ProtoObject and comparison for equality and ordering

[Mike Franklin]

On Wednesday, 15 May 2019 at 16:36:27 UTC, H. S. Teoh wrote:

> FYI, even with the above amusingly elaborate hack, you still 
> cannot achieve proper 4-way comparison results.  Consider: what 
> should OverengineeredCmpResult.opCmp return for payload == 
> ionno, such that <, <=, >, >=, ==, != would all produce the 
> correct result?
>
> Answer: it's not possible unless you return float, because x < 
> y translates to x.opCmp(y) < 0, and x > y translates to 
> x.opCmp(y) > 0, so the only way to represent an incomparable 
> state is for opCmp to return some value z for which z < 0 and z
> > 0 are *both* false.  There is no integer value that fits this
> description; the only candidate is float.nan. Substituting the 
> return value of opCmp with a custom struct doesn't fix this 
> problem; it only defers it to the custom struct's opCmp, which 
> suffers from the same problem.

I see.  I'm a little embarrassed I didn't realize that but happy 
you took the time to explain it.  Thank you.

Mike