A lightweight module for class extensions in D

[Gregor Richards]

Robert Fraser wrote:
> Gregor Richards wrote:
>> I ran into a situation where I needed (essentially) the visitor 
>> pattern, but the visitor pattern sucks, so I wanted to make something 
>> like class extensions instead (that is, methods added to a class 
>> outside of the class definition).
>>
>> Of course, it's not possible to do this particularly cleanly, but I 
>> made a system that works (albeit using gross string mixins). 
>> Essentially, if you have a class A, class B : A, class C : B, you 
>> could do something like this:
>>
>> mixin(extensions("A", "void", "doFoo", "", ""));
>>
>> mixin(extend("A", "doFoo"));
>> void A_doFoo(A pthis) {
>>     /* method for A */
>> }
>>
>> mixin(extend("B", "doFoo"));
>> void B_doFoo(B pthis) {
>>     /* method for B */
>> }
>>
>> Then if you call doFoo(new A()) the call will become A_doFoo(new A()), 
>> if you call doFoo(new B()) the call will become B_doFoo(new B()), if 
>> you call doFoo(new C()) the call will become B_doFoo(new C()).
>>
>> If anybody has some improvements, that'd be cool. Maybe you can get 
>> rid of the dependence on string mixins ... but I don't think templates 
>> quite cut it.
>>
>>  - Gregor Richards
>>
> 
> Pretty cool stuff, but I don't see how this is at all better than the 
> visitor pattern. It is not checked at compile-time (so if you forget to 
> implement one part of the hierarchy, you won't find that out until 
> runtime) and it's likely less efficient, especially for large enough 
> hierarchies (i.e. syntax trees). Where's the happy?

The visitor pattern requires annotating every class in the hierarchy. 
That's what annoys me about it. I'd like my AST nodes to just be AST nodes.

Yes, calling a function is less efficient. It can probably be improved, 
I'm just not sure how yet :P

Plus, of course, although the visitor pattern is the example I gave, 
this is a more general mechanism.

  - Gregor Richards