FYI I just collapsed the <params> tag into the <param> to make things slightly more homogeneous and thus easier to deal with in the implementation (which I'm thinking of and drafting some considerations and hints for).

We should also add in launch.frontend a way of allowing overriding exposed actions and substitutions (which is a little tricky).

Hmm, I see why, but I wonder if what this is actually telling us is that we need namespaced tags e.g. <ros::group/> vs. <group/> in XML.

I think that we can delete it from the design document at the moment, and address it in a follow up.

I'm also inclined to defer this till the next iteration, along with event handlers.

Hmm, I see why, but I wonder if what this is actually telling us is that we need namespaced tags e.g. <ros::group/> vs. <group/> in XML.

That sounds like a great idea. I would like to see a using tag too (<using name="ros"/>). Handling overrides will be easy.

I'm also inclined to defer this till the next iteration, along with event handlers.

Sure, I will delete event handlers and ros-namespaces from the document. We can add them in the next iteration.

So it just boil down to this:

<group ns="my_ns">
</group>

Versus this:

<group>
  <let name="ros_namespace" value="$(var ros_namespace)/my_namespace"/>
</group>

I think we could have something like this:

<group>
  <push_ros_namespace namespace="my_namespace"/>
  ...
</group>

And that would be sufficient.

If we really want to have it as an attribute of <group> then I think you guys have identified one way which is to have <group> and <ros::group> where the latter can override the first if you do something like <using name="ros::group"/> or <using namespace="ros"/>. Another option would be to extend <group> directly, so that if you have launch_ros_xml, it adds acceptable attributes to <group>, like the ns one, through some extensibility interface.

I think we could have something like this:

<group>
  <push_ros_namespace namespace="my_namespace"/>
  ...
</group>

Yes, that one would be easy to implement, and less verbose than my option.

If we really want to have it as an attribute of <group> then I think you guys have identified one way which is to have <group> and <ros::group> where the latter can override the first if you do something like <using name="ros::group"/> or <using namespace="ros"/>. Another option would be to extend <group> directly, so that if you have launch_ros_xml, it adds acceptable attributes to <group>, like the ns one, through some extensibility interface.

I think the namespace option would be really nice, because it avoids the possibility of name collisions. But, it's harder to implement.

For the moment I will add a push_ros_namespace action. I think we can merge the document as-is and update it later.

+1 to push_ros_namespace for now and deferring namespace/extension discussions to the next iteration.

With ros2/launch#272 and ros2/launch#273, we will have exactly the functionality described in this document.