• Java

record G<T>(Supplier<T> s) {
    T yield() { return s.get(); }
}
Random r = new Random();
G<?> g = new G<>(r::nextInt);
int x = g.yield();

• Java

class G<T> {
    T t;
    Function<T, T> f;
    G(T t, Function<T, T> f) {
        this.t = t;
        this.f = f;
    }
    T yield() {
        return t = f.apply(t);
    }
}

• C++

using namespace std;
template <typename T>
auto x(function<T(T)> f, T t = {}) {
    return [=] {
        T static x = t;
        return x = f(x);
    };
}
function f {[] (int x) { return x + 1; }};
function g {x(f)};

• Dart

List<int> g(int start) sync* {
  while (true) {
    yield start;
    start++;
  }
}

• Go

func generator() chan int {
	ch := make(chan int, 16)
	go func() {
		defer close(ch)
		timeout := time.After(2 * time.Minute)
		
		for i := 0; i < 1024; i++ {
			select {
			case ch <- i:
			case <-timeout:
				return
			}
		}
	}()
	return ch
}

• Perl

sub _upto (&) { return $_[0]; }  # prototype & is for a function param
sub upto {
    my ($start, $end) = @_;

    my $n = $start;

    return _upto {
        return
            wantarray ? $start .. $end    :
            $n > $end ? ($n = $start, ()) :
            $n++
            ;
        };
}
my $it = upto(3, 5);
foreach ( $it->() ) { print ' ' . $_ }

• Python

def g():
    for i in range(6):
        yield i

• Ruby

g = (1..6).each