On 9/15/18 12:04 PM, Neia Neutuladh wrote:
On Saturday, 15 September 2018 at 15:31:00 UTC, Steven Schveighoffer wrote:
The problem I had was that it wasn't clear to me which constraint was
failing. My bias brought me to "it must be autodecoding again!". But
objectively, I should have examined all the constraints to see what
was wrong. All C++ concepts seem to do (haven't used them) is help
identify easier which requirements are failing.
They also make it so your automated documentation can post a link to
something that describes the type in more cases. std.algorithm would
still be relatively horked, but a lot of functions could be declared as
yielding, for instance, ForwardRange!(ElementType!(TRange)).
True, we currently rely on convention there. But this really is simply
documentation at a different (admittedly more verified) level.
We can fix all these problems by simply identifying the constraint
clauses that fail. By color coding the error message identifying which
ones are true and which are false, we can pinpoint the error without
changing the language.
I wish. I had a look at std.algorithm.searching.canFind as the first
thing I thought to check. Its constraints are of the form:
bool canFind(Range)(Range haystack)
if (is(typeof(find!pred(haystack))))
The compiler can helpfully point out that the specific constraint that
failed was is(...), which does absolutely no good in trying to track
down the problem.
is(typeof(...)) constraints might be useless here, but we have started
to move away from such things in general (see for instance isInputRange
and friends).
But there could actually be a solution -- just recursively play out the
items at compile time (probably with the verbose switch) to see what
underlying cause there is.
Other than that, you can then write find(myrange) and see what comes up.
In my case even, the problem was hasSlicing, which itself is a
complicated template, and wouldn't have helped me diagnose the real
problem. A recursive display of what things failed would help, but even
if I could trigger a way to diagnose hasSlicing, instead of copying all
the constraints locally, it's still a much better situation.
I'm really thinking of exploring how this could play out, just toying
with the compiler to do this would give me experience in how the thing
works.
-Steve
