Today we’re excited to announce lorri, a new tool for NixOS, Linux, and macOS that makes developing with Nix even nicer:

When people try lorri, we often hear that it is more magical than they expected.

What is lorri?

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

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.

How is lorri different from a nix-shell?

Nix’s shells are a fantastic tool for environment management, but have some pain points.

Let’s look at three ways lorri changes the experience.

Channel updates are no big deal

Do you use import <nixpkgs> {}? Update your channels and … Oops! Your Nix shell is stale! Opening a new nix-shell means downloading all new dependencies before you’re able to get back to work.

Not with lorri. When your channel updates, lorri watch automatically begins re-evaluating your project’s dependencies in the background, outside of your work shell. If you enter the shell before the evaluation completes, the last completed evaluation is loaded instead. When the new one is ready, your environment updates automatically.

nix-collect-garbage before a flight? Sure.

Nix shells are not protected from garbage collection. This is good for one-off shells (nix-shell -p fortune --run fortune) but not so nice for your large project which integrates dozens of Nixpkgs dependencies. Having your gigabytes of project tooling disappear on a low-bandwidth connection is a pain most Nix users know.

lorri captures development dependencies and creates garbage collection roots automatically. Switching to lorri means you are always ready to get to work on your projects.

Editor integration is a snap

A big hassle for Nix users is editor integration and tools like language servers.

Adding direnv to the tool stack makes this better: direnv is like Nix’s secret weapon for editor integration. Many editors support direnv, direnv supports Nix, and boom: many editors support Nix!

Out of the box, however, direnv’s Nix integration is slow — continuously, unnecessarily evaluating Nix expressions. In some editors the expressions are re-evaluated on every file switch. This is painful, especially if the evaluation takes even half a second!

lorri’s direnv integration doesn’t ever call Nix. Instead, the integration always selects the last cached evaluation, ensuring a lightning-fast editor and shell experience.

When entering a project directory your tools just appear, ready to use.

Using lorri

lorri is used in two parts:

• direnv integration sets up your shell and editor integration with project-specific dependencies
• lorri watch monitors your project and automatically regenerates your environment

A typical workflow for us is to spawn a terminal running lorri watch, minimize it, and open a second terminal for my work shell. If I’m not editing dependencies, sometimes I’ll sometimes even skip the lorri watch and just use the cached evaluation.

lorri was built by Tweag for Target, and we are so excited to introduce it to the public. lorri is beta, open source (Apache-2.0) and a tutorial is available at Target/lorri. Give it a try, and let us know what you think =)!