On Monday, 23 April 2018 at 07:49:39 UTC, Simen Kjærås wrote:
On Monday, 23 April 2018 at 04:58:38 UTC, Alex wrote:
On Monday, 23 April 2018 at 00:26:23 UTC, Simen Kjærås wrote:
There is a limited set of lowerings, and they are defined in
the language, not in user code. They include operator
overloading (where `a op b` is translated to
`a.opBinary!op(b)`), foreach over ranges (where `foreach(e;
range) { }` becomes `for (auto e = range.front;
!range.empty(); range.popFront()) { }`), string switching
(which is forwarded to a template in druntime), and more that
I can't recall right now. This is compiler magic meant to
make implementating new features and reasoning about existing
features, easier. They are briefly described in the article
you linked, but I agree it offers limited insight. I've not
found any other great sources of information about it, sadly.
This is not true, in my opinion.
As an example, Walter gives the rewrite of a while-loop and a
foreach-loop into a for-loop, stating that by the ability to
do this, the for-loop is more basic than both of the former.
So, in my mind, every action of rewriting of something into
something else is "lowering". And if you can do this (of
course, maintaining the full semantic equivalence), available
actions have to remain the same.
That's not the definition of lowering used elsewhere, and so
will lead to confusion and misunderstanding. I would strongly
suggest you rethink your definition of lowering.
There is no official definition. That's because some natural
rewrite rules are implied, which are very general, I assume...
Said this, I'm not against the new getOverloads. On the
contrary, I find the feature cool. But if it is present, it
has to yield the same results for foo1 and foo2, just like the
pragmas.
In the general case, this is impossible. Even just limiting
ourselves to simple usages of static if it gets unwieldy. This
template is from Phobos (all unnecessary code removed). It has
224 different possible combinations of features:
private struct _Cache(R, bool bidir)
{
static if (bidir) {}
else {}
static if (isInfinite!R) {}
else {}
static if (hasLength!R) {}
version(assert) {}
static if (isForwardRange!R) {}
static if (hasSlicing!R) {
static if (hasEndSlicing) {}
static if (!isInfinite!R) {}
else static if (hasEndSlicing) {}
}
}
And that's before we even instantiate any templates that this
template references - any templates used inside _Cache could
increase the number of combinations.
But wait, there's more! How many does this have?
struct Bar(string s) {
mixin(s);
}
If that's just one overload because of the impossibility of
generating the options, what if we introduce a single static if
inside it? Is that still one, or is it two? Do we count the
possibilities introduced by a template mixin, but not by a
string mixin? What if the template mixin comes from a template
argument?
Sorry if I come off as very dismissive right now - I kind of
like the idea, but it seems impossible in practice.
I'm not sure about the procedure... should I post this as a
question to the GitHub, so it can be discussed? pull 2351 or
8195?
What you are suggesting would be a fundamental change in the
language, and should be discussed in the digitalmars.D forum.
Either PR is not the right place to discuss it. If there's any
interest, you will have to write a DIP for how the change will
work.
--
Simen
My point, is that if it is impossible to catch all cases of
template rewriting (which I'm advocating from the beginning)
getOverloads should not be extended to templates, as this would
be a fundamental change in the language.
