multiple_arity_constructors.rb

module MultipleConstuctorsFromArity
  def self.included(klass)
    klass.extend(ClassMethods)
  end

  module ClassMethods
    def method_added(method_name)
      return unless method_name == :initialize

      if const_defined?("IGNORE_INITIALIZER_DEFINITION")
        remove_const("IGNORE_INITIALIZER_DEFINITION")
      else
        method = instance_method(method_name)

        @@_arity_initializers ||= {}
        @@_arity_initializers[method.arity] = method

        const_set("IGNORE_INITIALIZER_DEFINITION", true)

        define_method(:initialize) do |*args|
          if arity_initializer = @@_arity_initializers[args.size]
            arity_initializer.bind(self).call(*args)
          else
            raise ArgumentError, "wrong number of arguments"
          end
        end
      end
    end
  end
end

class Foo
  include MultipleConstuctorsFromArity

  attr_reader :foo, :bar

  def initialize(foo)
    @foo = foo
  end

  def initialize(foo, bar)
    initialize(foo)
    @bar = bar
  end
end

describe MultipleConstuctorsFromArity do
  it "allows definition of multiple constructors with different arity" do
    one = Foo.new("a")
    expect(one.foo).to eq("a")

    two = Foo.new("a", 1)
    expect(two.foo).to eq("a")
    expect(two.bar).to eq(1)
  end

  it "raises ArgumentError when no initializer takes the given number of arguments" do
    expect do
      Foo.new(1, 2, 3)
    end.to raise_error(ArgumentError, "wrong number of arguments")
  end
end