Forward ranges in Phobos v2

[Andrei Alexandrescu]

On 2021-11-02 19:07, Paul Backus wrote:
> On Tuesday, 2 November 2021 at 21:58:20 UTC, Andrei Alexandrescu wrote:
>> On 2021-11-02 17:44, Dukc wrote:
>>>
>>>> What's a value range?
>>>
>>> Opposite of a reference range - copying implies `save()`.
>>
>> Yah, one simple improvement we could make is to assume all forward 
>> ranges copy their iteration state when copying the range. Then input 
>> ranges do NOT do that, i.e. all copies of an input range refer to the 
>> same stream and iterate it together (advancing one advances all).
>>
>> The differentiation can be made with a nested enum tag:
>>
>> struct MyInputRange {
>>     enum inputRangeTag = true;
>>     ...
>> }
>>
>> Client code can inspect R.inputRangeTag to figure whether the range is 
>> input (if present) or forward (if missing).
> 
> Not sure this is the best idea--it means new-style algorithms will 
> silently treat old-style input ranges as though they were forward 
> ranges, which could lead to incorrect behavior at runtime. If we are 
> going to make incompatible changes to the range API, we should do it in 
> such a way that version mismatches are caught at compile time.

Good point. Maybe have all ranges define that enum with values true and 
false respectively?