ProtoObject and comparison for equality and ordering

[Mike Franklin]

On Wednesday, 15 May 2019 at 00:32:32 UTC, Andrei Alexandrescu 
wrote:

>> Although it would be much more work, perhaps what is needed is 
>> a new type (e.g. `struct CmpResult`) with 4 immutable 
>> instances representing each result and an `opCmp` and 
>> `opEquals` implementation that does the right thing comparing 
>> against 0 or whatever else is needed.  Yes, it's more 
>> complicated, but I think it would scale better.
>
> Not sure there's much to gain there. a < b is lowered to 
> a.opCmp(b) < 0. So then... you define opCmp to return an 
> instance of this:
>
> ---
> import std.stdio;
>
> struct OverengineeredCmpResult {
>     enum R { lt, eq, gt, ionno }
>     private R payload;
>     int opCmp(int alwaysZero) {
>         writeln("b");
>         return 0;
>     }
> }
>
> struct A {
>     OverengineeredCmpResult opCmp(A rhs) {
>         writeln("a");
>         return 
> OverengineeredCmpResult(OverengineeredCmpResult.R.ionno);
>     }
> }
>
> void main() {
>     A a, b;
>     if (a < b) {}
> }
> ---
>
> Much ado about nothing.

Cool! It actually looks much simpler than I imagined.

Mike