BidirectionalRange switching direction

[John Colvin via Digitalmars-d-learn]

On Wednesday, 23 September 2015 at 02:10:22 UTC, Tofu Ninja wrote:
> Trying to implement a bi directional range and it is slightly 
> unclear what the semantics are supposed to be and just wanted 
> some clarification.
>
> Are bidirectional ranges supposed to be able to support 
> switching direction mid iteration? Like if I do popFront 
> popFront popBack should that be equal to just a single 
> popFront? Or once you start on a direction should switching be 
> considered an error? Or is popFront and popBack supposed to 
> consume from both ends of the range and the range is empty when 
> they meet?
>
> -tofu

The last one. E.g. for arrays (except narrow strings... ugh)

auto popFront(T)(ref T[] a)
{
     a = a[1 .. $];
}

auto popBack(T)(ref T[] a)
{
     a = a[0 .. $-1];
}