std.v2020.algorithm etc[ WAS: Is run.d going to be expand for runtime and the phobos library?]

[Stanislav Blinov]

On Saturday, 27 June 2020 at 18:55:39 UTC, Paul Backus wrote:
> On Saturday, 27 June 2020 at 17:30:54 UTC, Stanislav Blinov 
> wrote:
>>
>> Thereby result of (input.tail == input.tail) is a coin flip? I 
>> don't think that's a sensible design.
>
> Equality is not part of the range interface (current or 
> proposed), so there is no guarantee that comparing two ranges 
> with `==` will give you a useful result.

Equality is very much part of the struct interface though. I'll 
restate. What about input.tail.empty == input.tail.empty? Or 
input.tail.front == input.tail.front? Or, speaking of ranges, a 
input.tail.equal(input.tail)?..
I thought you were concerned about having to explain something to 
beginners ;)

If the explanation is "`tail() const` is impure, and calling it 
repeatedly with the same `this` produces different results, even 
when `this` is immutable", "WTF???" would be quite a reasonable 
follow-up question. This is not malloc. If we do define a `tail` 
primitive, it better yield the same output for the same input, 
meaning it would only be a primitive of forward ranges.