Allowing arbitrary types for a function's argument and return type

[pineapple via Digitalmars-d-learn]

I'm just starting to hammer D's very pleasant syntax into my 
head. After "Hello world", the first thing I do when learning any 
language is to write a simple program which generates and outputs 
the Collatz sequence for an arbitrary number. (I also like to 
golf it.) This is what I wrote in D:

import std.stdio;void main(){int i;readf(" 
%d",&i);while(i>1){writeln(i=i%2?i*3+1:i/2);}}

Any ways I could shorten it further?

Anyway, then I thought I should try something that was less of a 
mess, too, and wrote this:

import std.concurrency;
Generator!int sequence(int i){
     return new Generator!int({
         yield(i);
         while(i > 1){
             yield(i = (i % 2) ? (i * 3 + 1) : (i >> 1));
         }
     });
}

Which can be used like so:

import std.stdio;
void main(){
     foreach(i; sequence(11)){
         writeln(i);
     }
}

And now I'd like to make one more improvement, but this I haven't 
been able to figure out. What if I wanted the argument and output 
types to be longs instead of ints? Or some other, arbitrary 
discrete numeric type? Is there any template-like syntax I can 
use here instead of just copypasting for each numeric type I can 
think of? I've been spoiled by the likes of Python to be thinking 
in this duck-typing way.

Thanks!