[dmd-internals] Fixing forward ref bugs for good

[Don Clugston]

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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