‘x = <expr>’ is short for ‘:a { x = <expr>; }’.  Note that the
right-hand side refers to the original scope, so you get:

  nix-repl> x = 1
  nix-repl> x = x + 1
  nix-repl> x
  2

rather than an infinite recursion.

Example:

$ nix-repl '<nixos>'

> config.services.xserver.desktop<TAB>

comletes to

> config.services.xserver.desktopManager

You also get suggestions if there are multiple matches:

> config.services.xserver.desktopManager.kde4
config.services.xserver.desktopManager.kde4.enable
config.services.xserver.desktopManager.kde4.phononBackends

By default, we don't recurse into attribute sets or lists when
printing a value.  However, the new :p command does recurse.

For example:

  $ nix-repl '<nixpkgs>' '<nixos>'
  Welcome to Nix version 1.6pre3215_2c1ecf8. Type :? for help.

  Loading ‘<nixpkgs>’...
  Added 3337 variables.

  Loading ‘<nixos>’...
  Added 7 variables.

  nix-repl>

However, this may leave thunks in black-holed state, so it's not
really safe.

Example:

nix-repl> :l <nixpkgs>

nix-repl> lib.range 0 10
[ 0 1 2 3 4 5 6 7 8 9 10 ]

nix-repl> :l <nixos>

nix-repl> config.boot.kernelModules
[ "vboxdrv" "vboxnetadp" ... ]

The command ":a <expr>" evaluates <expr> and adds the attributes in
the resulting attribute set to the interpreter scope.  For instance:

nix-repl> :a import <nixpkgs> {}

nix-repl> lib.range 0 10
[ 0 1 2 3 4 5 6 7 8 9 10 ]

This program interactively reads a Nix expression from the user,
evaluates it, and prints the result.