YAML parser: support mixed types

[christianrauch]

Bug report

Required Info:

• Operating System: Ubuntu 18.04
• Installation type: binary
• Version or commit hash: dashing
• DDS implementation: Fast-RTPS
• Client library: rclcpp

Steps to reproduce issue

1. create a yaml configuration file containing:

my_param_lists:
  - frame: base
    id: 9
  - frame: object
    id: 14

2. provide configuration file to node (e.g. via mapping __params)

Expected behavior

The yaml parser should parse the YAML structure.

Actual behavior

The parser fails with:
Failed to parse yaml params file '[configuration].yaml': Sequence should be of same type. Value type 'integer' do not belong at line_num [line with "id: 9"], at /tmp/binarydeb/ros-dashing-rcl-yaml-param-parser-0.7.5/src/parser.c:931
I.e. it won't accept that frame and id are of different type (string, int).

Additional information

This has been supported in ROS1 (e.g. in the configuration of apriltags_ros) and should be made available for ROS2.

[nuclearsandwich]

The error message is somewhat misleading here and there's more preventing the above from working than just the YAML parsing.

There is not currently a supported parameter datatype for dictionaries/associative arrays. The supported datatypes are listed in the design doc describing parameters.
Heterogeneous arrays are also not supported currently.

As a start, improving the user experience so the error text points out the conceptual issue rather than a parser error would be most welcome. Is that something you would consider contributing @christianrauch?

If there are additional parameter datatypes that you would like to see, I think a PR to the design article linked above would be a a good start.

[christianrauch]

Is it a conceptual issue that this kind of parameter is not supported? I.e. is it possible to support this later or will this never happen?

These kinds of parameters definitely worked in ROS1. For example, this:

<rosparam param="tag_descriptions">[
    {id: 0, size: 0.163513},
    {id: 1, size: 0.163513, frame_id: a_frame},
    {id: 2, size: 0.163513, frame_id: tag_2},
    {id: 3, size: 0.163513},
    {id: 4, size: 0.163513},
    {id: 5, size: 0.163513}]
</rosparam>

could be added to a launch file and represented and used in the node as XmlRpc::XmlRpcValue: https://github.com/RIVeR-Lab/apriltags_ros/blob/62fbdc5797e67058a88a2495b19664a455903b30/apriltags_ros/src/apriltag_detector.cpp#L153-L180

I am fine with the error message per se, but I think that ROS2 needs an equivalent representation for this kind of heterogeneous key value maps.

[jacobperron]

I don't believe there's a conceptual issue. As @nuclearsandwich points out, what needs to be done is to add support for a new datatype to handle dictionaries. In order to do this we should extend the rcl_variant_s struct. It might make sense to define a new array of variant type, rcl_variant_array_s, and add that to the rcl_variant_s struct.

A agree that a PR to the design article that @nuclearsandwich referenced is the place to start.

[felixdivo]

This would really make things easier, and people should rightfully be able to expect (at least that degree of) YAML support for something that is called a YAML file.

[wieset]

I was surprised today to find that ros2 supports only basic yaml syntax and had to resort to parsing by myself to avoid changing the whole structure. In addition to complex and mixed type lists it would also be nice to see anchors / aliases supported.

Any reason ros2 is using its own parser instead of e.g. yaml-cpp?

[jacobperron]

The ROS 2 parameter parser uses the third-party parser from libyaml. I think we are not using yaml-cpp specifically because rcl is written in C not C++.

If you'd also like to see other YAML features supported, feel free open a feature request, or even better a pull request :)

[wieset]

That makes sense. I must have missed that, sorry. Will open a feature request.

[LANB0]

It's so stupid that the issue has existed for three years and will exist in all ros2's life. Shouldn't it be an urgent and high priority task that rcl support the full yaml syntax？I'll try to work to make all the YAML features supported in my free time.But for my job ,I must switch the param parser to a yaml lib in my every node, that's so bad.