lorri

https://github.com/target/lorri

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

:point_right: 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.

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 both the lorri command and direnv
   (both required for the next steps).

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/target/lorri/archive/master.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).

Support & Questions

Please use the issue tracker
for any problems or bugs you encounter. We are on #lorri on
freenode (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
   https://github.com/target/lorri/issues/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

Upgrading

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

By default, the upgrade command will upgrade from the master 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 Target
License: Apache 2.0 (see LICENSE file)

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