[dmd-internals] Fw: Fixing forward ref bugs for good

[Don Clugston]

> ----- Forwarded Message -----
> From: Steve Schveighoffer <schveiguy at yahoo.com>
> What I mean is, it feels like the wrong approach to ordering.  I'd rather
> ordering be dependent on ordering, not nesting.  It seems like a
> counter-intuitive way to do ordering that has the same issues using the
> order of declaration.
>
> People are going to look at some file that has static if(true) (or several
> levels of them!) and scratch their heads, potentially removing it.
> Are we trying to say here, ordering matters only for static ifs?

The problem isn't static if, or even ordering, per se. It's the
combination of (a) reflection, and (b) the fact that declarations can
be conditionally added via static if and mixin; the two concepts are
fundamentally incompatible.
Any reflection that checks for existence of a symbol has a result
which can change with time.

So, although the spec says that order of declarations doesn't matter, it isn't:

enum b1 = is(typeof(foo));
static if (!b2)  int foo;
enum b2 = is(typeof(foo));

Is b1 true, or false? Currently, it's false, but becomes true if moved
to the bottom of the file.

You could say that any use of a non-existent symbol potentially
depends on all non-evaluated static ifs; you don't know it doesn't
exist until you've expanded them ALL.
So one idea would be to evaluate every static if that had a condition
that didn't involve reflection. If you find a condition which is
blocked, you stop, and go onto the next one. Moderately difficult to
implement, but possible.
But often you get multiple static ifs which are blocked. They are
probably independent, but you don't know. What do you do? Well, you
could arbitrarily say you do them in the order the static ifs appear
in the file.
But it's not very predictable, because you can't just look at a static
if statement and see if it is blocked or not -- the reflection could
be happening anywhere in the file. I don't think that's viable.

>  Then why
> can't the ordering just be the order they appear?  Is there any advantage to
> use nesting?

The nesting behaviour is a natural consequence of saying that the
order doesn't matter. It's not a design decision. It does give an
advantage over order-in-file in the case of things like struct members
(where order in the file DOES matter) because you can control the
order the static ifs are evaluated (you can make the first static if
in the struct only be evaluated after all the others have finished).

>  What about some sort of pragma or @annotation to define
> ordering?

That's possible, too. (That's what I meant by syntax sugar).

> And what about repeatedly trying evaluating a static if until it can be
> evaluated, or you are stuck?  Has that been considered and rejected for some
> reason?

See above. It's not feasible.

> Again, not understanding all the issues, so maybe it's just the only way
> possible.  But it sure looks bad.

Fundamentally we're doing something which is contradictory (but so
useful in practice that we desperately want to keep it!) I think a
perfect solution is impossible.

I think we have four feasible solutions:
(1)
static ifs and mixins are expanded in the order they appear in the
file. This is applied recursively until none are left.
Finally, everything else is evaluated in parallel.
(2)
static ifs and mixins are expanded in parallel. This is repeated until
none are left.
Finally, everything else is evaluated in parallel.
(3)
 Everything is evaluated in parallel, except for static ifs and mixins.
 If no static ifs or mixins, quit.
 static ifs and mixins are evaluated & expanded in the order they
appear in the file. Repeat.
(4)
 Everything is evaluated in parallel, including static ifs and mixins
(but they aren't expanded).
 If no static ifs or mixins, quit.
static ifs and mixins are expanded. Repeat.

All of these have surprising behaviour in some ways.
-Don.

> From: Don Clugston <dclugston at googlemail.com>
> On 16 September 2011 13:25, Steve Schveighoffer <schveiguy at yahoo.com> wrote:
>> Not really understanding all the issues here, but this looks horrible :(
>> I hope this isn't the solution.
>> -Steve
>
> Are you reacting to the concept, or only to the syntax?
> Bear in mind that it occurs ONLY when you have dependent static ifs
> within a single scope.
> For example, if you have a 'static if' inside a template, struct, or
> function, it has access to ALL symbols in that module.
> In my opinion, this normally (not always, but normally) indicates a
> poor design. It can almost always be factored out (eg, by creating a
> bool enum).  Worst case, we could add some syntax sugar.
> I think that if you really want to have semantics which are
> independent of the ordering of declarations, then by definition you
> can't have dependent static ifs in the same scope.
> The second option is to allow ordering to be significant only for
> mixins and static if. And in that case, the ordering has to be strict:
> even if something near the end of the file is 'static if(1)', the
> declarations in it are not available anywhere else in the scope.
> I think any other option (like evaluating every static if that doesn't
> use is(typeof()) or other forms of reflection, before those that do)
> leads to madness.
>
>
>> From: Don Clugston <dclugston at googlemail.com>
>> On 15 September 2011 17:56, Andrei Alexandrescu <andrei at erdani.com> wrote:
>>> On 9/15/11 6:53 AM, Don Clugston wrote:
>>>>
>>>> On 15 September 2011 13:14,<mrmocool at gmx.de>  wrote:
>>>>>
>>>>> Am 15.09.2011, 11:44 Uhr, schrieb Don
>>>>> Clugston<dclugston at googlemail.com>:
>>>>>>
>>>>>> I think there is a simple solution to 'static if'.
>>>>>> Do const folding on every static if condition, giving a boolean
>>>>>> result.
>>>>>> Do NOT evaluate any static if bodies yet.
>>>>>> This would have the same effect as if every 'static if' were evaluated
>>>>>> simultaneously.
>>>>>
>>>>> What if the condition includes symbols from another static if's body or
>>>>> mixin or whatever?
>>>>
>>>> I think that should be disallowed.
>>>
>>> I see an issue here with cross-module use. For example, it's nice to
>>> have:
>>>
>>> import some.module;
>>>
>>> static if (is(typeof(some.module.foobar) == int)) {
>>>   alias some.module.foobar baz;
>>> } else {
>>>   enum baz = 42; // or whatever
>>> }
>>>
>>> So far so good. The problem now is that some.module uses a similar
>>> technique
>>> to introduce that symbol foobar, the code won't work anymore.
>>>
>>> I also realized that code relying on enumerating symbols in a module
>>> (like
>>> benchmark does) or a class (like an introspection library does) will miss
>>> all symbols guarded by static if. And, for example, ranges define plenty
>>> of
>>> those. This erodes the power of static if substantially.
>>
>> Not so. The thing is, static ifs can be nested. Only one level of
>> static if is removed at a time.
>> If you simply wrap the static if inside static if(true) {...}
>> it won't be evaluated until all the first-level static ifs have added
>> their symbols to the scope.
>>
>> So the current:
>> static if (cond1)
>> {
>>     A;
>> }
>> static if (cond2)
>> {
>>   B;
>> }
>> where cond2 depends on A, can be rewritten in the 'parallel execution'
>> paradigm as:
>> static if (cond1)
>> {
>>     A;
>> }
>> static if (true)
>> {
>>   static if (cond2)
>>   {
>>     B;
>>   }
>> }
>> Order of execution is controlled by depth of nesting, instead of by
>> order in the file.
>> Note that in your first example, the question of which module
>> instantiates the symbol is determined not even by order within the
>> file, but by which module is first on the command line -- ie, it's
>> determined by the makefile!
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