Proposal: allow to specify Union type for the module-self

[tk0miya]

It would be very helpful to use Union type for the module-self.

In my project, I have a Commentable concern module used across multiple ActiveRecord models. This Commentable module uses methods commonly defined in the modules where it is included. For example, it references methods like #comments and attributes such as #user_id.

Currently, in RBS, only classes or interfaces can be specified for module-self. As a workaround, I have defined an interface as follows:

interface _Commentable
  def comments: () -> Comment::ActiveRecord_Relation
  def user_id: () -> Integer
  ...
end

module Commentable : _Commentable
end

This interface allows for proper type checking, but as the number of methods grows, the interface definition becomes more cumbersome.

If it were possible to specify a Union type for module-self, it would allow for type definitions of modules without the need to define an interface.

module Commentable : (Article | Comment | ...)
end

However, generating an interface dynamically through Union types might negatively impact runtime performance. Do you think this idea is practical?

[ParadoxV5]

If module-self is a union type, then the types that include this module must be a branch of the union.

module Commentable : (Article | Comment | ...)
end

This is reminiscent of a sealed class in Java, which prevents users from includeing the module to their own types without tampering your interface.

---

Your intent is to save the need to create an intermediate module-self interface. https://github.com/ruby/rbs/blob/d9000d23c2c1d032a194848562806896c54fd0af/core/comparable.rbs#L55

I agree with the cumbersomeness, but the duck-typing nature of Ruby means that these interfaces can also make exact requirements as well as concrete types. https://github.com/ruby/rbs/blob/d9000d23c2c1d032a194848562806896c54fd0af/core/set.rbs#L200-L202