doc: Turn "Creating a Channel" into a step-by-step guide.

* doc/guix.texi (Creating a Channel): Rewrite as a step-by-step guide.
Move warning below and shorten it.
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@ -5771,18 +5771,99 @@ machines with limited resources.
Let's say you have a bunch of custom package variants or personal packages Let's say you have a bunch of custom package variants or personal packages
that you think would make little sense to contribute to the Guix project, but that you think would make little sense to contribute to the Guix project, but
would like to have these packages transparently available to you at the would like to have these packages transparently available to you at the
command line. You would first write modules containing those package command line. By creating a @dfn{channel}, you can use and publish such
definitions (@pxref{Package Modules}), maintain them in a Git repository, and a package collection. This involves the following steps:
then you and anyone else can use it as an additional channel to get packages
from. Neat, no? @enumerate
@item
Channels live in a Git repository so the first step, when creating a
channel, is to create its repository:
@example
mkdir my-channel
cd my-channel
git init
@end example
@item
The next step is to create files containing package modules
(@pxref{Package Modules}), each of which will contain one or more
package definitions (@pxref{Defining Packages}). A channel can provide
things other than packages, such as build systems or services; we're
using packages as most common use case.
For example, Alice might want to provide a module called @code{(alice
packages greetings)} that will provide her favorite ``hello world''
implementations. To do that Alice will create a directory corresponding
to that module name.
@example
mkdir -p alice/packages
$EDITOR alice/packages/greetings.scm
git add alice/packages/greetings.scm
@end example
You can name your package modules however you like; the main constraint
to keep in mind is to avoid name clashes with other package collections,
which is why our hypothetical Alice wisely chose the @code{(alice
packages @dots{})} name space.
Note that you can also place modules in a sub-directory of the
repository; @pxref{Package Modules in a Sub-directory}, for more info on
that.
@item
With this first module in place, the next step is to test the packages
it provides. This can be done with @command{guix build}, which needs to
be fold to look for modules in the Git checkout. For example, assuming
@code{(alice packages greetings)} provides a package called
@code{hi-from-alice}, Alice will run this command from the Git checkout:
@example
guix build -L. hi-from-alice
@end example
@noindent
... where @code{-L.} adds the current directory to Guile's load path
(@pxref{Load Paths,,, guile, GNU Guile Reference Manual}).
@item
It might take Alice a few iterations to obtain satisfying package
definitions. Eventually Alice will commit this file:
@example
git commit
@end example
As a channel author, consider bundling authentication material with your
channel so that users can authenticate it. @xref{Channel
Authentication}, and @ref{Specifying Channel Authorizations}, for info
on how to do it.
@item
To use Alice's channel, anyone can now add it to their channel file
(@pxref{Specifying Additional Channels}) and run @command{guix pull}
(@pxref{Invoking guix pull}):
@example
$EDITOR ~/.config/guix/channels.scm
guix pull
@end example
Guix will now behave as if the root directory of that channel's Git
repository had been permanently added to the Guile load path. In this
example, @code{(alice packages greetings)} will automatically be found
by the @command{guix} command.
@end enumerate
Voilà!
@c What follows stems from discussions at @c What follows stems from discussions at
@c <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22629#134> as well as @c <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22629#134> as well as
@c earlier discussions on guix-devel@gnu.org. @c earlier discussions on guix-devel@gnu.org.
@quotation Warning @quotation Warning
Before you, dear user, shout---``woow this is @emph{soooo coool}!''---and Before you publish your channel, we would like to share a few words of
publish your personal channel to the world, we would like to share a few words caution:
of caution:
@itemize @itemize
@item @item
@ -5793,13 +5874,11 @@ available to all Guix users and benefit from the project's quality assurance
process. process.
@item @item
When you maintain package definitions outside Guix, we, Guix developers, Package modules and package definitions are Scheme code that uses
consider that @emph{the compatibility burden is on you}. Remember that various programming interfaces (APIs). We, Guix developers, never
package modules and package definitions are just Scheme code that uses various change APIs gratuitously, but we do @emph{not} commit to freezing APIs
programming interfaces (APIs). We want to remain free to change these APIs to either. When you maintain package definitions outside Guix, we consider
keep improving Guix, possibly in ways that break your channel. We never that @emph{the compatibility burden is on you}.
change APIs gratuitously, but we will @emph{not} commit to freezing APIs
either.
@item @item
Corollary: if you're using an external channel and that channel breaks, please Corollary: if you're using an external channel and that channel breaks, please
@ -5813,21 +5892,6 @@ share your improvements, which are basic tenets of
email us at @email{guix-devel@@gnu.org} if you'd like to discuss this. email us at @email{guix-devel@@gnu.org} if you'd like to discuss this.
@end quotation @end quotation
To create a channel, create a Git repository containing your own package
modules and make it available. The repository can contain anything, but a
useful channel will contain Guile modules that export packages. Once you
start using a channel, Guix will behave as if the root directory of that
channel's Git repository has been added to the Guile load path (@pxref{Load
Paths,,, guile, GNU Guile Reference Manual}). For example, if your channel
contains a file at @file{my-packages/my-tools.scm} that defines a Guile
module, then the module will be available under the name @code{(my-packages
my-tools)}, and you will be able to use it like any other module
(@pxref{Modules,,, guile, GNU Guile Reference Manual}).
As a channel author, consider bundling authentication material with your
channel so that users can authenticate it. @xref{Channel
Authentication}, and @ref{Specifying Channel Authorizations}, for info
on how to do it.
@node Package Modules in a Sub-directory @node Package Modules in a Sub-directory