Why must a bidirectional range also be a forward range?

[Paul Backus]

On Thursday, 19 September 2019 at 09:31:32 UTC, Joseph Rushton 
Wakeling wrote:
> For context, the use-case I have is a data structure which 
> stores an internal buffer as an array.  A robust `save` method 
> would therefore have to duplicate the array (or at least the 
> active subset of its contents).  This means a fresh heap 
> allocation per `save`, which has some nasty implications for 
> phobos algorithms that eagerly `.save` when they can.

In this case, it is probably better to separate the range from 
the data structure it refers to. For example:

struct Container {
     int[] data;

     private static struct Range {
         int[] contents;
         bool empty()    { return contents.length == 0; }
         int front()     { return contents[0]; }
         void popFront() { contents = contents[1 .. $]; }
         int back()      { return contents[$ - 1]; }
         void popBack()  { contents = contents[0 .. $ - 1]; }
         Range save()    { return this; }
     }

     Range getRange() {
         return Range(data[]);
     }
}