Private members of interfaces

[tjpalmer]

Are private members supposed to be allowed in interfaces? If so, we should make them work. If not, we should provide clear error messages.

Examples of current unclear behavior:

$ interface A {
    private b: Int = 1;
    public c(): Int { b }
  }
3: public c(): Int { b }
                     ⇧
[interactive#0:3+20-21]@G: Wrong number of arguments.  Expected 2
interactive#0: A__0
$ interface A {
    private b(): Int { 1 }
    public c(): Int { b() }
  }
interactive#1: A__0
$ class D extends A {
    public d(): Int { b() }
    public e(): Int { c() }
  }
2: public d(): Int { b() }
                     ┗━┛
[interactive#2:2+20-23]@G: Expected subtype of Int, but got Invalid
2: public d(): Int { b() }
                     ⇧
[interactive#2:2+20-21]@G: Expected function type, but got Invalid
2: public d(): Int { b() }
                     ⇧
[interactive#2:2+20-21]@G: No declaration for b
interactive#2: D__0
$ class D extends A {
    public e(): Int { c() }
  }
interactive#3: D__0
$ new D().e()
interactive#4: fail
$ interface A { public c(): Int { 1 } }
interactive#5: A__0
$ class D extends A { public e(): Int { c() } }
interactive#6: D__0
$ new D().e()
interactive#7: 1

[ben-509]

Prior art notes: Java allows private methods in interfaces to factor code out of default methods. C# allows them, and protected methods. I don’t think Rust has them, but you chatgpt thinks you can hide the details using a private module,

I would not be against disallowing them entirely as complicated default methods seem like a code smell to me, and some backend implementations may be less than ideal. It wouldn't break anything if we change our minds in the future.

[mikesamuel]

My inclination is private members are useful, but probably need to be turned into non-members on most backends.

[mikesamuel]

Because private members are not overridable, and need not be visible outside the scope of the class body, you can pull them out into non-exported top level functions and then any privileged access they perform can be done via the equivalent of JVM access$123 methods.