On Sunday, 28 June 2015 at 00:25:47 UTC, Jonathan M Davis wrote:
[snip]
it's an open question as to whether any kind of property
enforcement will ever be in
I should probably note that a long term consequence of not having
full property enforcement is that whether parens are used or not
in generic code is going to be a problem. For, instance,
template isInputRange(R)
{
enum bool isInputRange = is(typeof(
(inout int = 0)
{
R r = R.init; // can define a range object
if (r.empty) {} // can test for empty
r.popFront(); // can invoke popFront()
auto h = r.front; // can get the front of the range
}));
}
requires that empty and front be called without parens, which
means that front and empty can be enums or variables or anything
else which is going to fit the syntax in the code above and not
just functions. So, you can't _ever_ be calling front or empty
with parens in generic code if you want your code to work with
ranges in general rather than the small set of ranges that you
might be using at the moment. But the compiler doesn't enforce
that right now at all. It just enforces that the ranges
themselves be such that front and empty be usable without parens.
In addition, it requires that popFront be used _with_ parens, but
the only case where it would break code if you didn't would be if
someone had declared popFront as callable rather than a function,
since using it without parens as a function would work - but not
as callable. Now, realistically, pretty much no one is going to
do that, but technically, according to the API requirements, it's
perfectly legitimate to do so, and the compiler doesn't protect
against anyone from writing code which calls popFront without
parens.
Full property enforcement would fix these issues, but we're never
going to get it. So, we will always have to worry about generic
code which uses parens where it shouldn't or doesn't use them
where it should. How big an issue this will be is an open
question, but being lax with syntax (like we are with parens on
functions) doesn't jive well with generic code, so it's something
that we're going to have to watch out for.
- Jonathan M Davis
