Program size, linking matter, and static this()

[Andrei Alexandrescu]

On 12/16/11 5:50 PM, Jonathan M Davis wrote:
> On Friday, December 16, 2011 17:13:49 Andrei Alexandrescu wrote:
>> Maybe there's an issue with the design. Maybe Singleton (the most damned
>> of all patterns) is not the best choice here. Or maybe the use of an
>> inheritance hierarchy with a grand total of 4 classes. Or maybe the
>> encapsulation could be rethought.
>>
>> The general point is, a design lives within a language. Any language is
>> going to disallow a few designs or make them unsuitable for particular
>> situation. This is, again, multiplied by the context: it's the standard
>> library.
>
> I don't know what's wrong with singletons.

http://en.wikipedia.org/wiki/Singleton_pattern

Second paragraph.

> It's a great pattern in certain
> circumstances. In this case, it avoids unnecessary allocations every time that
> you do something like Clock.currTime(). There's no reason to keep allocating
> new instances of LocalTime and wasting memory. The data in all of them would
> be identical. And since the time zone has to be dynamic, it requires either a
> class or function pointers (or delegates). And since multiple functions are
> involved per time zone, it's far cleaner to use class. It has the added
> benefit of giving you a nice place to do stuff like ask the time zone its name.
> So, I don't see what could be better than using classes for the time zones
> like it does now. And given the fact that it's completely unnecessary and
> wasteful to allocate multiple instances of UTC and LocalTime, it seems to me
> that the singleton pattern is exactly the correct solution for this problem.

You're using a stilted version of it. Most often the singleton object is 
created lazily upon the first access, whereas std.datetime creates the 
object (and therefore shotguns linkage with the garbage collector) even 
if never needed.

But what I'm trying here is to lift the level of discourse. The 
Singleton sounds like the solution of choice already presupposing that 
inheritance and polymorphism are good decisions. What I'm trying to say 
is that D should be rich enough to allow you considerable freedom in the 
design space, so we should have enough means to navigate around this one 
particular issue. I don't think we can say with a straight face we can't 
avoid use of static this inside std.datetime.

> There would be fewer potential issues with circular dependencies if
> std.datetime were broken up, but the consensus seems to be that we don't want
> to do that. Regardless, if I find a way to lazily load the singletons in spite
> of immutable and pure, then there won't be any more need for the static
> constructors for them. There's still one for the unit tests, but worse comes
> to worst, that functionality could be moved to a function which is called by
> the first unittest block.

Maybe the choice of immutable and pure is too restrictive. How about 
making the object returned const?

>> But what's the appropriate order then? :o)
>
> It doesn't matter. The static constructors in std.datetime has no dependencies
> on other modules at all aside from object and the core module which holds the
> declaration for tzset. In neither case does it depend on any other static
> constructors. In my experience, that's almost always the case. But because of
> how circular dependencies are treated, the compiler/runtime considers it a
> circular dependency as soon as two modules which import each other directly -
> or worse, indirectly - both have module constructors, regardless of whether
> there is anything even vaguely interdependent about those static constructors
> and what they initialize. So, you're forced to move stuff into other modules,
> and in some cases (such as when pure or immutable is being used), that may not
> work.

Under what circumstances it doesn't work, and how would adding _more_ 
support for _less_ safety would be better than a glorified cast that you 
can use _today_?

> Clearly, I'm not going to win any arguments on this, given that both you and
> Walter are definitely opposed, but I definitely think that the current situation
> with circular dependencies is one of D's major warts.

I'm not nailed to the floor. Any good arguments would definitely change 
my opinion.


Andrei