Allow default methods/instance variables in interfaces

[dead-claudia]

This would be epic to have. Java itself had a problem with forcing the implementing class to implement the abstract methods, and solved it with default interface implementations. Sometimes, multiple inheritance is necessary to have. For example, I have a sea of crisscrossing is-a relationships to handle, with a ton of things like the following:

class Identifier extends Node implements Assignable, ImportPatternEntry,
        IdentifierLike, PropertyLike, Subtypable {
    // stuff...
}

class MemberExpression extends Node implements Assignable,
        IdentifierLike, PropertyLike, Subtypable {
    // stuff...
}

It's not pretty, but I also have to check many of these relationships at runtime as well. I can't use traditional single inheritance for this, either, because none of these are mutually exclusive. I would love to have ways to set default properties on these classes, to where I don't have to make a build script to generate all this. (I have 30+ base classes and 20+ interfaces with very intertwined relationships like the above.)

What I was thinking of was something like this:

interface IFoo {
  bar = true;
}

class Foo {}

class Bar implements IFoo {}

new Foo().bar; // undefined
new Bar().bar; // true

[danquirk]

Definitely feels like part of a mixin story.

[dead-claudia]

I'm going to close this for now, since I'm looking at the fact it's not as nice as the original proposal on the other thread.

[izogfif]

Now that mixins were added in the language, could someone provide an example of the interface with default method via mixin? How to write

interface Foo {
  getValue: number;
}
interface Bar extends Foo {
  getValue(): number { // Default implementation, may be overridden in children
    return 42;
  }
}