At Tue, 28 Sep 2010 10:12:29 -0700, Steven Watanabe wrote:
AMDG
On 9/27/2010 11:28 PM, David Abrahams wrote:
At Mon, 27 Sep 2010 18:49:06 -0700, Steven Watanabe wrote:
...but I'm not even sure there is a problem yet, even though I was one of the first people to recognize that concepts clash with TMP as practiced today. The question is whether a different approach can accomplish the same ends. As far as I'm concerned, that's still an open question. Until someone finds a way, it might as well not be possible. That might be a little like saying, "until someone demonstrates that you can write the Sieve of Eratosthenes algorithm in BASIC, it might as well not be possible." At some level, all programming paradigms are equivalent.
You, yourself, just said that it was an open question.
Yes. That doesn't make it as good as impossible. There are things that are known/proven to be impossible, you know.
Would you describe writing the sieve of Eratosthenes in BASIC as an open question? FWIW, I'm sure that there /exists/ another way to do the things that we do now with TMP, which will work with concepts. I'm not at all convinced that it won't have more problems than current TMP.
Me neither; I think we just don't know. AFAIK nobody has tried very hard to look for concepts-compatible solutions to the same problems we currently use TMP for.
Thus, once you actually instantiate the template, there can be no errors whatsoever. We don't apply this level of type-checking to runtime code, because it's simply impractical to guarantee. I think you mean we don't apply this level of runtime checking to runtime code? No, I really do mean type checking. I don't see how that's consistent with your analogy. When I map back into the runtime world, why doesn't type checking become runtime (value) checking again?
This would be true without C++0x concepts. Half of concepts is type checking the bodies of templates when they are defined. These checks are equivalent to ordinary type checking in the runtime world.
If you say so. I still don't see how. [I think you mean "analogous," not "equivalent," right? They're certainly not substitutable for one another.]
The problem is that concepts force all checks into this realm.
I don't know what you're getting at, but it might be related to this: the fact that concepts do early checking is problematic for many of our current TMP idioms, which rely on having checks only at instantiation time. For example, foo<mpl::_1> couldn't be written if foo were a constrained template, unless mpl::_1 just happened to satisfy its requirements. Anyway, yeah, that's a clash. But maybe in a "concepts" world we wouldn't want to do that in the first place. -- Dave Abrahams BoostPro Computing http://www.boostpro.com