From development environments to continuous integration—the ultimate guide to software development with Guix

Ludovic Courtès — 5. Juni 2023

Guix is a handy tool for developers; guix shell, in particular, gives a standalone development environment for your package, no matter what language(s) it’s written in. To benefit from it, you have to initially write a package definition and have it either in Guix proper, in a channel, or directly upstream as a guix.scm file. This last option is appealing: all developers have to do to get set up is clone the project's repository and run guix shell, with no arguments—we looked at the rationale for guix shell in an earlier article.

Development needs go beyond development environments though. How can developers perform continuous integration of their code in Guix build environments? How can they deliver their code straight to adventurous users? This post describes a set of files developers can add to their repository to set up Guix-based development environments, continuous integration, and continuous delivery—all at once.

Update: Check out the Cookbook for an up-to-date and translated version of this tutorial!

Getting started

How do we go about “Guixifying” a repository? The first step, as we’ve seen, will be to add a guix.scm at the root of the repository in question. We’ll take Guile as an example in this post: it’s written in Scheme (mostly) and C, and has a number of dependencies—a C compilation tool chain, C libraries, Autoconf and its friends, LaTeX, and so on. The resulting guix.scm looks like the usual package definition, just without the define-public bit:

;; The ‘guix.scm’ file for Guile, for use by ‘guix shell’.

(use-modules (guix)
             (guix build-system gnu)
             ((guix licenses) #:prefix license:)
             (gnu packages autotools)
             (gnu packages base)
             (gnu packages bash)
             (gnu packages bdw-gc)
             (gnu packages compression)
             (gnu packages flex)
             (gnu packages gdb)
             (gnu packages gettext)
             (gnu packages gperf)
             (gnu packages libffi)
             (gnu packages libunistring)
             (gnu packages linux)
             (gnu packages pkg-config)
             (gnu packages readline)
             (gnu packages tex)
             (gnu packages texinfo)
             (gnu packages version-control))

(package
  (name "guile")
  (version "3.0.99-git")                          ;funky version number
  (source #f)                                     ;no source
  (build-system gnu-build-system)
  (native-inputs
   (append (list autoconf
                 automake
                 libtool
                 gnu-gettext
                 flex
                 texinfo
                 texlive-base                 ;for "make pdf"
                 texlive-epsf
                 gperf
                 git
                 gdb
                 strace
                 readline
                 lzip
                 pkg-config)

           ;; When cross-compiling, a native version of Guile itself is
           ;; needed.
           (if (%current-target-system)
               (list this-package)
               '())))
  (inputs
   (list libffi bash-minimal))
  (propagated-inputs
   (list libunistring libgc))

  (native-search-paths
   (list (search-path-specification
          (variable "GUILE_LOAD_PATH")
          (files '("share/guile/site/3.0")))
         (search-path-specification
          (variable "GUILE_LOAD_COMPILED_PATH")
          (files '("lib/guile/3.0/site-ccache")))))
  (synopsis "Scheme implementation intended especially for extensions")
  (description
   "Guile is the GNU Ubiquitous Intelligent Language for Extensions,
and it's actually a full-blown Scheme implementation!")
  (home-page "https://www.gnu.org/software/guile/")
  (license license:lgpl3+))

Quite a bit of boilerplate, but now someone who’d like to hack on Guile just needs to run:

guix shell

That gives them a shell containing all the dependencies of Guile: those listed above, but also implicit dependencies such as the GCC tool chain, GNU Make, sed, grep, and so on. The chef’s recommendation:

guix shell --container --link-profile

That gives a shell in an isolated container, and all the dependencies show up in $HOME/.guix-profile, which plays well with caches such as config.cache and absolute file names recorded in generated Makefiles and the likes. The fact that the shell runs in a container brings peace of mind: nothing but the current directory and Guile’s dependencies is visible inside the container; nothing from the system can possibly interfere with your development.

Level 1: Building with Guix

Now that we have a package definition, why not also take advantage of it so we can build Guile with Guix? We had left the source field empty, because guix shell above only cares about the inputs of our package—so it can set up the development environment—not about the package itself.

To build the package with Guix, we’ll need to fill out the source field, along these lines:

(use-modules (guix)
             (guix git-download)  ;for ‘git-predicate’
             …)

(define vcs-file?
  ;; Return true if the given file is under version control.
  (or (git-predicate (current-source-directory))
      (const #t)))                                ;not in a Git checkout

(package
  (name "guile")
  (version "3.0.99-git")                          ;funky version number
  (source (local-file "." "guile-checkout"
                      #:recursive? #t
                      #:select? vcs-file?))
  …)

Here’s what we changed:

We added (guix git-download) to our set of imported modules, so we can use its git-predicate procedure.
We defined vcs-file? as a procedure that returns true when passed a file that is under version control. For good measure, we add a fallback case for when we’re not in a Git checkout: always return true.
We set source to a local-file—a recursive copy of the current directory ("."), limited to files under version control (the #:select? bit).

From there on, our guix.scm file serves a second purpose: it lets us build the software with Guix. The whole point of building with Guix is that it’s a “clean” build—you can be sure nothing from your working tree or system interferes with the build result—and it lets you test a variety of things. First, you can do a plain native build:

guix build -f guix.scm

But you can also build for another system (possibly after setting up offloading or transparent emulation):

guix build -f guix.scm -s aarch64-linux -s riscv64-linux

… or cross-compile:

guix build -f guix.scm --target=x86_64-w64-mingw32

You can also use package transformation options to test package variants:

# What if we built with Clang instead of GCC?
guix build -f guix.scm \
  --with-c-toolchain=guile@3.0.99-git=clang-toolchain

# What about that under-tested configure flag?
guix build -f guix.scm \
  --with-configure-flag=guile@3.0.99-git=--disable-networking

Handy!

Level 2: The repository as a channel

We now have a Git repository containing (among other things) a package definition. Can’t we turn it into a channel? After all, channels are designed to ship package definitions to users, and that’s exactly what we’re doing with our guix.scm.

Turns out we can indeed turn it into a channel, but with one caveat: we must create a separate directory for the .scm file(s) of our channel so that guix pull doesn’t load unrelated .scm files when someone pulls the channel—and in Guile, there are lots of them! So we’ll start like this, keeping a top-level guix.scm symlink for the sake of guix shell:

mkdir -p .guix/modules
mv guix.scm .guix/modules/guile-package.scm
ln -s .guix/modules/guile-package.scm guix.scm

To make it usable as part of a channel, we need to turn our guix.scm file into a module: we do that by changing the use-modules form at the top to a define-module form. We also need to actually export a package variable, with define-public, while still returning the package value at the end of the file so we can still use guix shell and guix build -f guix.scm. The end result looks like this (not repeating things that haven’t changed):

(define-module (guile-package)
  #:use-module (guix)
  #:use-module (guix git-download)   ;for ‘git-predicate’
  …)

(define vcs-file?
  ;; Return true if the given file is under version control.
  (or (git-predicate (dirname (dirname (current-source-directory))))
      (const #t)))                                ;not in a Git checkout

(define-public guile
  (package
    (name "guile")
    (version "3.0.99-git")                          ;funky version number
    (source (local-file "../.." "guile-checkout"
                        #:recursive? #t
                        #:select? vcs-file?))
    …))

;; Return the package object define above at the end of the module.
guile

We need one last thing: a .guix-channel file so Guix knows where to look for package modules in our repository:

;; This file lets us present this repo as a Guix channel.

(channel
  (version 0)
  (directory ".guix/modules"))  ;look for package modules under .guix/modules/

To recap, we now have these files:

.
├── .guix-channel
├── guix.scm → .guix/modules/guile-package.scm
└── .guix
    └── modules
       └── guile-package.scm

And that’s it: we have a channel! (We could do better and support channel authentication so users know they’re pulling genuine code. We’ll spare you the details here but it’s worth considering!) Users can pull from this channel by adding it to ~/.config/guix/channels.scm, along these lines:

(append (list (channel
                (name 'guile)
                (url "https://git.savannah.gnu.org/git/guile.git")
                (branch "main")))
        %default-channels)

After running guix pull, we can see the new package:

$ guix describe
Generation 264  May 26 2023 16:00:35    (current)
  guile 36fd2b4
    repository URL: https://git.savannah.gnu.org/git/guile.git
    branch: main
    commit: 36fd2b4920ae926c79b936c29e739e71a6dff2bc
  guix c5bc698
    repository URL: https://git.savannah.gnu.org/git/guix.git
    commit: c5bc698e8922d78ed85989985cc2ceb034de2f23
$ guix package -A ^guile$
guile   3.0.99-git      out,debug       guile-package.scm:51:4
guile   3.0.9           out,debug       gnu/packages/guile.scm:317:2
guile   2.2.7           out,debug       gnu/packages/guile.scm:258:2
guile   2.2.4           out,debug       gnu/packages/guile.scm:304:2
guile   2.0.14          out,debug       gnu/packages/guile.scm:148:2
guile   1.8.8           out             gnu/packages/guile.scm:77:2
$ guix build guile@3.0.99-git
[…]
/gnu/store/axnzbl89yz7ld78bmx72vpqp802dwsar-guile-3.0.99-git-debug
/gnu/store/r34gsij7f0glg2fbakcmmk0zn4v62s5w-guile-3.0.99-git

That’s how, as a developer, you get your software delivered directly into the hands of users! No intermediaries, yet no loss of transparency and provenance tracking.

With that in place, it also becomes trivial for anyone to create Docker images, Deb/RPM packages, or a plain tarball with guix pack:

# How about a Docker image of our Guile snapshot?
guix pack -f docker -S /bin=bin guile@3.0.99-git

# And a relocatable RPM?
guix pack -f rpm -R -S /bin=bin guile@3.0.99-git

Bonus: Package variants

We now have an actual channel, but it contains only one package. While we’re at it, we can define package variants in our guile-package.scm file, variants that we want to be able to test as Guile developers—similar to what we did above with transformation options. We can add them like so:

;; This is the ‘.guix/modules/guile-package.scm’ file.

(define-module (guile-package)
  …)

(define-public guile
  …)

(define (package-with-configure-flags p flags)
  "Return P with FLAGS as additional 'configure' flags."
  (package/inherit p
    (arguments
     (substitute-keyword-arguments (package-arguments p)
       ((#:configure-flags original-flags #~(list))
        #~(append #$original-flags #$flags))))))

(define-public guile-without-threads
  (package
    (inherit (package-with-configure-flags guile
                                           #~(list "--without-threads")))
    (name "guile-without-threads")))

(define-public guile-without-networking
  (package
    (inherit (package-with-configure-flags guile
                                           #~(list "--disable-networking")))
    (name "guile-without-networking")))


;; Return the package object defined above at the end of the module.
guile

We can build these variants as regular packages once we’ve pulled the channel. Alternatively, from a checkout of Guile, we can run a command like this one from the top level:

guix build -L $PWD/.guix/modules guile-without-threads

Level 3: Setting up continuous integration

This channel becomes even more interesting once we set up continuous integration (CI). There are several ways to do that.

You can use one of the mainstream continuous integration tools, such as GitLab-CI. To do that, you need to make sure you run jobs in a Docker image or virtual machine that has Guix installed. If we were to do that in the case of Guile, we’d have a job that runs a shell command like this one:

guix build -L $PWD/.guix/modules guile@3.0.99-git

Doing this works great and has the advantage of being easy to achieve on your favorite CI platform.

That said, you’ll really get the most of it by using Cuirass, a CI tool designed for and tightly integrated with Guix. Using it is more work than using a hosted CI tool because you first need to set it up, but that setup phase is greatly simplified if you use its Guix System service. Going back to our example, we give Cuirass a spec file that goes like this:

;; Cuirass spec file to build all the packages of the ‘guile’ channel.
(list (specification
        (name "guile")
        (build '(channels guile))
        (channels
         (append (list (channel
                         (name 'guile)
                         (url "https://git.savannah.gnu.org/git/guile.git")
                         (branch "main")))
                 %default-channels))))

It differs from what you’d do with other CI tools in two important ways:

Cuirass knows it’s tracking two channels, guile and guix. Indeed, our own guile package depends on many packages provided by the guix channel—GCC, the GNU libc, libffi, and so on. Changes to packages from the guix channel can potentially influence our guile build and this is something we’d like to see as soon as possible as Guile developers.
Build results are not thrown away: they can be distributed as substitutes so that users of our guile channel transparently get pre-built binaries!

From a developer’s viewpoint, the end result is this status page listing evaluations: each evaluation is a combination of commits of the guix and guile channels providing a number of jobs—one job per package defined in guile-package.scm times the number of target architectures.

As for substitutes, they come for free! As an example, since our guile jobset is built on ci.guix.gnu.org, which runs guix publish in addition to Cuirass, one automatically gets substitutes for guile builds from ci.guix.gnu.org; no additional work is needed for that.

Bonus: Build manifest

The Cuirass spec above is convenient: it builds every package in our channel, which includes a few variants. However, this might be insufficiently expressive in some cases: one might want specific cross-compilation jobs, transformations, Docker images, RPM/Deb packages, or even system tests.

To achieve that, you can write a manifest. The one we have for Guile has entries for the package variants we defined above, as well as additional variants and cross builds:

;; This is ‘.guix/manifest.scm’.

(use-modules (guix)
             (guix profiles)
             (guile-package))   ;import our own package module

(define* (package->manifest-entry* package system
                                   #:key target)
  "Return a manifest entry for PACKAGE on SYSTEM, optionally cross-compiled to
TARGET."
  (manifest-entry
    (inherit (package->manifest-entry package))
    (name (string-append (package-name package) "." system
                         (if target
                             (string-append "." target)
                             "")))
    (item (with-parameters ((%current-system system)
                            (%current-target-system target))
            package))))

(define native-builds
  (manifest
   (append (map (lambda (system)
                  (package->manifest-entry* guile system))

                '("x86_64-linux" "i686-linux"
                  "aarch64-linux" "armhf-linux"
                  "powerpc64le-linux"))
           (map (lambda (guile)
                  (package->manifest-entry* guile "x86_64-linux"))
                (cons (package
                        (inherit (package-with-c-toolchain
                                  guile
                                  `(("clang-toolchain"
                                     ,(specification->package
                                       "clang-toolchain")))))
                        (name "guile-clang"))
                      (list guile-without-threads
                            guile-without-networking
                            guile-debug
                            guile-strict-typing))))))

(define cross-builds
  (manifest
   (map (lambda (target)
          (package->manifest-entry* guile "x86_64-linux"
                                    #:target target))
        '("i586-pc-gnu"
          "aarch64-linux-gnu"
          "riscv64-linux-gnu"
          "i686-w64-mingw32"
          "x86_64-linux-gnu"))))

(concatenate-manifests (list native-builds cross-builds))

We won’t go into the details of this manifest; suffice to say that it provides additional flexibility. We now need to tell Cuirass to build this manifest, which is done with a spec slightly different from the previous one:

;; Cuirass spec file to build all the packages of the ‘guile’ channel.
(list (specification
        (name "guile")
        (build '(manifest ".guix/manifest.scm"))
        (channels
         (append (list (channel
                         (name 'guile)
                         (url "https://git.savannah.gnu.org/git/guile.git")
                         (branch "main")))
                 %default-channels))))

We changed the (build …) part of the spec to '(manifest ".guix/manifest.scm") so that it would pick our manifest, and that’s it!

Wrapping up

We picked Guile as the running example in this post and you can see the result here:

.guix-channel;
.guix/modules/guile-package.scm with the top-level guix.scm symlink;
.guix/manifest.scm.

These days, repositories are commonly peppered with dot files for various tools: .envrc, .gitlab-ci.yml, .github/workflows, Dockerfile, .buildpacks, Aptfile, requirements.txt, and whatnot. It may sound like we’re proposing a bunch of additional files, but in fact those files are expressive enough to supersede most or all of those listed above.

With a couple of files, we get support for:

development environments (guix shell);
pristine test builds, including for package variants and for cross-compilation (guix build);
continuous integration (with Cuirass or with some other tool);
continuous delivery to users (via the channel and with pre-built binaries);
generation of derivative build artifacts such as Docker images or Deb/RPM packages (guix pack).

At the Guix headquarters, we’re quite happy about the result. We’ve been building a unified tool set for reproducible software deployment; this is an illustration of how you as a developer can benefit from it!

Acknowledgments

Thanks to Attila Lendvai, Brian Cully, and Ricardo Wurmus for providing feedback on an earlier draft of this post.

About GNU Guix

GNU Guix is a transactional package manager and an advanced distribution of the GNU system that respects user freedom. Guix can be used on top of any system running the Hurd or the Linux kernel, or it can be used as a standalone operating system distribution for i686, x86_64, ARMv7, AArch64 and POWER9 machines.

In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. When used as a standalone GNU/Linux distribution, Guix offers a declarative, stateless approach to operating system configuration management. Guix is highly customizable and hackable through Guile programming interfaces and extensions to the Scheme language.

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