[swift-evolution] Universal Equatability, Hashability, and Comparability

[Joe Groff]

> On Mar 9, 2016, at 11:41 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
>> or a desired one. 
> 
> That's true.  But what's important is that it's consistent with == and
> allows lookup in a sorted container.  The core ordering operation should
> not be spelled "<" though; we should use "<=>" for that so that types
> can keep their domain-specific "<" if necessary.  Too bad the same trick
> doesn't work for "==" :-).
> 
>> Many people have complained that 'nil < .Some(1)' works for optionals,
>> for instance, ordering 'nil' below Some values. We could use pointer
>> identity to order class instances and types, but this wouldn't be a
>> stable ordering across process runs. 
> 
> Oh, that's what you mean by "stable."  So what?

What if we separated "ordering for a collection" from the < == > family altogether? Even Floats can be given a true equivalence relationship and assigned a stable total ordering for the purposes of container identity. I think it's reasonable for types to be able to provide an abstract ordering without making '<' and friends casually work. Programmers may have had "don't rely on hash order" drilled into their heads over the decades, and "don't rely on container order" might be a reasonable abstraction step from that, but "don't rely on < behavior" strikes me as unintuitive and going against the intuition users build up from common concrete manifestations of '<', such as numeric types.

-Joe
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