What is a "yield return" equivalent in the D programming language?

Question

Here is a simple generator in C#.

    IEnumerable<int> Foo()
    {
        int a = 1, b = 1;
        while(true)
        {
            yield return b;
            int temp = a + b;
            a = b;
            b = temp;
        }
    }

How do I write a similar generator in Digital Mars D?

(The question is about the yield return statement)

Thanks!

---

Update. That's interesting. Since I'm just generating a mathematical sequence, using recurrence may be a good option.

auto fib = recurrence!("a[n-1] + a[n-2]")(1, 1);

foreach (e; take(fib, 10)) // <- prints first ten numbers from the sequence
{ 
    writeln(e); 
}

Answer 1

There's no exact equivalent in D. Here are some rough equivalents:

Using opApply-style internal iteration. This doesn't allow iterating over two iterators in lockstep, though:

struct Foo {
    int opApply(int delegate(ref int) dg) {
        int a = 1, b = 1;
        int result;
        while(true) {
            result = dg(b);
            if(result) break;
            int temp = a + b;
            a = b;
            b = temp;
        }

        return result;
    }
}

void main() {
    // Show usage:
    Foo foo;
    foreach(elem; foo) {
        // Do stuff.
    }
}

Use ranges. These are slightly harder to write in some cases, but are very efficient and allow lockstep iteration. This can also be iterated over with a foreach loop, exactly like the opApply version:

struct Foo {
    int a = 1, b = 1;

    int front() @property {
        return b;
    }

    void popFront() {
        int temp = a + b;
        a = b;
        b = temp;
    }

    // This range is infinite, i.e. never empty.
    enum bool empty = false;

    typeof(this) save() @property { return this; }
}

If you really need coroutine-style stuff you can combine ranges and opApply together using core.thread.Fiber, but you'll probably find that either ranges or opApply does what you need almost all the time.