= The Nix REPL

The Nix REPL footnote:[REPL is an acronym for (Read-Eval-Print-Loop).]
is an interactive environment for evaluating and debugging Nix code.
It's also a good place to begin learning Nix.
Enter it using the command `nix repl`.
Within the REPL, type `:?` to see a list of available commands.

[source]
....
$ nix repl
Welcome to Nix 2.18.1. Type :? for help.

nix-repl> :?
The following commands are available:

  <expr>                       Evaluate and print expression
  <x> = <expr>                 Bind expression to variable
  :a, :add <expr>              Add attributes from resulting set to scope
  :b <expr>                    Build a derivation
  :bl <expr>                   Build a derivation, creating GC roots in the
                               working directory
  :e, :edit <expr>             Open package or function in $EDITOR
  :i <expr>                    Build derivation, then install result into
                               current profile
  :l, :load <path>             Load Nix expression and add it to scope
  :lf, :load-flake <ref>       Load Nix flake and add it to scope
  :p, :print <expr>            Evaluate and print expression recursively
  :q, :quit                    Exit nix-repl
  :r, :reload                  Reload all files
  :sh <expr>                   Build dependencies of derivation, then start
                               nix-shell
  :t <expr>                    Describe result of evaluation
  :u <expr>                    Build derivation, then start nix-shell
  :doc <expr>                  Show documentation of a builtin function
  :log <expr>                  Show logs for a derivation
  :te, :trace-enable [bool]    Enable, disable or toggle showing traces for
                               errors
  :?, :help                    Brings up this help menu
....

A command that is useful to beginners is `:t`, which tells you the type of an expression.

Note that the command to exit the REPL is `:q` (or `:quit` if you prefer).