How Nested Functions Work, part 1

[Don]

Michiel Helvensteijn wrote:
> Rainer Deyke wrote:
> 
>>> I still find it silly that it was built that way. Seems to me you should
>>> be able to forward-reference *any* symbol that has a value that can't
>>> change over its lifetime. Functions, const/immutable vars, typedefs,
>>> classes, etc.
>> That can lead to subtle problems in the case of functions:
>>
>> int i = f();
>> int f() { return i; }
> 
> You're right, I forgot about that. You can conservatively statically forbid
> this. But I admit it makes the 'constant data should obviously be
> forward-referenceable' thing less convincing. :-)
> 
I can't see how it can be done in the general case, other than by 
constructing a dependency tree of top-level symbols.
Pass over the tree, evaluating everything which is a leaf of the tree.
If you have a pass where you don't succeed in removing any leaf nodes, 
you have a circular dependency.
DMD sort of does this, I think, but it only does two passes, and it's 
not very good at backing out when it discovers something is not a leaf.
It'd be hard to make that efficient when there are potentially infinite 
passes.