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.