foreach, an analogy

[Andy Knowles]

Karen Lanrap wrote:
> Sean Kelly wrote:
> 
>> The iterator would have to contain a stack of pointers to
>> previously visited nodes.
> 
> But that holds for "foreach" also.

No, it doesn't:

class Tree {
     Tree left;
     Tree right;
     int data;

     int opApply(int delegate(inout Tree n) dg) {
        int ret = dg(this);
        if(!ret && left != null)
          ret = left.opApply(dg);
        if(!ret && right != null)
          ret = right.opApply(dg);
        return ret;
     }
}

The call stack stores our stack implicitly.  This is the difference 
between for loops and foreach - it doesn't much change the way one 
writes a loop but it can drastically change the way one writes an iterator.