How to use eval in ROS2 XML launch file

edit

Based on a comment posted in the issue https://github.com/ros2/launch/issues/469, one way to make the eval substitution work is by adding extra quotes in the evaluated expression, as described below:

  <let name="foo" value="$(eval '\'a\' + \'_description\'')"/>

For the evaluation to work with a predefined variable, the expression would look like this (you will need to adapt for your own expression):

  <let name="bar" value="$(eval '\'$(var bar)\' == \'b\'')" />

I couldn't find any other documentation about this other than in the ticket mentioned above.