Bad interaction between launch frontend and YAML parameters

[hidmic]

Both launch XML and YAML frontends currently coexist with the YAML parsing of substitution text in Node parameters. This has caused us issues in the past (e.g. ros2/launch#226 (comment)), forcing additional quotes as escape mechanisms. At the time we deemed this as a situation we'd rarely find in the field.

---

Well, as you can see in RobotWebTools/rosbridge_suite#433, specifically on commit RobotWebTools/rosbridge_suite@d466c89, it's not as rare as we thought.

There, I'm forced to surround a * with two ' on each side.

• Why a * needs this? Because it's later used as a Node parameter and parameters are parsed as YAML, that parses that * as a malformed alias (see PyYAML documentation about aliases).

• Why two '? The inner pair prevents * from being parsed as a YAML alias, and the outer pair is removed by the frontend for consistency -- otherwise one has to remember that some values given in e.g. XML are parsed using YAML under the hood and some are not.

IMHO the deeper, unresolved issue that #31 partially addressed is that Substitutions in launch are always text and it is up to the receiving end to do any additional parsing, opening the door to things like this.

[dirk-thomas]

Since this affects how you have to write a launch file I would think this needs to be fixes asap before more packages start using this.

[ivanpauno]

I think we should have a custom class for a parameter description, instead of using a native type to represent them. There are a bunch of reasons:

• Conditional inclusion of parameters isn't now possible.
  As I commented here it would be much easier if we have a custom class for that.
• If we would have a class that could be constructed like:
  Parameter('name', VALUE_HERE, type=int)
  We could avoid all this problem.

Also, we currently don't provide a way of coercing the result of a substitution to another thing that's not an string. So each class is doing a custom thing for doing this conversions (e.g.: for converting from str to bool). I did a comment about "typed substitutions" in ros2/ros2_documentation#302 (comment), though that's not the only solution.

[ivanpauno]

After #137, the rules used for coercing type after substitutions and by launch frontend are the same.