Add StringJoinSubstitution substituion

[baconbrot]

Add a substitution wrapper for python str.join method.
Supports concatenation of substitutions as implemented in #838

This PR would complement PathJoinSubstitution for general use cases like file names with separators (e.g. some.long.urdf.xacro), naming of entities (e.g. the_name_of_the_robot) or even namespaces (e.g. /env/group/robot/joint)

Usecases

Domains
e.g. https://docs.ros.org with subdomain launch configuration equal to docs

StringJoinSubstitution(
    [
        ['https://', LaunchConfiguration('subdomain')],
        'ros',
        'org'
    ],
    delimiter='.'
)

File names
e.g. robot.a.urdf.xacro with model launch configuration equal to a

StringJoinSubstitution(
    [
        'robot'
        LaunchConfiguration('model'),
        'urdf',
        'xacro'
    ],
    delimiter='.'
)

[christophebedard]

This seems fine, but could you give a real-world example? Ah, sorry, I glossed over the last example in the PR description, although I kind of think it could simply be ['robot.', LaunchConfiguration('model'), '.urdf.xacro'].

Also, do you think we should support frontends, i.e., by implementing parse()?

[baconbrot]

@christophebedard frontend support would definitly be a great addition for #838 because of cross-platform path handling.

For both cases, PathJoinSubstitution and StringJoinSubstitution, I come across following problem:
Consider previous example for python launch

StringJoinSubstitution(
    [
        'robot'
        LaunchConfiguration('model'),
        'urdf',
        'xacro'
    ],
    delimiter='.'
)

I dont know how this would translate to yaml launch. Maybe to something like this

- arg:
  - name: "model"
- join-string:
      delimiter: "."
        children:
          - "robot"
          - "$(var model)"
          - "urdf"
          - "xacro"

but the join-string result is not accessible. Perhaps something like this

- arg:
  - name: "model"
- let:
    name: "some_string"
    value:
      join-string:
      delimiter: "."
      children:
        - "robot"
        - "$(var model)"
        - "urdf"
        - "xacro"

could work

[christophebedard]

Substitutions in frontends look like $(<name> <args>): $(var ...) is the LaunchConfiguration substitution for frontends, see

launch/launch/launch/substitutions/launch_configuration.py

Lines 33 to 34 in 7a79d6b

	@expose_substitution('var')
	class LaunchConfiguration(Substitution):

So for PathJoinSubstitution, it would be $(path-join ...) and look like this:

launch:
- let:
    name: 'model'
    value: 'a'
- log:
    message: '$(path-join robot $(var model) urdf xacro)'

This would log robot/a/urdf/xacro. Everything after path-join in $() is provided to the *Substitution class' parse() function as a sequence, see the data param here for example:

launch/launch/launch/substitutions/for_loop_var.py

Line 77 in 7a79d6b

def parse(cls, data: Sequence[SomeSubstitutionsType]):

It gets a bit weird if there are spaces, but it works if you quote the item that has spaces: you can do $(path-join robot $(var model) "ur df" xacro) which results in robot/a/ur df/xacro. You can also nest substitutions, which is kind of the point of #838 (but for Python). In frontends, it looks like:

launch:
- let:
    name: 'model'
    value: 'a'
- let:
    name: 'id'
    value: '2'
- log:
    message: '$(path-join "robot$(var id)" $(var model) "ur df" xacro)'

which outputs robot2/a/ur df/xacro. I think having the correct/platform-specific path separator might not actually matter in launch files, because the underlying Python code probably translates it into the correct path string, but it might still be worth supporting. I'll open a PR to add this to PathJoinSubstitution once #838 is merged.

For StringJoinSubstitution, the first element in $(str-join ...) would have to be the delimiter, then the rest would be the components to join. This can be added in a follow-up PR, though. Once delimiter supports substitutions, I think this PR is good to go.

[christophebedard]

Just a small suggestion, otherwise this looks good to me!

[baconbrot]

@christophebedard Alright, thanks for your review! :D

[christophebedard]

Pulls: #843
Gist: https://gist.githubusercontent.com/christophebedard/5aae799688178d79a6898bdefa08f849/raw/224dcbb05e9fa97b38d47aa82b744b2d642e36cf/ros2.repos
BUILD args: --packages-above-and-dependencies launch
TEST args: --packages-above launch
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/15605

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[christophebedard]

The test failures seem to be unrelated.

[christophebedard]

Would you be willing to add support for frontends, i.e., by implementing parse()? Otherwise, that's fine, I can open an issue and someone can eventually get to it.

[baconbrot]

Yes! I can do that

[christophebedard]

Great!