Re: [vim9script] On extending interfaces

I find Life's proposed syntax to be more how I would expect it to work where a child interface should be able to just add stuff without having to redeclare inherited methods. Also, if a concrete class has to implement both i1 and i2 explicitly, then i2 might as well not subclass i1.

Salman

On Wed, Jul 19, 2023, 11:53 Lifepillar <lifepillar@lifepillar.me> wrote:

Vim 9 script allows the following definitions, in which I2 extends I1 by
"implementing" it:

    interface I1
      def Foo()
    endinterface

    interface I2 implements I1
      def Foo()
      def Bar()
    endinterface

The above compiles: I don't know whether it is intentional, but it is
pretty cool! Or, it would be if the following code worked:

    def Echo(obj: I1)
      obj.Foo()
    enddef

    class C2 implements I2
      def Foo()
        echo 'C2'
      enddef

      def Bar()
      enddef
    endclass

    const c2 = C2.new()
    Echo(c2)  # ERROR

This results in:

    type mismatch, expected object<I1> but got object<C2>

But C2 does conform to I1! To fix the error, it is necessary to declare
all the interfaces implemented by C2, that is:

    class C2 implements I1, I2
      # etc.

I will mention two other minor issues:

- I2 must declare Foo() again: it would be nice if that definition could
  be inferred.
- "implements" is not a very accurate description: "extends" would make
  more sense, intuitively.

In summary, what I am asking is whether Vim could (or should) support
this syntax:

    interface I1
      def Foo()
    endinterface

    interface I2 extends I1
      def Bar()
    endinterface

with the following implications:

1. any class implementing I2 must implement both Foo() and Bar().
2. any object of a class implementing I2 may be used wherever an object
   with type I1 is expected.

Thanks,
Life.

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