Forward ranges in Phobos v2

[Paul Backus]

On Tuesday, 2 November 2021 at 00:05:48 UTC, Alexandru Ermicioi 
wrote:
> On Monday, 1 November 2021 at 23:46:07 UTC, H. S. Teoh wrote:
>> Good question, ask Andrei. ;-)
>
> Well, I hope he will check this thread and comment on it.
>
>> Presumably, if we standardize on structs/classes, it could be 
>> as simple as:
>>
>> 	auto myFunc(R)(R range) if (is(R == struct)) {
>> 		... // forward range
>> 	}
>>
>> 	auto myFunc(R)(R range) if (is(R == class)) {
>> 		... // input range
>> 	}
>>
>> But given that Andrei thinks it's a mistake for ranges to be 
>> implemented as classes, I've no idea.
>
> That would work, if you have templated funcs, but what if you 
> need it in an interface?
>
> If class based ranges are to be in D language, I doubt it will 
> be possible to avoid .save function completely. At least for 
> range interfaces, the save of forward range will have to be 
> expressed through a method, such as .save.

You can always wrap a class/interface method in a struct that 
calls .save on copy:


struct ClassRangeWrapper(T)
     if (is(T == class) || is(T == interface))
{
     T payload;
     alias payload this;

     this(ref inout typeof(this) other) inout
     {
         this.payload = other.payload.save;
     }
}

This way, range algorithms don't need to know about .save, so it 
can be removed from the official forward range requirements even 
though it still exists as an implementation detail.