rclcpp VS nickel

I think there's an overload of rclcpp::action::create_client that will accept an rcl_action_client_options_t (see here https://github.com/ros2/rclcpp/pull/1133/files) but it looks like MoveGroupInterface doesn't pass anything in when it creates the action clients:

So one of the bigger issues I've encountered is that re-entrant services can break with the multi-threaded executor https://github.com/ros2/rclcpp/issues/1212 Services in ROS2 by default are not reentrant meaning they need to be called asynchronously. Since my team was porting ROS1 nodes over we wanted to keep the old blocking service call behavior in ROS2 to avoid changing decision logic. Unfortunately, we found out that this can cause the node to crash per the issue above. The fix for that issue was implemented in Humble irc, but can't be ported back to foxy. That's one specific example, but we've seen a few other issues like that as well.

Alternatively, you can use the default client libraries like rclpy or rclcpp for writing custom code that reads and writes ROS2 messages.

Thanks for the response!

> This is difficult to answer without knowing more details.

The situation specifically is the ROS ecosystem, where metadata is managed in these package.xml files:

https://github.com/ros2/rclcpp/blob/master/rclcpp/package.xm...

The federated nature of the ecosystem has led to a culture where it's very normal to be building dozens of these at once, in the same workspace together, often from multiple repos (the repo above has four in it). So there are several build tools which automate the work of examining a source workspace and building all the packages within it in the correct topological order, respecting build_depend tags. The newest of these tools (colcon) has actually made the package.xml optional in many cases, as it can examine CMakelists, setup.py, BUILD, etc, and discover for itself what the dependencies are.

Your "distribution" of ROS is formed by listing all the packages and repos in this big file, for which there is other tooling to manage pulling dependency sources, whatever: https://github.com/ros/rosdistro/blob/master/foxy/distributi...

Anyway, so the existing ROS/nix efforts (1) seem to basically consume all of this package/distribution metadata at once and generate a giant parallel structure of nix definitions (eg https://github.com/lopsided98/nix-ros-overlay/blob/master/di...), which I fear would be completely opaque to users and any system which required everyone to leave behind these existing workflows would be an immediate non-starter.

I think the ideal scenario (and what it would look like if I built this myself based on debs) would be that you could source the "base" workspace as usual (enter the nix-shell?), and check out source, build packages as usual with colcon, the usual workspace-building tool, but there'd be an extra plugin/verb/flag for it, which would make it build each package as a nix package instead of into the usual installspace. The verb would generate the nix definitions on the fly, and probably handle the invocation and build-parallelism side of it as well.

[1]: https://github.com/acowley/ros2nix, https://github.com/lopsided98/nix-ros-overlay

So, its key features are:

1. domain-specific: designed for conveniently creating and composing derivations. This reason alone already justifies a new language, or an embedded domain-specific language (such as the Guile/Scheme for guix), or a mix of both (Starlark, the build language of Bazel embedded in a restricted Python-variant).

2. purely functional: this ties well into the philosophical backing of Nix the package manager, which aims to be purely functional, also known as hermeticity in other build systems (Bazel).

3. lazily evaluated: similar to other build systems (including Bazel), so that you can build only what you need on demand.

4. dynamically typed: this one is controversial. Being dynamically typed—in other words, not developing a type system—gets Nix out of the door first. But users often complain about the lack of proper types and modularity. There are experiments to address this, such as Nickel (https://github.com/tweag/nickel).

It is understandable that a one-pager may not have space for the whys.

Nickel：Nickel is a straightforward configuration language aimed at automatically generating static configuration files. Essentially, it's akin to JSON with the addition of functions and types.

Also look at nickel which is an evolution of nix. It's my favorite in this space.

nickel-lang.org

https://github.com/tweag/nickel

Or this https://github.com/tweag/nickel

Nickel is a programming language. While HCL is just a configuration format, so not really comparable.

Here's a comparison with similar tools: https://github.com/tweag/nickel#comparison

As for 'providence', I suppose you meant provenance :) it's been delayed because this was less critical for 1.0 to decide on or to implement (as it: it doesn't break backward compatibility in any way to add this feature in the short term), but this is very much on the roadmap: Issue #235. That's a must-have in a language with merging like Nickel.

Have you considered using a different language for templating? this could be a BIG selling point. Some good ones are cue-lang (though I haven't seen support for rust), kcl or nickel-lang.