Discussion Thread: DIP 1042--ProtoObject--Community Review Round 1

Adam D Ruppe destructionator at gmail.com
Sat Jan 15 13:19:10 UTC 2022

On Saturday, 15 January 2022 at 09:49:43 UTC, Alexandru Ermicioi 
> That was an example ofc. Equals method could be overloaded with 
> const and immutable version in there too, or have separate 
> iface.

So just like today...

>> Note that opEquals like this already works on dmd master with 
>> the specific class and attributes.
> From what I'm aware, with couple of compiler hacks, while this 
> offers no hacks.

No, there's no compiler hacks, this is a direct consequence of 
the Liskov substitution principle allowing you to tighten 
constraints in specializations.

How do you think it works today?

> Anyway, if there is no possibility to make friends inheritance 
> and method attributes easily, then best is to just remove them 
> from Object and be done with it.

Not only is there a possibility, it *already works*.

The DIP authors just don't know very much about D's classes.

> P.S. Can't we enhance compiler and bake the attribute info into 
> interface itself, and then allow downcasting it to an interface 
> with specific subset of attribtues?
> I.e. say we have safe nothrow nogc equals interface. We then in 
> some random code could downcast safely to safe nogc equals 
> interface?

Have you tried this?

interface GcEqual(T) {
         bool opEquals(T rhs);

interface NoGcEqual(T) {
         bool opEquals(T rhs) @nogc;

class A : GcEqual!A, NoGcEqual!A {
         override bool opEquals(Object rhs) {
                 return this.opEquals(cast(A) rhs);
         override bool opEquals(A rhs) @nogc {
                 if(rhs is null) return false;
                 return true;

void main() {
         A a = new A;

         GcEqual!A g = a;
         NoGcEqual!A n = a;

Works today. Restricted functions can implicitly cast to less 
restricted functions.

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