[dmd-internals] Fixing forward ref bugs for good

[Steve Schveighoffer]

Not really understanding all the issues here, but this looks horrible :(

I hope this isn't the solution.

-Steve




>________________________________
>From: Don Clugston <dclugston at googlemail.com>
>To: Discuss the internals of DMD <dmd-internals at puremagic.com>
>Sent: Friday, September 16, 2011 2:04 AM
>Subject: Re: [dmd-internals] Fixing forward ref bugs for good
>
>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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