lorri

https://github.com/nix-community/lorri

lorri is a nix-shell replacement for project development. lorri is based around fast direnv integration for robust CLI and editor integration.

When changes are made that would affect a project's developement shell, lorri builds the new shell in the background, and applies the result on the next shell prompt. The result is that development tools are kept in sync with the current Nix shell configuration (even e.g. as you switch branches) without blocking your use of the terminal.

👉 Check out our blog post to see how lorri improves on the nix-shell experience during everyday development as well as in common scenarios like channel updates and Nix garbage collection.

lorri supports both shell.nix and devShells in flake.nix files.

The project is about experimenting with and improving the developer's experience with Nix. A particular focus is managing your project's external dependencies, editor integration, and quick feedback.

lorri supports Linux and macOS.

Demo

This screencast shows lorri and direnv working together to reload the development environment as shell.nix is updated:

Setup on NixOS or with home-manager on Linux

If you are using NixOS or home-manager on Linux and a Nixpkgs channel at least as recent as nixos-19.09, you can get started with lorri as follows. Otherwise see the next section, Setup on other platforms.

1. Enable the daemon service. Set services.lorri.enable = true; in your NixOS configuration.nix or your home-manager home.nix.

   This will automatically install the lorri command.

   Note: There's a known issue preventing the lorri daemon from starting automatically upon installation. Until it's resolved, you'll have to reload the user daemon by hand by running systemctl --user daemon-reload, or reboot.

2. Install direnv. Add pkgs.direnv to environment.systemPackages in your NixOS configuration.nix or to home.packages in your home-manager home.nix.

3. Set up the direnv hook for your shell. See this section of the direnv documentation.

4. Activate the lorri integration. Run lorri init in your project directory to create a shell.nix and .envrc file. This will not overwrite existing files.

   In your shell, you will now see the following message from direnv:

   direnv: error .envrc is blocked. Run `direnv allow` to approve its content.

   Activate the integration by running direnv allow.

From this point on, lorri monitors your shell.nix and its dependencies and triggers builds as required. Whenever a build succeeds, direnv automatically reloads your environment.

See Usage for more details.

Setup on other platforms

If you are running Nix on a Linux distribution other than NixOS or on macOS, the following instructions will help you get started with lorri.

1. Install lorri. If you are using a Nixpkgs channel at least as recent as nixos-19.09, you can install lorri using nix-env -i lorri.

   Otherwise, install lorri from the repository as follows:

   $ nix-env -if https://github.com/nix-community/lorri/archive/canon.tar.gz

2. Start the daemon. For testing, you can start the daemon in a separate terminal by running lorri daemon.

   See contrib/daemon.md for ways to start the daemon automatically in the background.

3. Install direnv v2.19.2 or later. If you are using a Nixpkgs channel at least as recent as nixos-19.03, you can install a compatible version of direnv using nix-env -i direnv.

   Otherwise, you can install direnv from source as follows:

   $ nix-env -if https://github.com/direnv/direnv/archive/master.tar.gz

4. Set up the direnv hook for your shell. See this section of the direnv documentation.

5. Activate the lorri integration. Run lorri init in your project directory to create a shell.nix and .envrc file. This will not overwrite existing files.

   In your shell, you will see the following message from direnv:

   direnv: error .envrc is blocked. Run `direnv allow` to approve its content.

   Activate the integration by running direnv allow.

From this point on, lorri monitors your shell.nix and its dependencies and triggers builds as required. Whenever a build succeeds, direnv automatically reloads your environment.

See Usage for more details.

Usage

Once the daemon is running and direnv is set up, the daemon process will continue monitoring and evaluating the Nix expressions in your project's shell.nix, and direnv will automatically reload the environment as it changes.

direnv will continue to load the cached environment when the daemon is not running. However, the daemon must be running for direnv to reload the environment based on the current shell.nix and its dependencies.

Editor integration

With the right setup, you can use lorri and direnv to customize your development environment for each project.

If you use Emacs, our direnv-mode tutorial is there to help you get started.

This section needs to be fleshed out more (#244).

Associated Projects

lorri embodies a Unix philosophy of doing one thing well. As a result, it works very well with other tools to provide a powerful and streamlined development experience.

direnv is lorri's chief collaborator. From its website:

[direnv] augments existing shells with a new feature that can load and unload environment variables depending on the current directory.

As a foundation, lorri relies on Nix and Nixpkgs to install and manage software packages.

For pinning versions of software during development, niv is very helpful.

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Support & Questions

Please use the issue tracker for any problems or bugs you encounter.

• We have monthly Office Hours on Google Meet
• We are on IRC/Matrix, libera/#lorri (Webchat), though we might not be responsive at all times.

Why is lorri not on crates.io?

Command line tools written in Rust are commonly available as Rust crates on crates.io. lorri is not distributed in this way, for good reasons.

lorri can only be built within a Nix environment, and it can only be installed via Nix. This is because lorri specifies its runtime dependencies as a Nix closure, and because Nix is itself a runtime dependency of lorri.

In addition to these technical reasons, there is simply no point in running lorri if you don't have Nix installed. And if you have Nix installed, then you're best off installing lorri via Nix.

How To Help

All development on lorri happens on the Github repository, in the open. You can propose a change in an issue, then create a pull request after some discussion. Some issues are marked with the “good first issue” label, those are a good place to start. Just remember to leave a comment when you start working on something.

Debugging

Set these environment variables when debugging:

RUST_LOG=lorri=debug RUST_BACKTRACE=1 lorri watch

lorri reevaluates more than expected

lorri sometimes recursively watches a directory that the user did not expect. This can happen for a number of reasons:

1. When using a local checkout instead of a channel for nixpkgs, lorri watches that directory recursively, and will trigger on any file change.
2. When specifying src via a path, (like the much-used src = ./.;) lorri watches that path recursively (see target/lorri#6 for details). To get around this, use a builtins.filterSource-based function to filter src, e.g., use nix-gitignore: src = pkgs.nix-gitignore.gitignoreSource [] ./., or one of the functions in nixpkgs/lib/sources.nix

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Upgrading

Upgrading lorri is easy with the lorri self-upgrade command.

By default, the upgrade command will upgrade from the canon branch.

Other upgrade options are available, including upgrading from a local clone. See lorri self-upgrade --help for more details.

Evaluator + watch design

The evaluator should eagerly reevaluate the Nix expressions as soon as anything material to their output changes. This takes place in a few stages.

Initial evaluation

builder::run() instantiates (and builds) the Nix expression with nix-build -vv. The evaluator prints each imported Nix file, and each copied source file. builder::run() parses the log and notes each of these paths out as an "input" path.

Each input path is the absolute path which Nix examined.

Each input path is then passed to PathReduction which examines each path referenced, and reduces it to a minimum set of paths with the following rules:

1. Discard any store paths which isn't a symlink to outside the store: they are immutable.
2. Replace any store path which is a symlink to outside the store to the destination of the symlink.
3. Replace a reference to a Nix Channel with the updateable symlink root of the channel. Concretely, replace the path /nix/var/nix/profiles/per-user/root/channels/nixos/default.nix with /nix/var/nix/profiles/per-user/root/ to watch for the channels symlink to change.

Initial testing collapses over 2,000 paths to just five.

Loop

Each identified path is watched for changes with inotify (Linux) or fsevent (macOS). If the watched path is a directory, all of its sub-directories are also watched for changes.

Each new batch of change notifications triggers a fresh evaluation. Newly discovered paths are added to the watch list.

Garbage Collection Roots

lorri creates an indirect garbage collection root for each .drv in $XDG_CACHE_HOME/lorri (~/.cache/lorri/ by default) each time it evaluates your project.

License & Copyright

Copyright 2019–2020 Target, Copyright 2021 The Nix Community License: Apache 2.0 (see LICENSE file)

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(Nix as observed by LORRI on 2015-07-13)