opCmp, [partial/total/pre]orders, custom floating point types etc.

[Andrei Alexandrescu via Digitalmars-d]

On 01/12/2016 03:01 PM, John Colvin wrote:
> On Tuesday, 12 January 2016 at 19:28:36 UTC, Andrei Alexandrescu wrote:
>> On 01/12/2016 02:13 PM, John Colvin wrote:
>>> a<=b and b<=a must also be false.
>>
>> Would the advice "Only use < and == for partially-ordered data" work?
>> -- Andrei
>
> If by that you mean "Only use <= or >= on data that defines a total
> ordering"* I guess it would work, but it has some pretty big downsides:
>
> 1) Annoying to use.
> 2) You have to use the opCmp return 0 (which normally means a[<>]=b &&
> b[<>]=a) to mean "not comparable".
> 3) Not enforceable. Because of 2 you'll always get true if you use >= or
> <= on any a pair that doesn't have a defined ordering.
> 4) inefficient (have to do both < and == separately which can be a lot
> more work than <=).
>
> *would be safer to say "types that define", but strictly speaking...

I'd be in favor of giving people the option to disable the use of <= and 
 >= for specific data. It's a simple and logical approach. -- Andrei