libexpr: extend Value::print to allow limited depth

[inclyc]

Motivation

This is an extension for setting limited depth for nix::Value printing. Sometimes this list is fairly large because of recursive set inside nix, and printing in a desired depth make things much easier for users who just want to see limited depth (for example, when using nix repl, we may only want to see depth = 1 attribute fields, instead of digging into it (basically unreadable). Maybe we can extend nix repl after this change, for limited depth printing.

Context

I'm writting a nix language server which would like to provide consistent developer experience between editors and official nix implementation, and nix::Value::print seems a good way to show values (there is no other good public method to do so). And this method may take long time to finish.

Related PR: nix-community/nixd#149

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[roberth]

I'd say a depth parameter is a last resource, but a valid solution that's fairly easy to implement and maintain. You might want to consider, although you probably already have:

1. only explore one layer at a time, lazily, on-demand, interactive, etc
2. a mix of "depth first" and "breadth first" until you reach an output size limit; some clever balancing act. Should work reasonably even for pkgs.
3. depth limit. The minimum limit is still terrible for something like pkgs

Assuming 1 is not feasible for you and 2 is a "research project" and a lot more to test and review, let's go ahead with this.

This needs testing. Ideally I'd like to see a new unit test in src/libexpr/tests (I hate to say that this function is not unit tested yet), but perhaps it would also be nice to have a depth parameter for nix-instantiate, which uses this method.

[inclyc]

Thanks for reviewing my PRs ❤️ .

a mix of "depth first" and "breadth first" until you reach an output size limit; some clever balancing act. Should work reasonably even for pkgs.

Before addressing review comments, would you like to accept patches that trying to stop printing to ostream if it reaches some size limit also? Because pkgs is fairly big even when depth = 1.

I'd say a depth parameter is a last resource, but a valid solution that's fairly easy to implement and maintain.

Yes, it gives us reasonable outputs because nix language itself is structural and nested, so given a "depth" argument somehow preserve enough information instead of digging in some irrelavent attribute paths.

This needs testing. Ideally I'd like to see a new unit test in src/libexpr/tests (I hate to say that this function is not unit tested yet), but perhaps it would also be nice to have a depth parameter for nix-instantiate, which uses this method.

Thanks, I would like to see how to add these tests then.

[roberth]

Thanks, I would like to see how to add these tests then.

You can refer to the existing tests as examples of how to set them up.

• nix-instantiate is tested in tests/eval.sh
• evaluator unit tests are in src/libexpr/tests