@node Contributing
@chapter Contributing

This project is a cooperative effort, and we need your help to make it
grow!  Please get in touch with us on @email{guix-devel@@gnu.org} and
@code{#guix} on the Freenode IRC network.  We welcome ideas, bug
reports, patches, and anything that may be helpful to the project.  We
particularly welcome help on packaging (@pxref{Packaging Guidelines}).

@menu
* Building from Git::           The latest and greatest.
* Running Guix Before It Is Installed::  Hacker tricks.
* The Perfect Setup::           The right tools.
* Coding Style::                Hygiene of the contributor.
* Submitting Patches::          Share your work.
@end menu

@node Building from Git
@section Building from Git

If you want to hack Guix itself, it is recommended to use the latest
version from the Git repository.  When building Guix from a checkout,
the following packages are required in addition to those mentioned in
the installation instructions (@pxref{Requirements}).

@itemize
@item @url{http://gnu.org/software/autoconf/, GNU Autoconf};
@item @url{http://gnu.org/software/automake/, GNU Automake};
@item @url{http://gnu.org/software/gettext/, GNU Gettext};
@item @url{http://gnu.org/software/texinfo/, GNU Texinfo};
@item @url{http://www.graphviz.org/, Graphviz};
@item @url{http://www.gnu.org/software/help2man/, GNU Help2man (optional)}.
@end itemize

Run @command{./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.

@noindent
If you get an error like this one:

@example
configure.ac:46: error: possibly undefined macro: PKG_CHECK_MODULES
@end example

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

@example
export ACLOCAL_PATH=/usr/share/aclocal
@end example

See @pxref{Macro Search Path,,, automake, The GNU Automake Manual} for
more information.

Then, run @command{./configure} as usual.

Finally, you have to invoke @code{make check} to run tests.  If anything
fails, take a look at installation instructions (@pxref{Installation})
or send a message to the @email{guix-devel@@gnu.org, mailing list}.


@node Running Guix Before It Is Installed
@section Running Guix Before It Is Installed

In order to keep a sane working environment, you will find it useful to
test the changes made in your local source tree checkout without
actually installing them.  So that you can distinguish between your
``end-user'' hat and your ``motley'' costume.

To that end, all the command-line tools can be used even if you have not
run @code{make install}.  To do that, prefix each command with
@command{./pre-inst-env} (the @file{pre-inst-env} script lives in the
top build tree of Guix), as in:

@example
$ sudo ./pre-inst-env guix-daemon --build-users-group=guixbuild
$ ./pre-inst-env guix build hello
@end example

However, note that @command{./pre-inst-env guix pull} will not upgrade your
local source tree@footnote{@code{guix pull} fetches the latest guix source,
compiles it, put it to the store and link "~/.config/guix/latest" to it.}.
You should run @command{git pull} instead
if you want to upgrade your local source tree.

@noindent
Similarly, for a Guile session using the Guix modules:

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

;;; ("x86_64-linux")
@end example

@noindent
@cindex REPL
@cindex read-eval-print loop
@dots{} and for a REPL (@pxref{Using Guile Interactively,,, guile, Guile
Reference Manual}):

@example
$ ./pre-inst-env guile
scheme@@(guile-user)> ,use(guix)
scheme@@(guile-user)> ,use(gnu)
scheme@@(guile-user)> (define snakes
                       (fold-packages
                         (lambda (package lst)
                           (if (string-prefix? "python"
                                               (package-name package))
                               (cons package lst)
                               lst))
                         '()))
scheme@@(guile-user)> (length snakes)
$1 = 361
@end example

The @command{pre-inst-env} script sets up all the environment variables
necessary to support this, including @env{PATH} and @env{GUILE_LOAD_PATH}.


@node The Perfect Setup
@section The Perfect Setup

The Perfect Setup to hack on Guix is basically the perfect setup used
for Guile hacking (@pxref{Using Guile in Emacs,,, guile, Guile Reference
Manual}).  First, you need more than an editor, you need
@url{http://www.gnu.org/software/emacs, Emacs}, empowered by the
wonderful @url{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,
@kbd{M-.} to jump to an object definition, a REPL to try out your code,
and more (@pxref{Introduction,,, geiser, Geiser User Manual}).  For
convenient Guix development, make sure to augment Guile’s load path so
that it finds source files from your checkout:

@lisp
;; @r{Assuming the Guix checkout is in ~/src/guix.}
(add-to-list 'geiser-guile-load-path "~/src/guix")
@end lisp

To actually edit the code, Emacs already has a neat Scheme mode.  But in
addition to that, you must not miss
@url{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.

GNU Guix also comes with a minor mode that provides some additional
functionality for Scheme buffers (@pxref{Emacs Development}).


@node Coding Style
@section Coding Style

In general our code follows the GNU Coding Standards (@pxref{Top,,,
standards, GNU Coding Standards}).  However, they do not say much about
Scheme, so here are some additional rules.

@menu
* Programming Paradigm::        How to compose your elements.
* Modules::                     Where to store your code?
* Data Types and Pattern Matching::  Implementing data structures.
* Formatting Code::             Writing conventions.
@end menu

@node Programming Paradigm
@subsection 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 @code{memoize} procedure.

@node Modules
@subsection Modules

Guile modules that are meant to be used on the builder side must live in
the @code{(guix build @dots{})} 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
@code{(gnu @dots{})} name space rather than @code{(guix @dots{})}.

@node Data Types and Pattern Matching
@subsection 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 @code{car}, @code{cdr},
@code{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
@code{define-record-type*}) rather than abuse lists.  In addition, it
should use pattern matching, via Guile’s @code{(ice-9 match)} module,
especially when matching lists.

@node Formatting Code
@subsection Formatting Code

When writing Scheme code, we follow common wisdom among Scheme
programmers.  In general, we follow the
@url{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 @code{substitute*}
macro, have special indentation rules.  These are defined in the
@file{.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
@code{(guix build @dots{})} name space, though.

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


@node Submitting Patches
@section 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 @code{git
format-patch} sent to the @email{guix-devel@@gnu.org, mailing list}.
Please write commit logs in the ChangeLog format (@pxref{Change Logs,,,
standards, GNU Coding Standards}); you can check the commit history for
examples.

Before submitting a patch that adds or modifies a package definition,
please run through this check list:

@enumerate
@item
Take some time to provide an adequate synopsis and description for the
package.  @xref{Synopses and Descriptions}, for some guidelines.

@item
Run @code{guix lint @var{package}}, where @var{package} is the
name of the new or modified package, and fix any errors it reports
(@pxref{Invoking guix lint}).

@item
Make sure the package builds on your platform, using @code{guix build
@var{package}}.

@item
Take a look at the profile reported by @command{guix size}
(@pxref{Invoking guix size}).  This will allow you to notice references
to other packages unwillingly retained.  It may also help determine
whether to split the package (@pxref{Packages with Multiple Outputs}),
and which optional dependencies should be used.

@item
For important changes, check that dependent package (if applicable) are
not affected by the change; @code{guix refresh --list-dependent
@var{package}} will help you do that (@pxref{Invoking guix refresh}).

@item
Check whether the package's build process is deterministic.  This
typically means checking whether an independent build of the package
yields the exact same result that you obtained, bit for bit.

A simple way to do that is with @command{guix challenge}
(@pxref{Invoking guix challenge}).  You may run it once the package has
been committed and built by @code{hydra.gnu.org} to check whether it
obtains the same result as you did.  Better yet: Find another machine
that can build it and run @command{guix publish}.

@end enumerate

When posting a patch to the mailing list, use @samp{[PATCH] @dots{}} as a
subject.  You may use your email client or the @command{git send-mail}
command.