On 12/14/10 2:45 AM, Don wrote:
Walter Bright wrote:
bearophile wrote:
Andrei:
D's constrained templates were introduced on 17 June 2008 and
they've been
a smashing hit. Virtually all of Phobos uses them, and I'm sure a
lot of
generic client code. They take half a minute to explain and solve a
difficult problem in a very simple manner.<
A template with two constrains makes it less handy to add error
messages that
explain why the instantiation has failed:
Adding such error messages misses the point of constraints. The idea
is that template overloading is based on constraints. Given 3
templates with 3 constraints, overload resolution picks the one with
constraints that pass. Giving error messages for the others is wrong.
If you want it to produce error messages if the instantiation is
wrong, the correct way is to use static asserts within the template
body. No additional features serve any purpose.
Actually I think there's considerable potential for better error messages.
Currently, if constraints fail for all templates, the error message
prints the full constraint for the first template, which is not very
helpful.
A huge improvement would be to distinguish "no templates match at all",
(eg the number of parameters is different)
from "there are templates which would match, except that the template
constraints fail".
Yes, printing the first constraint is quite random. This should be
listed as a bug.
Andrei
