interfaces_in_ruby.rb

module IPoint
  attr_accessor :x, :y
end

module IReadonlyPoint
  attr_reader :x, :y
end

module IReadonlyLine
  attr_reader :source, :destination
end

class Point
  attr_accessor :x, :y
end

class HalfPoint
  attr_accessor :x
end

class ReadonlyPoint
  attr_reader :x, :y
end

class StraightLine
  attr_accessor :source, :destination
end

class EmailAnalyzer
  def source user, address
  end

  def destination user, address
  end
end

module Interface
  InterfaceArityError = Class.new StandardError

  def self.implements interface, *objects
    objects.each do |object|
      interface.instance_methods(false).each do |method|
        fail NotImplementedError, "#{object}##{method}" unless object.respond_to? method

        interface_arity = interface.instance_method(method).arity
        implementation_arity = object.method(method).arity

        fail InterfaceArityError, method if interface_arity != implementation_arity
      end
    end
  end
end

describe Interface do
  it "it should not raise an error for full implementations" do
    Interface.implements IPoint, Point.new
    Interface.implements IReadonlyPoint, Point.new, ReadonlyPoint.new
    Interface.implements IReadonlyLine, StraightLine.new
  end

  it "should fail on non-implementations" do
    expect {
      Interface.implements IPoint, HalfPoint.new
    }.to raise_error(NotImplementedError)

    expect {
      Interface.implements IPoint, ReadonlyPoint.new
    }.to raise_error(NotImplementedError)
  end

  it "should fail on implementations with incorrect arity" do
    expect {
      Interface.implements IReadonlyLine, EmailAnalyzer.new
    }.to raise_error(Interface::InterfaceArityError)
  end

  it "can be used in classes" do
    class Drawer
      def draw point1, point2
        Interface.implements IPoint, point1, point2
        # draws a line ...
      end
    end

    drawer = Drawer.new
    point1 = Point.new
    point2 = Point.new

    drawer.draw(point1, point2)

    expect {
      drawer.draw("5,3", point2)
    }.to raise_error
  end
end

module Interface
  InterfaceArityError = Class.new StandardError

  def self.implements interface, *objects
    objects.each do |object|
      interface.instance_methods(false).each do |method|
        fail NotImplementedError, "#{object}##{method}" unless object.respond_to? method

        interface_arity = interface.instance_method(method).arity
        implementation_arity = object.method(method).arity

        fail InterfaceArityError, method if interface_arity != implementation_arity
      end
    end
  end

  def self.enforced(interface)
    Module.new do
      define_method(:initialize) do |*args|
        Interface.implements(interface, self)
      end
    end
  end
end

module IPoint
  attr_accessor :x, :y
end

module IHalfPoint
  attr_accessor :x
end

class HalfPoint
  attr_accessor :x

  include Interface.enforced(IHalfPoint)
end

class BadPoint
  attr_accessor :x

  include Interface.enforced(IPoint)
end

a = HalfPoint.new
b = BadPoint.new
# => interface.rb:7:in `block (2 levels) in implements': #<BadPoint:0x007f9eeb8965c0>#y (NotImplementedError)