ProtoObject and comparison for equality and ordering

[Andrei Alexandrescu]

On 5/14/19 9:37 PM, Mike Franklin wrote:
> On Tuesday, 14 May 2019 at 19:34:12 UTC, Andrei Alexandrescu wrote:
>> In designing ProtoObject and comparison for equality and ordering, 
>> we've assumed all class objects are supposed to be comparable 
>> (including ProtoObject themselves).
> 
> What's the rationale for that?

Pretty much principle of least surprise for existing D users.

> And are you talking about reference 
> equality or value equality?

Depends on how the user implements it. The intent is to support 
meaningful comparison of the content of objects.

> IMO, objects should only support reference equality out of the box.

So that means "x is y" works but not "x == y"?

>> This is in keeping with Java, C#, and existing D where Object has 
>> built-in means for comparison.
> 
> Classes in C# only support reference equality out of the box: 
> https://dotnetfiddle.net/AOQ0Ry

Affirmative. Same in Java, is that correct?

> I would like to distinguish between reference equality and value 
> equality.  Reference equality, I think, should be done with the `is` 
> operator, and should probably work out of the box. And I like the idea 
> of users opting into any feature.  If users want to support value 
> equality, they should implement `opEquals`.  If they want comparability 
> they should implement `opCmp`.  If they want to support hashability, the 
> should be required to implement a `getHash` or something along that line 
> of thinking.

Sounds good. If we go with the notion "you can't key a built-in 
hashtable on any class type" that should work.