Whats the proper way to write a Range next function

[JG]

On Wednesday, 15 June 2022 at 17:30:31 UTC, JG wrote:
> On Wednesday, 15 June 2022 at 13:52:24 UTC, Christian Köstlin 
> wrote:
>> the naive version would look like
>>
>> ```d
>> auto next(Range)(Range r) {
>>     r.popFront;
>>     return r.front;
>> }
>> ```
>>
>> But looking at a mature library e.g. 
>> https://github.com/submada/btl/blob/9cc599fd8495215d346ccd62d6e9f1f7ac140937/source/btl/vector/package.d#L229 is looks like there should be tons of annotations/attributes on it.
>>
>> Kind regards,
>> Christian
>
> I not sure of your use case. But you need to check if the range 
> is empty before and after calling popFront (and decide what to 
> if it is). Also unless you want the original range passed in 
> mutated you should call save (assuming you have a forward 
> range) before calling your next or you need to modify next so 
> it calls save.

This is what I would write for next (which should only be called 
after
checking if the range is empty).

It produces a range whose front points at the next element.
```d
auto next(Range)(Range r) {
   auto ret = r.save;
   ret.popFront;
   return ret;
}
```