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01/03: website: Add packaging tutorial.
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From: |
Ludovic Courtès |
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Subject: |
01/03: website: Add packaging tutorial. |
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Date: |
Wed, 10 Oct 2018 09:58:00 -0400 (EDT) |
civodul pushed a commit to branch master
in repository guix-artwork.
commit a065355e08b6eebf20dbf45e121d3393843e4622
Author: Pierre Neidhardt <address@hidden>
Date: Wed Oct 10 15:04:58 2018 +0200
website: Add packaging tutorial.
* website/posts/packaging-tutorial.md: New file.
---
website/posts/packaging-tutorial.md | 1072 +++++++++++++++++++++++++++++++++++
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diff --git a/website/posts/packaging-tutorial.md
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+title: A packaging tutorial for Guix
+date: 2018-10-09
+author: Pierre Neidhardt
+tags: Software development, Programming interfaces, Scheme API
+---
+
+
+# Introduction
+
+GNU Guix stands out as the *hackable* package manager, mostly because it uses
+[GNU Guile](https://www.gnu.org/software/guile/), a powerful high-level
programming language, one of the
[Scheme](https://en.wikipedia.org/wiki/Scheme_(programming_language))
+dialects from the [Lisp
family](https://en.wikipedia.org/wiki/Lisp_(programming_language)).
+
+Package definitions are also written in Scheme, which empowers Guix in some
very
+unique ways, unlike most other package managers that use shell scripts or
+simple languages.
+
+- Use functions, structures, macros and all of Scheme expressiveness for your
+ package definitions.
+
+- Inheritance makes it easy to customize a package by inheriting from it and
+ modifying only what is needed.
+
+- Batch processing: the whole package collection can be parsed, filtered and
+ processed. Building a headless server with all graphical interfaces
stripped
+ out? It's possible. Want to rebuild everything from source using specific
+ compiler optimization flags? Pass the `#:make-flags "..."` argument to the
+ list of packages. It wouldn't be a stretch to think [Gentoo USE
flags](https://wiki.gentoo.org/wiki/USE_flag) here,
+ but this goes even further: the changes don't have to be thought out
+ beforehand by the packager, they can be *programmed* by the user!
+
+The following tutorial covers all the basics around package creation with Guix.
+It does not assume much knowledge of the Guix system nor of the Lisp language.
+The reader is only expected to be familiar with the command line and to have
some
+basic programming knowledge.
+
+
+# A "Hello World" package
+
+The [Defining Packages section of the
manual](https://www.gnu.org/software/guix/manual/en/html_node/Defining-Packages.html)
introduces the basics of Guix
+packaging. In the following section, we will partly go over those basics
again.
+
+`GNU hello` is a dummy project that serves as an idiomatic example for
+packaging. It uses the GNU build system (`./configure && make && make
install`).
+Guix already provides a package definition which is a perfect example to start
+with. You can look up its declaration with `guix edit hello` from the
+command line. Let's see how it looks:
+
+```scheme
+ (define-public hello
+ (package
+ (name "hello")
+ (version "2.10")
+ (source (origin
+ (method url-fetch)
+ (uri (string-append "mirror://gnu/hello/hello-" version
+ ".tar.gz"))
+ (sha256
+ (base32
+ "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"))))
+ (build-system gnu-build-system)
+ (synopsis "Hello, GNU world: An example GNU package")
+ (description
+ "GNU Hello prints the message \"Hello, world!\" and then exits. It
+ serves as an example of standard GNU coding practices. As such, it
supports
+ command-line arguments, multiple languages, and so on.")
+ (home-page "https://www.gnu.org/software/hello/";)
+ (license gpl3+)))
+```
+
+As you can see, most of it is rather straightforward. But let's review the
+fields together:
+
+- **name:** The project name. Using Scheme conventions, we prefer to keep it
+ lower case, without underscore and using dash-separated words.
+- **source:** This field contains a description of the source code origin.
The
+ `origin` record contains these fields:
+ 1. The method, here `url-fetch` to download via HTTP/FTP, but other
methods
+ exist, such as `git-fetch` for Git repositories.
+ 2. The URI, which is typically some `https://` location for `url-fetch`.
Here
+ the special `mirror://gnu` refers to a set of well known locations,
all of
+ which can be used by Guix to fetch the source, should some of them
fail.
+ 3. The `sha256` checksum of the requested file. This is essential to
ensure
+ the source is not corrupted. Note that Guix works with base32 strings,
+ hence the call to the `base32` function.
+- **build-system:** This is where the power of abstraction provided by the
Scheme
+ language really shines: in this case, the `gnu-build-system`
+ abstracts away the famous `./configure && make && make
+ install` shell invocations. Other build systems include
the
+ `trivial-build-system` which does not do anything and requires
+ from the packager to program all the build steps, the
+ `python-build-system`, the `emacs-build-system`, [and many
+
more](https://www.gnu.org/software/guix/manual/en/html_node/Build-Systems.html).
+- **synopsis:** It should be a concise summary of what the package
+ does. For many packages a tagline from the
+ project's home page can be used as the synopsis.
+- **description:** Same as for the synopsis, it's fine to re-use the project
+ description from the homepage. Note that Guix uses Texinfo
+ syntax.
+- **home-page:** Use HTTPS if available.
+- **license:** See `guix/licenses.scm` in the project source for a full list.
+
+Time to build our first package! Nothing fancy here for now: we will stick to
a
+dummy "my-hello", a copy of the above declaration.
+
+As with the ritualistic "Hello World" taught with most programming languages,
+this will possibly be the most "manual" approach. We will work out an ideal
+setup later; for now we will go the simplest route.
+
+Save the following to a file `my-hello.scm`.
+
+```scheme
+ (use-modules (guix packages)
+ (guix download)
+ (guix build-system gnu)
+ (guix licenses))
+
+ (package
+ (name "my-hello")
+ (version "2.10")
+ (source (origin
+ (method url-fetch)
+ (uri (string-append "mirror://gnu/hello/hello-" version
+ ".tar.gz"))
+ (sha256
+ (base32
+ "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"))))
+ (build-system gnu-build-system)
+ (synopsis "Hello, Guix world: An example custom Guix package")
+ (description
+ "GNU Hello prints the message \"Hello, world!\" and then exits. It
+ serves as an example of standard GNU coding practices. As such, it
supports
+ command-line arguments, multiple languages, and so on.")
+ (home-page "https://www.gnu.org/software/hello/";)
+ (license gpl3+))
+```
+
+We will explain the extra code in a moment.
+
+Feel free to play with the different values of the various fields. If you
+change the source, you'll need to update the checksum. Indeed, Guix refuses to
+build anything if the given checksum does not match the computed checksum of
the
+source code. To obtain the correct checksum of the package declaration, we
+need to download the source, compute the sha256 checksum and convert it to
+base32.
+
+Thankfully, Guix can automate this task for us; all we need is to provide the
+URI:
+
+```sh
+ $ guix download mirror://gnu/hello/hello-2.10.tar.gz
+
+ Starting download of /tmp/guix-file.JLYgL7
+ From https://ftpmirror.gnu.org/gnu/hello/hello-2.10.tar.gz...
+ following redirection to
`https://mirror.ibcp.fr/pub/gnu/hello/hello-2.10.tar.gz'...
+ …10.tar.gz 709KiB 2.5MiB/s 00:00
[##################] 100.0%
+ /gnu/store/hbdalsf5lpf01x4dcknwx6xbn6n5km6k-hello-2.10.tar.gz
+ 0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i
+```
+
+Note in this specific case that the output tells us which mirror was chosen.
+
+If the result of the above command is not the same as in the above snippet,
+update your `my-hello` declaration accordingly.
+
+Now you can happily run
+
+```sh
+ $ guix package --install-from-file=my-hello.scm
+```
+
+You should now have `my-hello` in your profile!
+
+```sh
+ $ guix package --list-installed=my-hello
+ my-hello 2.10 out
+ /gnu/store/f1db2mfm8syb8qvc357c53slbvf1g9m9-my-hello-2.10
+```
+
+We've gone as far as we could without any knowledge of Scheme. Now is the
right
+time to introduce the minimum we need from the language before we can proceed.
+
+
+# A Scheme crash-course
+
+As we've seen above, basic packages don't require much Scheme knowledge, if
none
+at all. But as you progress and your desire to write more and more complex
+packages grows, it will become both necessary and empowering to hone your
Lisper
+skills.
+
+Since an extensive Lisp course is very much out of the scope of this tutorial,
+we will only cover some basics here.
+
+Guix uses the Guile implementation of Scheme. To start playing with the
+language, install it with `guix package --install guile` and start a
[REPL](https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop) by
+running `guile` from the command line.
+
+Alternatively you can also run `guix environment --ad-hoc guile -- guile` if
+you'd rather not have Guile installed in your user profile.
+
+In the following examples we use the `>` symbol to denote the REPL prompt, that
+is, the line reserved for user input. See [the Guile
manual](https://www.gnu.org/software/guile/manual/html_node/Using-Guile-Interactively.html)
for more details on
+the REPL.
+
+- Scheme syntax boils down to a tree of expressions (or *s-expression* in
Lisp
+ lingo). An expression can be a literal such numbers and strings, or a
+ compound which is a parenthesized list of compounds and literals. `#t` and
+ `#f` stand for the booleans "true" and "false", respectively.
+
+ Examples of valid expressions:
+
+ > "Hello World!"
+ "Hello World!"
+ > 17
+ 17
+ > (display (string-append "Hello " "Guix" "\n"))
+ "Hello Guix!"
+
+- This last example is a function call embedded in another function call.
When
+ a parenthesized expression is evaluated, the first term is the function and
+ the rest are the arguments passed to the function. Every function returns
the
+ last evaluated expression as value.
+
+- Anonymous functions are declared with the `lambda` term:
+
+ > (lambda (x) (* x x))
+ #<procedure 120e348 at <unknown port>:24:0 (x)>
+
+ The above lambda returns the square of its argument. Since everything is
an
+ expression, the `lambda` expression returns an anonymous function, which
can
+ in turn be applied to an argument:
+
+ > ((lambda (x) (* x x)) 3)
+ 9
+
+- Anything can be assigned a global name with `define`:
+
+ > (define a 3)
+ > (define square (lambda (x) (* x x)))
+ > (square a)
+ 9
+
+- Procedures can be defined more concisely with the following syntax:
+
+ (define (square x) (* x x))
+
+- A list structure can be created with the `list` procedure:
+
+ > (list 2 a 5 7)
+ (2 3 5 7)
+
+- The *quote* disables evaluation of a parenthesized expression: the first
term
+ is not called over the other terms. Thus it effectively returns a list of
+ terms.
+
+ > '(display (string-append "Hello " "Guix" "\n"))
+ (display (string-append "Hello " "Guix" "\n"))
+ > '(2 a 5 7)
+ (2 a 5 7)
+
+- The *quasiquote* disables evaluation of a parenthesized expression until a
+ colon re-enables it. Thus it provides us with fine-grained control over
what
+ is evaluated and what is not.
+
+ > `(2 a 5 7 (2 ,a 5 ,(+ a 4)))
+ (2 a 5 7 (2 3 5 7))
+
+ Note that the above result is a list of mixed elements: numbers, symbols
(here
+ `a`) and the last element is a list itself.
+
+- Multiple variables can be named locally with `let`:
+
+ > (define x 10)
+ > (let ((x 2)
+ (y 3))
+ (list x y))
+ (2 3)
+ > x
+ 10
+ > y
+ ERROR: In procedure module-lookup: Unbound variable: y
+
+ Use `let*` to allow later variable declarations to refer to earlier
+ definitions.
+
+ > (let* ((x 2)
+ (y (* x 3)))
+ (list x y))
+ (2 6)
+
+- The keyword syntax is `#:`, it is used to create unique identifiers. See
also
+ the [Keywords section in the Guile
manual](https://www.gnu.org/software/guile/manual/html_node/Keywords.html).
+
+- The percentage `%` is typically used for read-only global variables in the
+ build stage. Note that it is merely a convention, like `_` in C. Scheme
Lisp
+ treats `%` exactly the same as any other letter.
+
+- Modules are created with `define-module`. For instance
+
+ (define-module (guix build-system ruby)
+ #:use-module (guix store)
+ #:export (ruby-build
+ ruby-build-system))
+
+ defines the module `ruby` which must be located in
+ `guix/build-system/ruby.scm` somewhere in `GUILE_LOAD_PATH`. It depends on
+ the `(guix store)` module and it exports two symbols, `ruby-build` and
+ `ruby-build-system`.
+
+For a more detailed introduction, check out [Scheme at a
Glance](http://www.troubleshooters.com/codecorn/scheme_guile/hello.htm), by
Steve Litt.
+
+One of the reference Scheme books is the seminal *Structure and Interpretation
+of Computer Programs*, by Harold Abelson and Gerald Jay Sussman, with Julie
+Sussman. You'll find a free copy
[online](https://mitpress.mit.edu/sites/default/files/sicp/index.html),
together with [videos of the lectures
+by the
authors](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-001-structure-and-interpretation-of-computer-programs-spring-2005/video-lectures/).
The book is available in Texinfo format as the `sicp` Guix
+package. Go ahead, run `guix package --install sicp` and start reading with
+`info sicp` (or with the Emacs Info reader). An unofficial ebook [is also
+available](https://sarabander.github.io/sicp/).
+
+You'll find more books, tutorials and other resources at
<https://schemers.org/>.
+
+
+# Setup
+
+Now that we know some Scheme basics we can detail the different possible setups
+for working on Guix packages.
+
+There are several ways to set up a Guix packaging environment.
+
+We recommend you work directly on the Guix source checkout since it makes it
+easier for everyone to contribute to the project.
+
+But first, let's look at other possibilities.
+
+
+### Local file
+
+This is what we previously did with `my-hello`. Now that we know more Scheme,
+let's explain the leading chunks. As stated in `guix package --help`:
+
+```scheme
+ -f, --install-from-file=FILE
+ install the package that the code within FILE
+ evaluates to
+```
+
+Thus the last expression *must* return a package, which is the case in our
+earlier example.
+
+The `use-modules` expression tells which of the modules we need in the file.
+Modules are a collection of values and procedures. They are commonly called
+"libraries" or "packages" in other programming languages.
+
+
+### GUIX_PACKAGE_PATH
+
+*Note: Starting from Guix 0.16, the more flexible Guix "channels" are the
+preferred way and supersede `GUIX_PACKAGE_PATH`. See below.*
+
+It can be tedious to specify the file from the command line instead of simply
+calling `guix package --install my-hello` as you would do with the official
+packages.
+
+Guix makes it possible to streamline the process by adding as many "package
+declaration paths" as you want.
+
+Create a directory, say `~./guix-packages` and add it to the
`GUIX_PACKAGE_PATH`
+environment variable:
+
+```sh
+ $ mkdir ~/guix-packages
+ $ export GUIX_PACKAGE_PATH=~/guix-packages
+```
+
+To add several directories, separate them with a colon (`:`).
+
+Our previous `my-hello` needs some adjustments though:
+
+```scheme
+ (define-module (my-hello)
+ #:use-module (guix licenses)
+ #:use-module (guix packages)
+ #:use-module (guix build-system gnu)
+ #:use-module (guix download))
+
+ (define-public my-hello
+ (package
+ (name "my-hello")
+ (version "2.10")
+ (source (origin
+ (method url-fetch)
+ (uri (string-append "mirror://gnu/hello/hello-" version
+ ".tar.gz"))
+ (sha256
+ (base32
+ "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"))))
+ (build-system gnu-build-system)
+ (synopsis "Hello, Guix world: An example custom Guix package")
+ (description
+ "GNU Hello prints the message \"Hello, world!\" and then exits. It
+ serves as an example of standard GNU coding practices. As such, it
supports
+ command-line arguments, multiple languages, and so on.")
+ (home-page "https://www.gnu.org/software/hello/";)
+ (license gpl3+)))
+```
+
+Note that we have assigned the package value to an exported variable name with
+`define-public`. This is effectively assigning the package to the `my-hello`
+variable so that it can be referenced, among other as dependency of other
+packages.
+
+If you use `guix package --install-from-file=my-hello.scm` on the above file,
it
+will fail because the last expression, `define-public`, does not return a
+package. If you want to use `define-public` in this use-case nonetheless, make
+sure the file ends with an evaluation of `my-hello`:
+
+```scheme
+ ; ...
+ (define-public my-hello
+ ; ...
+ )
+
+ my-hello
+```
+
+This last example is not very typical.
+
+Now `my-hello` should be part of the package collection like all other official
+packages. You can verify this with:
+
+```sh
+ $ guix package --show=my-hello
+```
+
+
+### Guix channels
+
+Guix 0.16 features channels, which is very similar to `GUIX_PACKAGE_PATH` but
+provides better integration and provenance tracking. Channels are not
+necessarily local, they can be maintained as a public Git repository for
+instance. Of course, several channels can be used at the same time.
+
+See the [Channel secion in the
manual](http://guix.info/manual/en/Channels.html) for setup details.
+
+
+### Direct checkout hacking
+
+Working directly on the Guix project is recommended: it reduces the friction
+when the time comes to submit your changes upstream to let the community
benefit
+from your hard work!
+
+Unlike most software distributions, the Guix repository holds in one place both
+the tooling (including the package manager) and the package definitions. This
+choice was made so that it would give developers the flexibility to modify the
+API without breakage by updating all packages at the same time. This reduces
+development inertia.
+
+Check out the official [Git](https://git-scm.com/) repository:
+
+```sh
+ $ git clone https://git.savannah.gnu.org/git/guix.git
+```
+
+In the rest of this article, we use `$GUIX_CHECKOUT` to refer to the location
of
+the checkout.
+
+Follow the instruction from the ["Contributing"
chapter](https://www.gnu.org/software/guix/manual/en/html_node/Contributing.html)
in the manual to set up the
+repository environment.
+
+Once ready, you should be able to use the package definitions from the
+repository environment.
+
+Feel free to edit package definitions found in `$GUIX_CHECKOUT/gnu/packages`.
+
+The `$GUIX_CHECKOUT/pre-inst-env` script lets you use `guix` over the package
+collection of the repository.
+
+- Search packages, such as Ruby:
+
+ $ cd $GUIX_CHECKOUT
+ $ ./pre-inst-env guix package --list-available=ruby
+ ruby 1.8.7-p374 out gnu/packages/ruby.scm:119:2
+ ruby 2.1.6 out gnu/packages/ruby.scm:91:2
+ ruby 2.2.2 out gnu/packages/ruby.scm:39:2
+
+- Build a package, here Ruby version 2.1:
+
+ $ ./pre-inst-env guix build --keep-failed address@hidden
+ /gnu/store/c13v73jxmj2nir2xjqaz5259zywsa9zi-ruby-2.1.6
+
+- Install it to your user profile:
+
+ $ ./pre-inst-env guix package --install address@hidden
+
+- Check for common mistakes:
+
+ $ ./pre-inst-env guix lint address@hidden
+
+Guix strives at maintaining a high packaging standard; when contributing to the
+Guix project, remember to
+
+- follow the [coding
style](https://www.gnu.org/software/guix/manual/en/html_node/Coding-Style.html),
+- and review the [check
list](https://www.gnu.org/software/guix/manual/en/html_node/Submitting-Patches.html)
from the manual.
+
+Once you are happy with the result, you are welcome to send your contribution
to
+make it part of Guix. This process is also detailed in the
[manual](https://www.gnu.org/software/guix/manual/en/html_node/Contributing.html).
+
+It's a community effort so the more join in, the better Guix becomes!
+
+
+# Extended example
+
+The above "Hello World" example is as simple as it goes. Packages can be more
+complex than that and Guix can handle more advanced scenarios. Let's look at
+another, more sophisticated package (slightly modified from the source):
+
+```scheme
+ (define-module (gnu packages version-control)
+ #:use-module ((guix licenses) #:prefix license:)
+ #:use-module (guix utils)
+ #:use-module (guix packages)
+ #:use-module (guix git-download)
+ #:use-module (guix build-system cmake)
+ #:use-module (gnu packages ssh)
+ #:use-module (gnu packages web)
+ #:use-module (gnu packages pkg-config)
+ #:use-module (gnu packages python)
+ #:use-module (gnu packages compression)
+ #:use-module (gnu packages tls))
+
+ (define-public my-libgit2
+ (let ((commit "e98d0a37c93574d2c6107bf7f31140b548c6a7bf")
+ (revision "1"))
+ (package
+ (name "my-libgit2")
+ (version (git-version "0.26.6" revision commit))
+ (source (origin
+ (method git-fetch)
+ (uri (git-reference
+ (url "https://github.com/libgit2/libgit2/";)
+ (commit commit)))
+ (file-name (git-file-name name version))
+ (sha256
+ (base32
+ "17pjvprmdrx4h6bb1hhc98w9qi6ki7yl57f090n9kbhswxqfs7s3"))
+ (patches (search-patches "libgit2-mtime-0.patch"))
+ (modules '((guix build utils)))
+ (snippet '(begin
+ ;; Remove bundled software.
+ (delete-file-recursively "deps")
+ #t))))
+ (build-system cmake-build-system)
+ (outputs '("out" "debug"))
+ (arguments
+ `(#:tests? #t ; Run the test suite (this
is the default)
+ #:configure-flags '("-DUSE_SHA1DC=ON") ; SHA-1 collision detection
+ #:phases
+ (modify-phases %standard-phases
+ (add-after 'unpack 'fix-hardcoded-paths
+ (lambda _
+ (substitute* "tests/repo/init.c"
+ (("#!/bin/sh") (string-append "#!" (which "sh"))))
+ (substitute* "tests/clar/fs.h"
+ (("/bin/cp") (which "cp"))
+ (("/bin/rm") (which "rm")))
+ #t))
+ ;; Run checks more verbosely.
+ (replace 'check
+ (lambda _ (invoke "./libgit2_clar" "-v" "-Q")))
+ (add-after 'unpack 'make-files-writable-for-tests
+ (lambda _ (for-each make-file-writable (find-files "."
".*")))))))
+ (inputs
+ `(("libssh2" ,libssh2)
+ ("http-parser" ,http-parser)
+ ("python" ,python-wrapper)))
+ (native-inputs
+ `(("pkg-config" ,pkg-config)))
+ (propagated-inputs
+ ;; These two libraries are in 'Requires.private' in libgit2.pc.
+ `(("openssl" ,openssl)
+ ("zlib" ,zlib)))
+ (home-page "https://libgit2.github.com/";)
+ (synopsis "Library providing Git core methods")
+ (description
+ "Libgit2 is a portable, pure C implementation of the Git core
methods
+ provided as a re-entrant linkable library with a solid API, allowing you to
+ write native speed custom Git applications in any language with bindings.")
+ ;; GPLv2 with linking exception
+ (license license:gpl2))))
+```
+
+(In those cases were you only want to tweak a few fields from a package
+definition, you should rely on inheritance instead of copy-pasting everything.
+See below.)
+
+Let's discuss those fields in depth.
+
+
+## `git-fetch` method
+
+Unlike the `url-fetch` method, `git-fetch` expects a `git-reference` which
takes
+a Git repository and a commit. The commit can be any Git reference such as
+tags, so if the `version` is tagged, then it can be used directly. Sometimes
+the tag is prefixed with a `v`, in which case you'd use `(commit (string-append
+"v" version))`.
+
+To ensure that the source code from the Git repository is stored in a unique
+directory with a readable name we use `(file-name (git-file-name name
+version))`.
+
+Note that there is also a `git-version` procedure that can be used to derive
the
+version when packaging programs for a specific commit.
+
+
+## Snippets
+
+Snippets are quoted (i.e. non-evaluated) Scheme code that are a means of
patching
+the source. They are a Guix-y alternative to the traditional `.patch` files.
+Because of the quote, the code in only evaluated when passed to the Guix daemon
+for building.
+
+There can be as many snippet as needed.
+
+Snippets might need additional Guile modules which can be imported from the
+`modules` field.
+
+
+## Inputs
+
+First, a syntactic comment: See the quasi-quote / comma syntax?
+
+```scheme
+ (native-inputs
+ `(("pkg-config" ,pkg-config)))
+```
+
+is equivalent to
+
+```scheme
+ (native-inputs
+ (list (list "pkg-config" pkg-config)))
+```
+
+You'll mostly see the former because it's shorter.
+
+There are 3 different input types. In short:
+
+- **native-inputs:** Required for building but not runtime –
installing a package
+ through a substitute won't install these inputs.
+- **inputs:** Installed in the store but not in the profile, as well as being
+ present at build time.
+- **propagated-inputs:** Installed in the store and in the profile, as well
as
+ being present at build time.
+
+See [the package reference in the
manual](https://www.gnu.org/software/guix/manual/en/html_node/package-Reference.html)
for more details.
+
+The distinction between the various inputs is important: if a dependency can be
+handled as an *input* instead of a *propagated input*, it should be done so, or
+else it "pollutes" the user profile for no good reason.
+
+For instance, a user installing a graphical program that depends on a
+command line tool might only be interested in the graphical part, so there is
no
+need to force the command line tool into the user profile. The dependency is a
+concern to the package, not to the user. *Inputs* make it possible to handle
+dependencies without bugging the user by adding undesired executable files (or
+libraries) to their profile.
+
+Same goes for *native-inputs*: once the program is installed, build-time
+dependencies can be safely garbage-collected.
+It also matters when a substitute is available, in which case only the *inputs*
+and *propagated inputs* will be fetched: the *native inputs* are not required
to
+install a package from a substitute.
+
+
+## Outputs
+
+Just like how a package can have multiple inputs, it can also produce multiple
+outputs.
+
+Each output corresponds to a separate directory in the store.
+
+The user can choose which output to install; this is useful to save space or
+to avoid polluting the user profile with unwanted executables or libraries.
+
+Output separation is optional. When the `outputs` field is left out, the
+default and only output (the complete package) is referred to as `"out"`.
+
+Typical separate output names include `debug` and `doc`.
+
+It's advised to separate outputs only when you've shown it's worth it: if the
+output size is significant (compare with `guix size`) or in case the package is
+modular.
+
+
+## Build system arguments
+
+The `arguments` is a keyword-value list used to configure the build process.
+
+The simplest argument `#:tests?` can be used to disable the test suite when
+building the package. This is mostly useful when the package does not feature
+any test suite. It's strongly recommended to keep the test suite on if there
is
+one.
+
+Another common argument is `:make-flags`, which specifies a list of flags to
+append when running make, as you would from the command line. For instance,
the
+following flags
+
+```scheme
+ #:make-flags (list (string-append "prefix=" (assoc-ref %outputs "out"))
+ "CC=gcc")
+```
+
+translate into
+
+```sh
+ $ make CC=gcc prefix=/gnu/store/...-<out>
+```
+
+This sets the C compiler to `gcc` and the `prefix` variable (the installation
+directory in Make parlance) to `(assoc-ref %outputs "out")`, which is a
build-stage
+global variable pointing to the destination directory in the store (something
like
+`/gnu/store/...-my-libgit2-20180408`).
+
+Similarly, it's possible to set the "configure" flags.
+
+```scheme
+ #:configure-flags '("-DUSE_SHA1DC=ON")
+```
+
+The `%build-inputs` variable is also generated in scope. It's an association
+table that maps the input names to their store directories.
+
+The `phases` keyword lists the sequential steps of the build system. Typically
+phases include `unpack`, `configure`, `build`, `install` and `check`. To know
+more about those phases, you need to work out the appropriate build system
+definition in `$GUIX_CHECKOUT/guix/build/gnu-build-system.scm`:
+
+```scheme
+ (define %standard-phases
+ ;; Standard build phases, as a list of symbol/procedure pairs.
+ (let-syntax ((phases (syntax-rules ()
+ ((_ p ...) `((p . ,p) ...)))))
+ (phases set-SOURCE-DATE-EPOCH set-paths install-locale unpack
+ bootstrap
+ patch-usr-bin-file
+ patch-source-shebangs configure patch-generated-file-shebangs
+ build check install
+ patch-shebangs strip
+ validate-runpath
+ validate-documentation-location
+ delete-info-dir-file
+ patch-dot-desktop-files
+ install-license-files
+ reset-gzip-timestamps
+ compress-documentation)))
+```
+
+Or from the REPL:
+
+```scheme
+ > (add-to-load-path "/path/to/guix/checkout")
+ > ,module (guix build gnu-build-system)
+ > (map first %standard-phases)
+ (set-SOURCE-DATE-EPOCH set-paths install-locale unpack bootstrap
+ patch-usr-bin-file patch-source-shebangs configure
+ patch-generated-file-shebangs build check install patch-shebangs strip
+ validate-runpath validate-documentation-location delete-info-dir-file
+ patch-dot-desktop-files install-license-files reset-gzip-timestamps
+ compress-documentation)
+```
+
+If you want to know more about what happens during those phases, consult the
+associated procedures.
+
+For instance, as of this writing the definition of `unpack` for the GNU build
+system is
+
+```scheme
+ (define* (unpack #:key source #:allow-other-keys)
+ "Unpack SOURCE in the working directory, and change directory within the
+ source. When SOURCE is a directory, copy it in a sub-directory of the
current
+ working directory."
+ (if (file-is-directory? source)
+ (begin
+ (mkdir "source")
+ (chdir "source")
+
+ ;; Preserve timestamps (set to the Epoch) on the copied tree so
that
+ ;; things work deterministically.
+ (copy-recursively source "."
+ #:keep-mtime? #t))
+ (begin
+ (if (string-suffix? ".zip" source)
+ (invoke "unzip" source)
+ (invoke "tar" "xvf" source))
+ (chdir (first-subdirectory "."))))
+ #t)
+```
+
+Note the `chdir` call: it changes the working directory to where the source was
+unpacked.
+Thus every phase following the `unpack` will use the source as a working
+directory, which is why we can directly work on the source files.
+That is to say, unless a later phase changes the working directory to something
+else.
+
+We modify the list of `%standard-phases` of the build system with the
+`modify-phases` macro as per the list of specified modifications, which may
have
+the following forms:
+
+- `(add-before PHASE NEW-PHASE PROCEDURE)`: Run `PROCEDURE` named
`NEW-PHASE` before `PHASE`.
+- `(add-after PHASE NEW-PHASE PROCEDURE)`: Same, but afterwards.
+- `(replace PHASE PROCEDURE)`.
+- `(delete PHASE)`.
+
+The `PROCEDURE` supports the keyword arguments `inputs` and `outputs`. Each
+input (whether *native*, *propagated* or not) and output directory is
referenced
+by their name in those variables. Thus `(assoc-ref outputs "out")` is the
store
+directory of the main output of the package. A phase procedure may look like
+this:
+
+```scheme
+ (lambda* (#:key inputs outputs #:allow-other-keys)
+ (let (((bash-directory (assoc-ref inputs "bash"))
+ (output-directory (assoc-ref outputs "out"))
+ (doc-directory (assoc-ref outputs "doc"))
+ ; ...
+ #t)
+```
+
+The procedure must return `#t` on success. It's brittle to rely on the return
+value of the last expression used to tweak the phase because there is no
+guarantee it would be a `#t`. Hence the trailing `#t` to ensure the right
value
+is returned on success.
+
+
+## Code staging
+
+The astute reader may have noticed the quasi-quote and comma syntax in the
+argument field. Indeed, the build code in the package declaration should not
be
+evaluated on the client side, but only when passed to the Guix daemon. This
+mechanism of passing code around two running processes is called [code
staging](https://arxiv.org/abs/1709.00833).
+
+
+## "Utils" functions
+
+When customizing `phases`, we often need to write code that mimics the
+equivalent system invocations (`make`, `mkdir`, `cp`, etc.) commonly used
during
+regular "Unix-style" installations.
+
+Some like `chmod` are native to Guile. See the [Guile reference
manual](https://www.gnu.org/software/guile/manual/guile.html) for a
+complete list.
+
+Guix provides additional helper functions which prove especially handy in the
+context of package management.
+
+Some of those functions can be found in
+`$GUIX_CHECKOUT/guix/guix/build/utils.scm`. Most of them mirror the behaviour
+of the traditional Unix system commands:
+
+- **which:** Like the `which` system command.
+- **find-files:** Akin to the `find` system command.
+- **mkdir-p:** Like `mkdir -p`, which creates all parents as needed.
+- **install-file:** Similar to `install` when installing a file to a
(possibly
+ non-existing) directory. Guile has `copy-file` which works
+ like `cp`.
+- **copy-recursively:** Like `cp -r`.
+- **delete-file-recursively:** Like `rm -rf`.
+- **invoke:** Run an executable. This should be used instead of `system*`.
+- **with-directory-excursion:** Run the body in a different working
directory,
+ then restore the previous working directory.
+- **substitute\*:** A "sed-like" function.
+
+
+## Module prefix
+
+The license in our last example needs a prefix: this is because of how the
+`license` module was imported in the package, as `#:use-module ((guix licenses)
+#:prefix license:)`. The [Guile module import
mechanism](https://www.gnu.org/software/guile/manual/html_node/Using-Guile-Modules.html)
gives the user full
+control over namespacing: this is needed to avoid clashes between, say, the
+`zlib` variable from `licenses.scm` (a *license* value) and the `zlib` variable
+from `compression.scm` (a *package* value).
+
+
+# Other build systems
+
+What we've seen so far covers the majority of packages using a build system
+other than the `trivial-build-system`. The latter does not automate anything
+and leaves you to build everything manually. This can be more demanding and we
+won't cover it here for now, but thankfully it is rarely necessary to fall back
+on this system.
+
+For the other build systems, such as ASDF, Emacs, Perl, Ruby and many more, the
+process is very similar to the GNU build system except for a few specialized
+arguments.
+
+Learn more about build systems in
+
+- [the manual, section 4.2 Build
systems](https://www.gnu.org/software/guix/manual/en/html_node/Build-Systems.html#Build-Systems),
+- the source code in the `$GUIX_CHECKOUT/guix/build` and
+ `$GUIX_CHECKOUT/guix/build-system` directories.
+
+
+# Programmable and automated package definition
+
+We can't repeat it enough: having a full-fledged programming language at hand
+empowers us in ways that reach far beyond traditional package management.
+
+Let's illustrate this with some awesome features of Guix!
+
+
+## Recursive importers
+
+You might find some build systems good enough that there is little to do at all
+to write a package, to the point that it becomes repetitive and tedious after a
+while. A *raison d'être* of computers is to replace human beings at those
+boring tasks. So let's tell Guix to do this for us and create the package
+definition of an R package from CRAN (the output is trimmed for conciseness):
+
+```sh
+ $ guix import cran --recursive walrus
+
+ (define-public r-mc2d
+ ; ...
+ (license gpl2+)))
+
+ (define-public r-jmvcore
+ ; ...
+ (license gpl2+)))
+
+ (define-public r-wrs2
+ ; ...
+ (license gpl3)))
+
+ (define-public r-walrus
+ (package
+ (name "r-walrus")
+ (version "1.0.3")
+ (source
+ (origin
+ (method url-fetch)
+ (uri (cran-uri "walrus" version))
+ (sha256
+ (base32
+ "1nk2glcvy4hyksl5ipq2mz8jy4fss90hx6cq98m3w96kzjni6jjj"))))
+ (build-system r-build-system)
+ (propagated-inputs
+ `(("r-ggplot2" ,r-ggplot2)
+ ("r-jmvcore" ,r-jmvcore)
+ ("r-r6" ,r-r6)
+ ("r-wrs2" ,r-wrs2)))
+ (home-page "https://github.com/jamovi/walrus";)
+ (synopsis "Robust Statistical Methods")
+ (description
+ "This package provides a toolbox of common robust statistical tests,
including robust descriptives, robust t-tests, and robust ANOVA. It is also
available as a module for 'jamovi' (see <https://www.jamovi.org> for more
information). Walrus is based on the WRS2 package by Patrick Mair, which is in
turn based on the scripts and work of Rand Wilcox. These analyses are
described in depth in the book 'Introduction to Robust Estimation & Hypothesis
Testing'.")
+ (license gpl3)))
+```
+
+The recursive importer won't import packages for which Guix already has package
+definitions, except for the very first.
+
+Not all applications can be packaged this way, only those relying on a select
+number of supported systems. Read about the full list of importers in the
[guix
+import
section](https://www.gnu.org/software/guix/manual/en/html_node/Invoking-guix-import.html)
of the manual.
+
+
+## Automatic update
+
+Guix can be smart enough to check for updates on systems it knows. It can
+report outdated package definitions with
+
+```sh
+ $ guix refresh hello
+```
+
+In most cases, updating a package to a newer version requires little more than
+changing the version number and the checksum. Guix can do that automatically
as
+well:
+
+```sh
+ $ guix refresh hello --update
+```
+
+
+## Inheritance
+
+If you've started browsing the existing package definitions, you might have
+noticed that a significant number of them have a `inherit` field:
+
+```scheme
+ (define-public adwaita-icon-theme
+ (package (inherit gnome-icon-theme)
+ (name "adwaita-icon-theme")
+ (version "3.26.1")
+ (source (origin
+ (method url-fetch)
+ (uri (string-append "mirror://gnome/sources/" name "/"
+ (version-major+minor version) "/"
+ name "-" version ".tar.xz"))
+ (sha256
+ (base32
+ "17fpahgh5dyckgz7rwqvzgnhx53cx9kr2xw0szprc6bnqy977fi8"))))
+ (native-inputs
+ `(("gtk-encode-symbolic-svg" ,gtk+ "bin")))))
+```
+
+All unspecified fields are inherited from the parent package. This is very
+convenient to create alternative packages, for instance with different source,
+version or compilation options.
+
+
+# Getting help
+
+Sadly, some applications can be tough to package. Sometimes they need a patch
to
+work with the non-standard filesystem hierarchy enforced by the store.
+Sometimes the tests won't run properly. (They can be skipped but this is not
+recommended.) Other times the resulting package won't be reproducible.
+
+Should you be stuck, unable to figure out how to fix any sort of packaging
+issue, don't hesitate to ask the community for help.
+
+See the [Guix homepage](https://www.gnu.org/software/guix/contact/) for
information on the mailing lists, IRC, etc.
+
+
+# Conclusion
+
+This tutorial was an showcase of the sophisticated package management that Guix
+boasts. At this point we have mostly restricted this introduction to the
+`gnu-build-system` which is a core abstraction layer on which more advanced
+abstractions are based.
+
+Now where do we go from here? Next we ought to dissect the innards of the
build
+system by removing all abstractions, using the `trivial-build-system`: this
+should give us a thorough understanding of the process before investigating
some
+more advanced packaging techniques and edge cases.
+
+Other features worth exploring are the interactive editing and debugging
+capabilities of Guix provided by the Guile REPL.
+
+Those fancy features are no strict requirements and this is a good point to
stop
+for now. With what we've introduced here you should be well armed to package
+lots of programs. You can get started right away and hopefully we will see
your
+contributions soon!
+
+
+# References
+
+- The [package reference in the
manual](https://www.gnu.org/software/guix/manual/en/html_node/Defining-Packages.html)
+
+- [Pjotr’s hacking guide to GNU
Guix](https://gitlab.com/pjotrp/guix-notes/blob/master/HACKING.org)
+
+- "Guix Guix: Package without a scheme!", by Andreas Enge (in
+ `guix-maintenance.git/talks/ghm-2013/andreas/slides.pdf`)
+
+
+# About GNU Guix
+
+[GNUÂ Guix](https://www.gnu.org/software/guix) is a transactional package
manager for the GNU system. The Guix System
+Distribution or GuixSD is an advanced distribution of the GNU system that
relies
+on GNU Guix and [respects the user's
freedom](https://www.gnu.org/distros/free-system-distribution-guidelines.html).
+
+In addition to standard package management features, Guix supports
transactional
+upgrades and roll-backs, unprivileged package management, per-user profiles,
and
+garbage collection. Guix uses low-level mechanisms from the Nix package
+manager, except that packages are defined as native
[Guile](https://www.gnu.org/software/guile) modules, using
+extensions to the [Scheme](http://schemers.org) language. GuixSD offers a
declarative approach to
+operating system configuration management, and is highly customizable and
+hackable.
+
+GuixSD can be used on an i686, x86_64 and armv7 machines. It is also possible
+to use Guix on top of an already installed GNU/Linux system, including on
+mips64el and aarch64.