On 3/14/13 6:02 PM, H. S. Teoh wrote:
On Thu, Mar 14, 2013 at 05:54:05PM -0400, Andrei Alexandrescu wrote:
On 3/14/13 2:38 PM, Jonathan M Davis wrote:
On Thursday, March 14, 2013 13:26:18 Andrei Alexandrescu wrote:
Walter has measured coverage of Phobos unittests a couple of times,
it's very high. But I agree it would be nice to have it as a target.
Though this is exactly a case where 100% unit test coverage doesn't
mean much. All that means is that each path has been instantiated
and run. It doesn't mean that it's covered enough possible
instantiations to properly test the template.
Concepts won't help there either.
[...]
It does help. For example, if the code wrongly assumes mutability for a
particular template type, then it may work for most test cases (frankly,
I find const/immutable unittest coverage in Phobos very poor) but fail
when some daring user passes const(T) instead of T to the template. For
example, you may have accidentally written the equivalent of:
auto func(T)(T t) {
Unqual!T u = t; //<-- spot the bug
...
}
Under a concepts system, this would be caught early because the compiler
would detect a concept mismatch (Unqual!T != T) when analysing the
template code.
Currently, though, if there is no unittest that tries instantiating the
template with const(T), the bug goes undetected, because in all *tested*
instantiations, Unqual!T == T.
Template constraints start from the most permissive end of the spectrum:
by default there's no verification, and constraints add verification.
With typeclasses it's the other way around: by default nothing is
allowed, so code needs to add permissions explicitly.
I agree that in that regard typeclasses are better than template
constraints. Of course there are many other aspects to be considered
when comparing the two.
Andrei
