example.rb

require './ri'

class PrinterJobs < Interface
  def queue_job; end
  def cancel_job; end
end

class InkjetJobs
  def queue_job
    puts "Queuing job"
  end

  def cancel_job
    puts "Cancelling Job"
  end
end

class BadPrinterJobs
  def queue_job(job)
    puts "Queuing job: #{job}"
  end

  def cancel_job(job)

  end
end
  

ri.rb

# Ruby Interfaces

require 'set'

class InterfaceError < Exception; end

class Interface  
  def self.ensure(obj)
    unless conforms?(obj)
      raise InterfaceError,
        "#{obj.class.name} does not conform to #{self.name} interface"
    end
  end
  
  def self.conforms?(obj)
    same_methods?(obj) && all_same_arity?(obj)
  end

  def self.same_methods?(obj)
    obj_methods = obj.class.instance_methods(false)
    return false if own_methods.empty? ^ obj_methods.empty?
    own_methods.to_set.subset?(obj_methods.to_set)
  end

  def self.all_same_arity?(obj)
    return false unless same_methods?(obj)
    own_methods.inject(true) do |same_arity, method|
      same_arity && same_arity?(obj, method)
    end
  end

  def self.same_arity?(obj, method)
    return false unless same_methods?(obj)
    new.method(method).arity == obj.method(method).arity
  end  
  
  def self.own_methods
    instance_methods(false)
  end
end

ri_test.rb

require './ri'
require './example'

describe Interface, "#ensure" do
  it "throws an exception when obj non-conforming" do
    expect { Interface.ensure(Time.new) }.to raise_error(InterfaceError)
  end
end

describe Interface, "#conforms?" do
  it "returns false when obj non-conforming" do
    expect(Interface.conforms?(Time.new)).to be(false)
  end

  it "returns true when obj conforming" do
    expect(Interface.conforms?(Interface.new)).to be(true)
  end
end

describe Interface, "#all_same_arity?" do
  it "returns false when one or more methods do not have same arity" do
    expect(PrinterJobs.all_same_arity?(BadPrinterJobs.new)).to be(false)
  end
  
  it "returns true when all methods have same arity" do
    expect(PrinterJobs.all_same_arity?(InkjetJobs.new)).to be(true)
  end
end

describe Interface, "#same_arity?" do
  it "returns false when a method does not have the same arity" do
    expect(PrinterJobs.same_arity?(BadPrinterJobs.new, :queue_job)).to be(false)
  end

  it "returns true when a method does have the same arity" do
    expect(PrinterJobs.same_arity?(InkjetJobs.new, :queue_job)).to be(true)
  end
end