On 01/03/2017 03:10 AM, Joakim wrote:
On Monday, 2 January 2017 at 21:23:19 UTC, Andrei Alexandrescu wrote:
Regarding the ongoing doubts about the advantages of inline imports:
they are first and foremost a completion of the nested import feature.
As such, most, if not all, arguments against inline imports apply
equally to nested imports. Come to think of it, lazy imports vs nested
imports:
* same improvement in compilation speed? check
* no language changes? check
* no nasty bugs in the aftermath (such as the infamous
https://issues.dlang.org/show_bug.cgi?id=10378)? check
* scalable builds? check
Yet local imports are overwhelmingly superior to lazy imports because
of one thing: they localize dependencies. They introduce modularity
and its ancillary perks (fast and scalable builds, easier review and
refactoring) not by engineering, but by organically placing
dependencies spatially with their dependents. (The scope statement
does the same thing with temporal dependencies.) That the DIP does not
make it clear that it is a necessary and sufficient extension of local
imports is a problem with it.
I now am really glad we slipped local imports before the formalization
of DIPs. The feature could have been easily demeaned out of existence.
Except that almost nobody has argued against local imports.
I don't have time to research this, but my recollection is that at least
some framed the bugs regarding lookups as a fundamental problem of local
imports, not a simple matter of getting it right.
Overall, yes, local imports have been a success (really saving
scalability of large project builds which looked pretty bleak at a
time), which should increase trust in the authors of the feature...
hmmm... :o)
Rather, the
argument is that local imports mostly solved this problem, so why bother
adding new syntax for the dozen remaining symbols from 2-3 modules that
are commonly used in template constraints?
I understand. It is my humble opinion that we are "mostly pregnant" for
as long as we require top-level imports. The real change of phase occurs
when there are ZERO imports at top level. That's the prize DIP1005 is after.
Arguing that local imports have been successful so we should simply do
more of it is not a good argument, as there comes a point of diminishing
returns. You need to show that there are still worthile gains to be
made from changing the language again, which is why I want to benchmark
this feature with Walter before deciding.
You can be reasonably certain the benchmarks will improve as projected -
starting in proportion to the the transitive fanout of top-level imports
(10.5x for the standard library) and going down thereafter with the
size, complexity, and actual dependencies of the module being compiled.
All in all measurable but not dramatic. The same improvement will be
brought about by lazy imports, and it won't be the deal maker/breaker.
If you're waiting for the numbers to get convinced of anything, you
already consider DIP1005 useless.
Allow me to make an appeal regarding the review of any DIP. There
seems to be a tendency of some reviewers to get attached and
emotionally invested to their opinion, to the extent they'd be hurt by
being "wrong" and would go to great lengths to argue they're "right".
This has obvious negative effects on the entire process. Please don't.
There's no loss of face to worry about. The only commitment we all
should have is to the good of the D language. If DIP1005 reaches the
conclusion of its own uselessness, I'd be the first one to write it up
and close the PR.
We could level this analysis back at you: you consider this DIP so
"obvious" that you are not engaging with our concerns, making flip,
incorrect remarks about how we would have bikeshedded local imports also.
I apologize it my remarks seem flippant, though I honestly have
difficulty finding evidence of that. All I did was note that many of the
arguments pitching lazy imports as a better solution than DIP1005
(except of course for the Amdahl one) apply directly to local imports.
In the end, this is a minor DIP that is easily bikeshedded, as everybody
can grasp it and have an opinion on it. I have refrained from
commenting recently because I will let benchmarking settle it for me.
Obviously, that won't suffice for others.
I do agree that if framed as a modest improvement in build economics, it
is quite unimportant. But that's a problem with the DIP; its main
strength is better encapsulation, which is no small thing.
Andrei
