Feature request: Type Parameter Bounds

[tlubz]

It's currently difficult to write a well-typed generic container that calls any methods on members of the type parameter. Example:

class TestContainer
  attr_reader :obj

  def initialize(obj)
    @obj = obj
  end

  def ==(other)
    @obj == other.obj
  end
end

class TestContainer[T]
  attr_reader obj: T
  def initialize: (T obj) -> void
  def ==: (untyped other) -> bool
end

steep (default settings) output:

steep check domain/TestContainer.rb 
# Type checking files:

..........................................................F

domain/TestContainer.rb:9:9: [error] Type `T` does not have method `==`
│ Diagnostic ID: Ruby::NoMethod
│
└     @obj == other.obj
           ~~

Detected 1 problem from 1 file

Ideally we would be able to assert at least one-sided bounds on the type parameters, for example if we could assert that the parameter inherits from Numeric:

class TestContainer[T < Numeric]
  attr_reader obj: T
  def initialize: (T obj) -> void
  def ==: (untyped other) -> bool
end

[soutaro]

Hi @tlubz! 👋

Yes, we know the requirement of type parameter bounds and have it in our roadmap to RBS 2.0, which will be released with Ruby 3.1 hopefully.
I've started implementation of the feature and will share the progress on this repo soon.

[pocke]

This feature has been implemented in #844. So I'll close this issue.