in message
news:uvfqze84g.fsf_at_boost-consulting.com...
> "Joel de Guzman" <joel_at_[hidden]> writes:
> > [...]
> >     struct square
> >     {
> >         template <class X>
> >         struct sig { typedef X type; }
> >         X operator()(X x) { return x * x; }
> >     };

I don't understand this example.  Is operator() also supposed to
be templated?  If so, what connects sig<> with operator()?

> [...]
>     b. It should be called apply, because that just makes it
>        cheaper and easier to use: instead of
> [...]

But my understanding is that apply<> is the compile-time way
to spell "operator()" for function objects.  If sig<> is really
the definition of R for "R operator()", without being the result
of operator(), should it really be spelled "apply<>"?  It seems
to me that there is a confusion between runtime and compile
time here.  The fact that apply<> works syntactically adds to
the confusion.

In Joel's example above, square could be a metafunction
object as well as a function object.  But what kind of
metafunction is it?  It's the identity metafunction.  Clearly,
this is different from it's name and run-time purpose.  That's
because sig<> does not reflect it's intent, only its return
value to support lazy evaluation.

So in this case, I think calling the return type metafunction
"apply<>" is confusing, even though it makes it convenient
to use with MPL::lambda.  I think I agree with Joel that it
should be called result<>.  Even though it *could* be used
with lambda, how would you do so?  The result of apply<>
is not square, as one would hope, but X.  What are you
going to do with the return type by itself?

There definitely needs to be a convention.  And maybe
ReturnTypeMetafunction needs to be a formal concept
that encapsulates that convention and specifies a name.
But I don't think the name should be "apply<>".

Dave

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