Listener type needs to be parameterized in service object type

[jclark]

In order to make #611 useable, the Listener abstract object type needs to become a parameterized type Listener<S>, where S is the service object type.

If some listener, e.g. FooListener defines an attach method whose first argument is a FooService, which defines some remote methods, then FooListener will not be a subtype of Listener as currently defined (because of parameter type contravariance). Instead it will be a subtype of Listener<FooService>.

[jclark]

This change needs to happen in:

• https://ballerina.io/spec/lang/draft/v2020-09-22/#section_5.7.4
• https://ballerina.io/spec/lang/draft/v2020-09-22/#section_8.3
• https://ballerina.io/spec/lang/draft/v2020-09-22/#section_8.3.1
• https://ballerina.io/spec/lang/draft/v2020-09-22/#section_8.3.2

[jclark]

In order to write the type for Listener in lang.object, we need a way to express that an object type has a type parameter, e.g. by extending @typeParam. We also need this for #434.

[jclark]

It's not going to work to use @typeParam to describe Listener in lang.object, because there is no way for the user to reference this type (e.g. using Listener<T>). I think the best we can do is to define the service argument to attach as never i.e. object:Listener is Listener<never>. This ensures that the abstract type Listener<T> is a subtype of object:Listener for any T. We can do better probably when we do #609 or obviously if we add generic types.

The reason we need Listener in lang.object is so that all Listeners can belong to the same distinct type. So another possibility, perhaps less confusing, is just to define object:Listener as distinct object {}. Another possibility would be to not make Listener distinct (is there any need for it to be distinct?) and not define it in lang.object.

[jclark]

I chatted with @hasithaa and our conclusion was that the easiest solution was to make Listener like Iterator i.e.

• not a distinct type
• no declaration in lang.object
• purely spec-internal