2.2 The REPL as a power tool

Meet the Nix REPL — the fastest way to learn the language and a genuine power tool. Watch it patch the hello program live to print a custom greeting, reload with :r, and drop you straight into a shell running the patched build. Three Nix superpowers — overrides, the REPL, and the store — on screen at once, plus the colon-commands (:b, :p, :doc) that make the REPL your daily workbench.