guix/HACKING

-*- mode: org; coding: utf-8; -*-

#+TITLE: Hacking GNU Guix and Its Incredible Distro

Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>

  Copying and distribution of this file, with or without modification,
  are permitted in any medium without royalty provided the copyright
  notice and this notice are preserved.


* Building from Git

When building Guix from a checkout, the following packages are required in
addition to those mentioned in the installation instructions:

  - [[http://www.gnu.org/software/autoconf/][GNU Autoconf]]
  - [[http://www.gnu.org/software/automake/][GNU Automake]]
  - [[http://www.gnu.org/software/gettext/][GNU Gettext]]
  - [[http://www.graphviz.org/][Graphviz]]

Run ‘./bootstrap’ to download the Nix daemon source code and to generate the
build system infrastructure using autoconf.  It reports an error if an
inappropriate version of the above packages is being used.

The ‘bootstrap’ script, among other things, invokes ‘git submodule update’; if
you didn’t run it, you may get the following error:

  make: *** No rule to make target `nix/libstore/schema.sql', needed by
  `nix/libstore/schema.sql.hh'

If you get an error like this one:

  configure.ac:46: error: possibly undefined macro: PKG_CHECK_MODULES

it probably means that Autoconf couldn’t find ‘pkg.m4’, which is provided by
pkg-config.  Make sure that ‘pkg.m4’ is available.  For instance, if you
installed Automake in ‘/usr/local’, it wouldn’t look for ‘.m4’ files in
‘/usr/share’.  So you have to invoke the following command in that case

  $ export ACLOCAL_PATH=/usr/share/aclocal

See “info '(automake) Macro Search Path'” for more information.

Then, run ‘./configure’ as usual.

Finally, you have to invoke ‘make check’ to run tests.  If anything fails,
take a look at “info '(guix) Installation'” or send a message to
<guix-devel@gnu.org>.

* Running Guix before it is installed

Command-line tools can be used even if you have not run "make install".
To do that, prefix each command with ‘./pre-inst-env’, as in:

  ./pre-inst-env guix build --help

Similarly, for a Guile session using the Guix modules:

  ./pre-inst-env guile -c '(use-modules (guix utils)) (pk (%current-system))'

The ‘pre-inst-env’ script sets up all the environment variables
necessary to support this.

* The Perfect Setup

The Perfect Setup to hack on Guix is basically the perfect setup used
for Guile hacking (info "(guile) Using Guile in Emacs").  First, you
need more than an editor, you need [[http://www.gnu.org/software/emacs][Emacs]], empowered by the wonderful
[[http://nongnu.org/geiser/][Geiser]].

Geiser allows for interactive and incremental development from within
Emacs: code compilation and evaluation from within buffers, access to
on-line documentation (docstrings), context-sensitive completion, M-. to
jump to an object definition, a REPL to try out your code, and more.

To actually edit the code, Emacs already has a neat Scheme mode.  But in
addition to that, you must not miss [[http://www.emacswiki.org/emacs/ParEdit][Paredit]].  It provides facilities to
directly operate on the syntax tree, such as raising an s-expression or
wrapping it, swallowing or rejecting the following s-expression, etc.

* Submitting Patches

Development is done using the Git distributed version control system.  Thus,
access to the repository is not strictly necessary.  We welcome contributions
in the form of patches as produced by ‘git format-patch’ sent to
guix-devel@gnu.org.  Please write commit logs in the [[http://www.gnu.org/prep/standards/html_node/Change-Logs.html#Change-Logs][GNU ChangeLog format]].

As you become a regular contributor, you may find it convenient to have write
access to the repository (see below.)

* Coding Style

In general our code follows the [[info:standards][GNU Coding Standards]] (GCS).  However, the GCS
do not say much about Scheme, so here are some additional rules.

** Programming Paradigm

Scheme code in Guix is written in a purely functional style.  One exception is
code that involves input/output, and procedures that implement low-level
concepts, such as the ‘memoize’ procedure.

** Modules

Guile modules that are meant to be used on the builder side must live in the
(guix build …) name space.  They must not refer to other Guix or GNU modules.
However, it is OK for a “host-side” module to use a build-side module.

Modules that deal with the broader GNU system should be in the (gnu …) name
space rather than (guix …).

** Data Types and Pattern Matching

The tendency in classical Lisp is to use lists to represent everything, and
then to browse them “by hand” using ‘car’, ‘cdr’, ‘cadr’, and co.  There are
several problems with that style, notably the fact that it is hard to read,
error-prone, and a hindrance to proper type error reports.

Guix code should define appropriate data types (for instance, using
‘define-record-type*’) rather than abuse lists.  In addition, it should use
pattern matching, via Guile’s (ice-9 match) module, especially when matching
lists.

** Formatting Code

When writing Scheme code, we follow common wisdom among Scheme programmers.
In general, we follow the [[http://mumble.net/~campbell/scheme/style.txt][Riastradh's Lisp Style Rules]].  This document happens
to describe the conventions mostly used in Guile’s code too.  It is very
thoughtful and well written, so please do read it.

Some special forms introduced in Guix, such as the ‘substitute*’ macro, have
special indentation rules.  These are defined in the .dir-locals.el file,
which Emacs automatically uses.  If you do not use Emacs, please make sure to
let your editor know the rules.

We require all top-level procedures to carry a docstring.  This requirement
can be relaxed for simple private procedures in the (guix build …) name space,
though.

Procedures should not have more than four positional parameters.  Use keyword
parameters for procedures that take more than four parameters.

* Commit Access

For frequent contributors, having write access to the repository is
convenient.  When you deem it necessary, feel free to ask for it on the
mailing list.  When you get commit access, please make sure to follow the
policy below (discussions of the policy can take place on guix-devel@gnu.org.)

Non-trivial patches should always be posted to guix-devel@gnu.org (trivial
patches include fixing typos, etc.)

For patches that just add a new package, and a simple one, it’s OK to commit,
if you’re confident (which means you successfully built it in a chroot setup,
and have done a reasonable copyright and license auditing.)  Likewise for
package upgrades.  We have a mailing list for commit notifications
(guix-commits@gnu.org), so people can notice.  Before pushing your changes,
make sure to run ‘git pull --rebase’.

For anything else, please post to guix-devel@gnu.org and leave time for a
review, without committing anything.  If you didn’t receive any reply
after two weeks, and if you’re confident, it’s OK to commit.

That last part is subject to being adjusted, allowing individuals to commit
directly on non-controversial changes on parts they’re familiar with.