Iterators and Ranges: Comparing C++ to D to Rust

[Dukc]

On Tuesday, 15 June 2021 at 18:35:04 UTC, Walter Bright wrote:
> On 6/14/2021 7:29 AM, Steven Schveighoffer wrote:
>> I wonder if there is room for hobbled iterators (cursors) to 
>> be in D in some capacity.
>
> Have them be an index, rather than a pointer.

This is a good generic solution for random access ranges. But 
iterators are about any forward range.

For a forward range, I think something like this does it albeit 
verbosely:

```
auto midAndTail = haystack.find(/*...*/);
auto head = haystack.recurrence!((r,n)=>r[n-1].dropOne).until!"a 
is b"(midAndTail).map!"a.front";
```

But there is currently no way for a bidirectional range to 
iterate forward and then turn back. We could agree on a generic 
range primitive, say `turned`, that returns a range that iterates 
back to start that is optional. But then the bidirectional range 
that supports it will always be bigger because it needs to 
remember it's start. Hard question.