Delegate returning itself

[Jonathan Marler via Digitalmars-d-learn]

On Monday, 8 December 2014 at 14:38:37 UTC, Marc Schütz wrote:
> 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);`...

Nice addition! I can't think of a way to solve the implicit 
conversion from function pointer to struct, but not a big deal.  
I'm mostly glad I found a way to do this with no overhead and no 
awkward casting.  Adding the implicit conversion would be icing 
on the cake.