9.14 Using Guix Interactively

The guix repl command gives you access to a warm and friendly read-eval-print loop (REPL) (see Invoking guix repl). If you’re getting into Guix programming—defining your own packages, writing manifests, defining services for Guix System or Guix Home, etc.—you will surely find it convenient to toy with ideas at the REPL.

If you use Emacs, the most convenient way to do that is with Geiser (see The Perfect Setup), but you do not have to use Emacs to enjoy the REPL. When using guix repl or guile in the terminal, we recommend using Readline for completion and Colorized to get colorful output. To do that, you can run:

guix install guile guile-readline guile-colorized

... and then create a .guile file in your home directory containing this:

(use-modules (ice-9 readline) (ice-9 colorized))

(activate-readline)
(activate-colorized)

The REPL lets you evaluate Scheme code; you type a Scheme expression at the prompt, and the REPL prints what it evaluates to:

$ guix repl
scheme@(guix-user)> (+ 2 3)
$1 = 5
scheme@(guix-user)> (string-append "a" "b")
$2 = "ab"

It becomes interesting when you start fiddling with Guix at the REPL. The first thing you’ll want to do is to “import” the (guix) module, which gives access to the main part of the programming interface, and perhaps a bunch of useful Guix modules. You could type (use-modules (guix)), which is valid Scheme code to import a module (see Using Guile Modules in GNU Guile Reference Manual), but the REPL provides the use command as a shorthand notation (see REPL Commands in GNU Guile Reference Manual):

scheme@(guix-user)> ,use (guix)
scheme@(guix-user)> ,use (gnu packages base)

Notice that REPL commands are introduced by a leading comma. A REPL command like use is not valid Scheme code; it’s interpreted specially by the REPL.

Guix extends the Guile REPL with additional commands for convenience. Among those, the build command comes in handy: it ensures that the given file-like object is built, building it if needed, and returns its output file name(s). In the example below, we build the coreutils and grep packages, as well as a “computed file” (see computed-file), and we use the scandir procedure to list the files in Grep’s /bin directory:

scheme@(guix-user)> ,build coreutils
$1 = "/gnu/store/…-coreutils-8.32-debug"
$2 = "/gnu/store/…-coreutils-8.32"
scheme@(guix-user)> ,build grep
$3 = "/gnu/store/…-grep-3.6"
scheme@(guix-user)> ,build (computed-file "x" #~(mkdir #$output))
building /gnu/store/…-x.drv...
$4 = "/gnu/store/…-x"
scheme@(guix-user)> ,use(ice-9 ftw)
scheme@(guix-user)> (scandir (string-append $3 "/bin"))
$5 = ("." ".." "egrep" "fgrep" "grep")

At a lower-level, a useful command is lower: it takes a file-like object and “lowers” it into a derivation (see Derivations) or a store file:

scheme@(guix-user)> ,lower grep
$6 = #<derivation /gnu/store/…-grep-3.6.drv => /gnu/store/…-grep-3.6 7f0e639115f0>
scheme@(guix-user)> ,lower (plain-file "x" "Hello!")
$7 = "/gnu/store/…-x"

The full list of REPL commands can be seen by typing ,help guix and is given below for reference.

REPL command: build object

Lower object and build it if it’s not already built, returning its output file name(s).

REPL command: lower object

Lower object into a derivation or store file name and return it.

REPL command: verbosity level

Change build verbosity to level.

This is similar to the --verbosity command-line option (see Common Build Options): level 0 means total silence, level 1 shows build events only, and higher levels print build logs.

REPL command: run-in-store exp

Run exp, a monadic expresssion, through the store monad. See The Store Monad, for more information.

REPL command: enter-store-monad

Enter a new REPL to evaluate monadic expressions (see The Store Monad). You can quit this “inner” REPL by typing ,q.