code.rb

private def confirm_failure(error)
  self.error = error
  run_failure_callbacks
end


private def run_failure_callbacks
  # This line fails because on_failure responds_to #call ?!
  on_failure.call(self) if on_failure.respond_to?(:call)
end

spec.rb

context "when on_failure is callable" do
  it "calls the on_failure callback with itself" do
    expect(on_failure).to receive(:call).with(command)
    command.call
  end
end

context "when on_failure is not callable" do
  let(:nil_callback) { double("nil_callback") }
  let(:command) { Command.new(on_failure: nil_callback) }

  it "does not call on_failure" do
    expect(nil_callback).not_to receive(:call)
    command.call
  end
end