Delegate returning itself

[via Digitalmars-d-learn]

On Monday, 8 December 2014 at 14:31:53 UTC, Jonathan Marler wrote:
> On Monday, 8 December 2014 at 14:08:33 UTC, Jonathan Marler 
> wrote:
>> On Saturday, 6 December 2014 at 15:46:16 UTC, Adam D. Ruppe 
>> wrote:
>>> The problem is the recursive *alias* rather than the 
>>> delegate. Just don't use the alias name inside itself so like
>>>
>>> alias MyDelegate = void delegate() delegate();
>>>
>>> will work. The first void delegate() is the return value of 
>>> the MyDelegate type.
>>
>> Yes I tried that as well.  It still doesn't solve the issue.  
>> The delegate being returned doesn't return a delegate, it 
>> returns the "void" type.  You would need to write delegate() 
>> delegate() delegate() delegate() ...FOREVER.  I can't figure 
>> out a way to write this in the language even though the 
>> machine code it generates should be quite trivial.
>
> I did some digging and realized that C/C++ have the same 
> problem.
>  I found a nice post on it with 2 potential solutions 
> (http://c-faq.com/decl/recurfuncp.html).  I liked the second 
> solution so I wrote up an example in D.  If anyone has any 
> other ideas or can think of a way to improve my example feel 
> free to post and let me know, thanks.import std.stdio;
>
> struct StateFunc
> {
>   StateFunc function() func;
> }
> StateFunc state1()
> {
>   writeln("state1");
>   return StateFunc(&state2);
> }
> StateFunc state2()
> {
>   writeln("state2");
>   return StateFunc(&state3);
> }
> StateFunc state3()
> {
>   writeln("state3");
>   return StateFunc(null);
> }
> void main(string[] args)
> {
>   StateFunc state = StateFunc(&state1);
>
>   while(state.func != null) {
>     state = state.func();
>   }
> }

Nice! Using alias this, you can call the struct directly:

struct StateFunc
{
   StateFunc function() func;
   alias func this;
}
state = state();

Now there still needs to be a way to just `return &state2;` 
instead of `return StateFunc(&state2);`...