Tail-recursive factorial() in D.

factorial.d

import std.stdio;

ulong
factorial(int n)
{
  if(n < 2)
    return 1;
  else
    return n * factorial(n - 1);
}

int
main()
{
  int n = 10;
  ulong f = factorial(n);
	
  writefln("%d! = %d", n, f);
  return 0;
}