Quick musing on ADT in Ruby using Sorbet, name TBD. May gemify it later.

quick_adt.rb

require "sorbet-runtime"

module ADT
  module DeconstructableSorbetStruct
    def deconstruct
      self.class.props.map { send(_1) }
    end

    def deconstruct_keys(keys)
      return self.class.props.to_h { |k| [k, send(k)] }

      (self.class.props & keys).to_h { |k| [k, send(k)] }
    end
  end

  def self.type(**config, &fn)
    Class.new(T::Struct) do
      include DeconstructableSorbetStruct

      config.each do |field, type|
        const field, type
      end

      module_eval(&fn) if block_given?
    end
  end

  def self.sum(**types, &fn)
    container_module = Module.new do
      extend T::Helpers
      sealed!

      module_eval(&fn) if block_given?
    end

    types.each do |type, config|
      klass = Class.new(T::Struct) do
        include container_module
        include DeconstructableSorbetStruct

        config.each do |field, type|
          const field, type
        end
      end

      container_module.const_set(type.capitalize, klass)
    end

    container_module
  end
end

Coord = ADT.type(x: Integer, y: Integer, z: Integer)
Shape = ADT.sum(
  circle: { radius: Integer, center: Coord },
  square: { top_left: Coord, bottom_right: Coord }
)

Coord = ADT.type(x: Integer, y: Integer, z: Integer) do
  def translate(x: 0, y: 0, z: 0)
    Coord.new(x: @x + x, y: @y + y, z: @z + z)
  end
end

Shape = ADT.sum(
  circle: { radius: Integer, center: Coord },
  square: { top_left: Coord, bottom_right: Coord }
) do
  def translate(x: 0, y: 0, z: 0)
    case self
    when Shape::Circle
      Shape::Circle.new(radius:, center: center.translate(x:, y:, z:))
    when Shape::Square
      Shape::Square.new(
        top_left: top_left.translate(x:, y:, z:),
        bottom_right: bottom_right.translate(x:, y:, z:)
      )
    else
      T.absurd(self)
    end
  end
end

center = Coord.new(x: 0, y: 0, z: 0)
# => <Coord x=0, y=0, z=0>

circle = Shape::Circle.new(radius: 5, center:)
# => <Shape::Circle center=<Coord x=0, y=0, z=0>, radius=5>

circle.translate(z: 10)
# => <Shape::Circle center=<Coord x=0, y=0, z=10>, radius=5>