2.2 The Nix REPL as a power tool

The Nix REPL is the fastest way to test a Nix expression before it ends up buried in a thousand line file. This Nix REPL tutorial shows how to evaluate code interactively, read documentation without opening a browser, and inspect, build and even patch packages from nixpkgs, all from one prompt.

In this video

• Starting nix repl, evaluating expressions, and reading built-in docs with :doc and tab completion
• Loading all of nixpkgs with nix repl --file and using tab completion as a quick package search
• Inspecting package metadata: description, home page, license, and what the ... in the output hints at
• Building a package with :b and dropping into its shell with :u, straight from the REPL
• A live demo: patching GNU hello so it prints a different greeting, without editing a single file

After this lesson

You keep a REPL open next to your editor and test every idea there first, which saves you from writing Nix code that never had a chance to work.