Forward ranges in Phobos v2

[Andrei Alexandrescu]

On 2021-11-02 20:38, Paul Backus wrote:
> On Wednesday, 3 November 2021 at 00:24:11 UTC, H. S. Teoh wrote:
>> On Wed, Nov 03, 2021 at 12:18:59AM +0000, Paul Backus via 
>> Digitalmars-d wrote:
>>>
>>> Having input ranges implement `next` and forward ranges implement 
>>> `head` and `tail` would also make them easy to distinguish.
>>
>> That would work too, but makes the input range API no longer a subset 
>> of the forward range API.  This would lead to code duplication in 
>> algorithms that only require an input range but could work equally 
>> well with a forward range.
> 
> Not necessarily. It's possible to implement `next` as a UFCS function 
> for mutable forward ranges using the `head`/`tail` API:
> 
> auto next(R)(ref R r)
>      if (isForwardRangeV2!R && isMutable!R)
> {
>      alias E = ElementType!R;
>      if (r.empty)
>          return none!E();
>      else
>      {
>          auto result = some(r.head);
>          r = r.tail;
>          return result;
>      }
> }

OK, so the signature of next for all ranges is:

Option!(ElementType!R) next(Range)(ref Range);

Is that correct?