example_spec.rb

subject { App.new }

App::PURPOSES.each do |p,_|
  context do
    before { subject.purpose = p }
    its(:purpose){ should == p }
  end
end

App::PLATFORMS.each do |p,_|
  context do
    before { subject.platform = p }
    its(:platform){ should == p }
  end
end

original_spec.rb

subject { App.new }

%w(purpose platform).each do |meth|
  describe "#{meth}=" do
    App.const_get(meth.pluralize.upcase).keys.each do |attribute|

      it "accepts #{attribute} as a valid value" do
        app = App.new
        app.send "#{meth}=", attribute

        app.send(meth).should == attribute
      end
    end
  end
end