How do I find the arity of an Expression? (dmd hacking)

[Ary Borenszweig]

Ellery Newcomer wrote:
> On 11/30/2009 12:32 PM, Chad J wrote:
>> Ellery Newcomer wrote:
>>> On 11/30/2009 03:53 AM, Ary Borenszweig wrote:
>>>> Chad J wrote:
>>>>> Given an Expression object in dmd, I'd like to know how many
>>>>> subexpressions it contains and, even better, iterate over them. I'd
>>>>> like to do this in a general way, without having to create cases 
>>>>> for all
>>>>> of the different kinds of Expressions. Is there some way to do this
>>>>> that I've been missing?
>>>>>
>>>>> Thanks,
>>>>> - Chad
>>>>
>>>> Where's the Expression object defined in D? I don't think there's 
>>>> such a
>>>> thing (I mean, a reflection capability to get an expression out of
>>>> something).
>>>
>>> DMD source, eg parse.c, expression.c, etc
>>
>> Right.  That.
> 
> Not that I know anything about DMD outside parse.c, but in expression.h, 
> there be decls along the lines of
> 
> struct UnaExp{
>   Expression* e1;
> }
> struct BinExp{
>   Expression* e1;
>   Expression* e2;
> }
> 
> Iterating them would just be a tree walk. That takes care of 90% and ...
> 
> Oh. Yeah. There are a bunch of special cases.
> 
> But from what I've seen, iterating over an expression or any part of the 
> AST involves implementing a function (like semantic) at each subclass. 
> I'm betting there is no other way to do this.
> 
> I love ANTLR. It generates an arbitrary child/sibling AST, and walking 
> it is a piece of cake.

It would be better if dmd's source code implemented the visitor pattern. 
  Descent's port implements it and it uses it a a lot of places.