yigal chripun wrote:
Walter Bright Wrote:
Yigal Chripun wrote:
But that doesn't mean the idea itself isn't valid. Perhaps a
different language with different goals in mind can provide a
much simpler non convoluted implementation and semantics for the
same idea? You've shown in the past that you're willing to break
backward compatibility in the name of progress and experiment
with new ideas. You can make decisions that the C++ committee
will never approve.
Doesn't that mean that this is at least worth a shot?
I believe that D's template constraint feature fills the bill, it
does everything Concepts purported to do, and more, in a simple and
easily explained manner, except check the template body against the
constraint.
The latter is, in my not-so-humble opinion, a desirable feature but
its desirability is overwhelmed by the payment in complexity and
constrictions on the Concepts necessary to make it work.
could you please expand on what are the main issues with implementing
that check?
Because the constraint can have any computation in it. So how does one
verify that all computations in the template body are represented in the
constraint in the same way?
I also wonder what's the situation regarding this in
other languages - C# has constraints IIRC - is the check performed
there?
I don't know C# constraints.
IMO cconstraints are neat but they aren't perfect. For one, they need
to be repeated for each template, it would be very awesome if that
could be moved to the parameter of the template so instead of:
template foo(T) if isRange!T ... template bar(T) if isRange!T ...
you could write something like: struct Range if ... {} template foo(r
: Range) ... template bar(r : Range) ...
inside both templates the parameter r satisfies all the constraits of
Range.
does that sound reasonable at all?
The template parameter list was already fairly complicated, I thought
that adding in the constraints would make it impenetrable. Also, the
parameter list involves the individual parameters, whereas the
constraint involves any combination of them.
Adding in the constraints to the parameters also has unknown
consequences for figuring out partial ordering. Partial ordering is
based on abstract types, whereas constraints are based on actual types.
