<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="module-services-emacs">
 <title>Emacs</title>
<!--
    Documentation contributors:
      Damien Cassou @DamienCassou
      Thomas Tuegel @ttuegel
      Rodney Lorrimar @rvl
      Adam Hoese @adisbladis
  -->
 <para>
  <link xlink:href="https://www.gnu.org/software/emacs/">Emacs</link> is an
  extensible, customizable, self-documenting real-time display editor — and
  more. At its core is an interpreter for Emacs Lisp, a dialect of the Lisp
  programming language with extensions to support text editing.
 </para>
 <para>
  Emacs runs within a graphical desktop environment using the X Window System,
  but works equally well on a text terminal. Under
  <productname>macOS</productname>, a "Mac port" edition is available, which
  uses Apple's native GUI frameworks.
 </para>
 <para>
  <productname>Nixpkgs</productname> provides a superior environment for
  running <application>Emacs</application>. It's simple to create custom builds
  by overriding the default packages. Chaotic collections of Emacs Lisp code
  and extensions can be brought under control using declarative package
  management. <productname>NixOS</productname> even provides a
  <command>systemd</command> user service for automatically starting the Emacs
  daemon.
 </para>
 <section xml:id="module-services-emacs-installing">
  <title>Installing <application>Emacs</application></title>

  <para>
   Emacs can be installed in the normal way for Nix (see
   <xref linkend="sec-package-management" />). In addition, a NixOS
   <emphasis>service</emphasis> can be enabled.
  </para>

  <section xml:id="module-services-emacs-releases">
   <title>The Different Releases of Emacs</title>

   <para>
    <productname>Nixpkgs</productname> defines several basic Emacs packages.
    The following are attributes belonging to the <varname>pkgs</varname> set:
    <variablelist>
     <varlistentry>
      <term>
       <varname>emacs</varname>
      </term>
      <term>
       <varname>emacs25</varname>
      </term>
      <listitem>
       <para>
        The latest stable version of Emacs 25 using the
        <link
                xlink:href="http://www.gtk.org">GTK 2</link>
        widget toolkit.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>
       <varname>emacs25-nox</varname>
      </term>
      <listitem>
       <para>
        Emacs 25 built without any dependency on X11 libraries.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>
       <varname>emacsMacport</varname>
      </term>
      <term>
       <varname>emacs25Macport</varname>
      </term>
      <listitem>
       <para>
        Emacs 25 with the "Mac port" patches, providing a more native look and
        feel under macOS.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    If those aren't suitable, then the following imitation Emacs editors are
    also available in Nixpkgs:
    <link xlink:href="https://www.gnu.org/software/zile/">Zile</link>,
    <link xlink:href="http://homepage.boetes.org/software/mg/">mg</link>,
    <link xlink:href="http://yi-editor.github.io/">Yi</link>,
    <link xlink:href="https://joe-editor.sourceforge.io/">jmacs</link>.
   </para>
  </section>

  <section xml:id="module-services-emacs-adding-packages">
   <title>Adding Packages to Emacs</title>

   <para>
    Emacs includes an entire ecosystem of functionality beyond text editing,
    including a project planner, mail and news reader, debugger interface,
    calendar, and more.
   </para>

   <para>
    Most extensions are gotten with the Emacs packaging system
    (<filename>package.el</filename>) from
    <link
        xlink:href="https://elpa.gnu.org/">Emacs Lisp Package Archive
    (<acronym>ELPA</acronym>)</link>,
    <link xlink:href="https://melpa.org/"><acronym>MELPA</acronym></link>,
    <link xlink:href="https://stable.melpa.org/">MELPA Stable</link>, and
    <link xlink:href="http://orgmode.org/elpa.html">Org ELPA</link>. Nixpkgs is
    regularly updated to mirror all these archives.
   </para>

   <para>
    Under NixOS, you can continue to use
    <function>package-list-packages</function> and
    <function>package-install</function> to install packages. You can also
    declare the set of Emacs packages you need using the derivations from
    Nixpkgs. The rest of this section discusses declarative installation of
    Emacs packages through nixpkgs.
   </para>

   <para>
    The first step to declare the list of packages you want in your Emacs
    installation is to create a dedicated derivation. This can be done in a
    dedicated <filename>emacs.nix</filename> file such as:
    <example xml:id="ex-emacsNix">
     <title>Nix expression to build Emacs with packages (<filename>emacs.nix</filename>)</title>
<programlisting language="nix">
/*
This is a nix expression to build Emacs and some Emacs packages I like
from source on any distribution where Nix is installed. This will install
all the dependencies from the nixpkgs repository and build the binary files
without interfering with the host distribution.

To build the project, type the following from the current directory:

$ nix-build emacs.nix

To run the newly compiled executable:

$ ./result/bin/emacs
*/
{ pkgs ? import &lt;nixpkgs&gt; {} }: <co xml:id="ex-emacsNix-1" />

let
  myEmacs = pkgs.emacs; <co xml:id="ex-emacsNix-2" />
  emacsWithPackages = (pkgs.emacsPackagesGen myEmacs).emacsWithPackages; <co xml:id="ex-emacsNix-3" />
in
  emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [ <co xml:id="ex-emacsNix-4" />
    magit          # ; Integrate git &lt;C-x g&gt;
    zerodark-theme # ; Nicolas' theme
  ]) ++ (with epkgs.melpaPackages; [ <co xml:id="ex-emacsNix-5" />
    undo-tree      # ; &lt;C-x u&gt; to show the undo tree
    zoom-frm       # ; increase/decrease font size for all buffers %lt;C-x C-+&gt;
  ]) ++ (with epkgs.elpaPackages; [ <co xml:id="ex-emacsNix-6" />
    auctex         # ; LaTeX mode
    beacon         # ; highlight my cursor when scrolling
    nameless       # ; hide current package name everywhere in elisp code
  ]) ++ [
    pkgs.notmuch   # From main packages set <co xml:id="ex-emacsNix-7" />
  ])
</programlisting>
    </example>
    <calloutlist>
     <callout arearefs="ex-emacsNix-1">
      <para>
       The first non-comment line in this file (<literal>{ pkgs ? ...
       }</literal>) indicates that the whole file represents a function.
      </para>
     </callout>
     <callout arearefs="ex-emacsNix-2">
      <para>
       The <varname>let</varname> expression below defines a
       <varname>myEmacs</varname> binding pointing to the current stable
       version of Emacs. This binding is here to separate the choice of the
       Emacs binary from the specification of the required packages.
      </para>
     </callout>
     <callout arearefs="ex-emacsNix-3">
      <para>
       This generates an <varname>emacsWithPackages</varname> function. It
       takes a single argument: a function from a package set to a list of
       packages (the packages that will be available in Emacs).
      </para>
     </callout>
     <callout arearefs="ex-emacsNix-4">
      <para>
       The rest of the file specifies the list of packages to install. In the
       example, two packages (<varname>magit</varname> and
       <varname>zerodark-theme</varname>) are taken from MELPA stable.
      </para>
     </callout>
     <callout arearefs="ex-emacsNix-5">
      <para>
       Two packages (<varname>undo-tree</varname> and
       <varname>zoom-frm</varname>) are taken from MELPA.
      </para>
     </callout>
     <callout arearefs="ex-emacsNix-6">
      <para>
       Three packages are taken from GNU ELPA.
      </para>
     </callout>
     <callout arearefs="ex-emacsNix-7">
      <para>
       <varname>notmuch</varname> is taken from a nixpkgs derivation which
       contains an Emacs mode.
      </para>
     </callout>
    </calloutlist>
   </para>

   <para>
    The result of this configuration will be an <command>emacs</command>
    command which launches Emacs with all of your chosen packages in the
    <varname>load-path</varname>.
   </para>

   <para>
    You can check that it works by executing this in a terminal:
<screen>
<prompt>$ </prompt>nix-build emacs.nix
<prompt>$ </prompt>./result/bin/emacs -q
</screen>
    and then typing <literal>M-x package-initialize</literal>. Check that you
    can use all the packages you want in this Emacs instance. For example, try
    switching to the zerodark theme through <literal>M-x load-theme &lt;RET&gt;
    zerodark &lt;RET&gt; y</literal>.
   </para>

   <tip>
    <para>
     A few popular extensions worth checking out are: auctex, company,
     edit-server, flycheck, helm, iedit, magit, multiple-cursors, projectile,
     and yasnippet.
    </para>
   </tip>

   <para>
    The list of available packages in the various ELPA repositories can be seen
    with the following commands:
    <example xml:id="module-services-emacs-querying-packages">
     <title>Querying Emacs packages</title>
<programlisting><![CDATA[
nix-env -f "<nixpkgs>" -qaP -A emacsPackages.elpaPackages
nix-env -f "<nixpkgs>" -qaP -A emacsPackages.melpaPackages
nix-env -f "<nixpkgs>" -qaP -A emacsPackages.melpaStablePackages
nix-env -f "<nixpkgs>" -qaP -A emacsPackages.orgPackages
]]></programlisting>
    </example>
   </para>

   <para>
    If you are on NixOS, you can install this particular Emacs for all users by
    adding it to the list of system packages (see
    <xref linkend="sec-declarative-package-mgmt" />). Simply modify your file
    <filename>configuration.nix</filename> to make it contain:
    <example xml:id="module-services-emacs-configuration-nix">
     <title>Custom Emacs in <filename>configuration.nix</filename></title>
<programlisting><![CDATA[
{
 environment.systemPackages = [
   # [...]
   (import /path/to/emacs.nix { inherit pkgs; })
  ];
}
]]></programlisting>
    </example>
   </para>

   <para>
    In this case, the next <command>nixos-rebuild switch</command> will take
    care of adding your <command>emacs</command> to the <varname>PATH</varname>
    environment variable (see <xref linkend="sec-changing-config" />).
   </para>

<!-- fixme: i think the following is better done with config.nix
https://nixos.org/nixpkgs/manual/#sec-modify-via-packageOverrides
-->

   <para>
    If you are not on NixOS or want to install this particular Emacs only for
    yourself, you can do so by adding it to your
    <filename>~/.config/nixpkgs/config.nix</filename> (see
    <link xlink:href="http://nixos.org/nixpkgs/manual/#sec-modify-via-packageOverrides">Nixpkgs
    manual</link>):
    <example xml:id="module-services-emacs-config-nix">
     <title>Custom Emacs in <filename>~/.config/nixpkgs/config.nix</filename></title>
<programlisting><![CDATA[
{
  packageOverrides = super: let self = super.pkgs; in {
    myemacs = import /path/to/emacs.nix { pkgs = self; };
  };
}
]]></programlisting>
    </example>
   </para>

   <para>
    In this case, the next <literal>nix-env -f '&lt;nixpkgs&gt;' -iA
    myemacs</literal> will take care of adding your emacs to the
    <varname>PATH</varname> environment variable.
   </para>
  </section>

  <section xml:id="module-services-emacs-advanced">
   <title>Advanced Emacs Configuration</title>

   <para>
    If you want, you can tweak the Emacs package itself from your
    <filename>emacs.nix</filename>. For example, if you want to have a
    GTK 3-based Emacs instead of the default GTK 2-based binary and remove the
    automatically generated <filename>emacs.desktop</filename> (useful is you
    only use <command>emacsclient</command>), you can change your file
    <filename>emacs.nix</filename> in this way:
   </para>

   <example xml:id="ex-emacsGtk3Nix">
    <title>Custom Emacs build</title>
<programlisting><![CDATA[
{ pkgs ? import <nixpkgs> {} }:
let
  myEmacs = (pkgs.emacs.override {
    # Use gtk3 instead of the default gtk2
    withGTK3 = true;
    withGTK2 = false;
  }).overrideAttrs (attrs: {
    # I don't want emacs.desktop file because I only use
    # emacsclient.
    postInstall = (attrs.postInstall or "") + ''
      rm $out/share/applications/emacs.desktop
    '';
  });
in [...]
]]></programlisting>
   </example>

   <para>
    After building this file as shown in <xref linkend="ex-emacsNix" />, you
    will get an GTK 3-based Emacs binary pre-loaded with your favorite packages.
   </para>
  </section>
 </section>
 <section xml:id="module-services-emacs-running">
  <title>Running Emacs as a Service</title>

  <para>
   <productname>NixOS</productname> provides an optional
   <command>systemd</command> service which launches
   <link xlink:href="https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Server.html">
   Emacs daemon </link> with the user's login session.
  </para>

  <para>
   <emphasis>Source:</emphasis>
   <filename>modules/services/editors/emacs.nix</filename>
  </para>

  <section xml:id="module-services-emacs-enabling">
   <title>Enabling the Service</title>

   <para>
    To install and enable the <command>systemd</command> user service for Emacs
    daemon, add the following to your <filename>configuration.nix</filename>:
<programlisting>
<xref linkend="opt-services.emacs.enable"/> = true;
<xref linkend="opt-services.emacs.package"/> = import /home/cassou/.emacs.d { pkgs = pkgs; };
</programlisting>
   </para>

   <para>
    The <varname>services.emacs.package</varname> option allows a custom
    derivation to be used, for example, one created by
    <function>emacsWithPackages</function>.
   </para>

   <para>
    Ensure that the Emacs server is enabled for your user's Emacs
    configuration, either by customizing the <varname>server-mode</varname>
    variable, or by adding <literal>(server-start)</literal> to
    <filename>~/.emacs.d/init.el</filename>.
   </para>

   <para>
    To start the daemon, execute the following:
<screen>
<prompt>$ </prompt>nixos-rebuild switch  # to activate the new configuration.nix
<prompt>$ </prompt>systemctl --user daemon-reload        # to force systemd reload
<prompt>$ </prompt>systemctl --user start emacs.service  # to start the Emacs daemon
</screen>
    The server should now be ready to serve Emacs clients.
   </para>
  </section>

  <section xml:id="module-services-emacs-starting-client">
   <title>Starting the client</title>

   <para>
    Ensure that the emacs server is enabled, either by customizing the
    <varname>server-mode</varname> variable, or by adding
    <literal>(server-start)</literal> to <filename>~/.emacs</filename>.
   </para>

   <para>
    To connect to the emacs daemon, run one of the following:
<programlisting><![CDATA[
emacsclient FILENAME
emacsclient --create-frame  # opens a new frame (window)
emacsclient --create-frame --tty  # opens a new frame on the current terminal
]]></programlisting>
   </para>
  </section>

  <section xml:id="module-services-emacs-editor-variable">
   <title>Configuring the <varname>EDITOR</varname> variable</title>

<!--<title><command>emacsclient</command> as the Default Editor</title>-->

   <para>
    If <xref linkend="opt-services.emacs.defaultEditor"/> is
    <literal>true</literal>, the <varname>EDITOR</varname> variable will be set
    to a wrapper script which launches <command>emacsclient</command>.
   </para>

   <para>
    Any setting of <varname>EDITOR</varname> in the shell config files will
    override <varname>services.emacs.defaultEditor</varname>. To make sure
    <varname>EDITOR</varname> refers to the Emacs wrapper script, remove any
    existing <varname>EDITOR</varname> assignment from
    <filename>.profile</filename>, <filename>.bashrc</filename>,
    <filename>.zshenv</filename> or any other shell config file.
   </para>

   <para>
    If you have formed certain bad habits when editing files, these can be
    corrected with a shell alias to the wrapper script:
<programlisting>alias vi=$EDITOR</programlisting>
   </para>
  </section>

  <section xml:id="module-services-emacs-per-user">
   <title>Per-User Enabling of the Service</title>

   <para>
    In general, <command>systemd</command> user services are globally enabled
    by symlinks in <filename>/etc/systemd/user</filename>. In the case where
    Emacs daemon is not wanted for all users, it is possible to install the
    service but not globally enable it:
<programlisting>
<xref linkend="opt-services.emacs.enable"/> = false;
<xref linkend="opt-services.emacs.install"/> = true;
</programlisting>
   </para>

   <para>
    To enable the <command>systemd</command> user service for just the
    currently logged in user, run:
<programlisting>systemctl --user enable emacs</programlisting>
    This will add the symlink
    <filename>~/.config/systemd/user/emacs.service</filename>.
   </para>
  </section>
 </section>
 <section xml:id="module-services-emacs-configuring">
  <title>Configuring Emacs</title>

  <para>
   The Emacs init file should be changed to load the extension packages at
   startup:
   <example xml:id="module-services-emacs-package-initialisation">
    <title>Package initialization in <filename>.emacs</filename></title>
<programlisting><![CDATA[
(require 'package)

;; optional. makes unpure packages archives unavailable
(setq package-archives nil)

(setq package-enable-at-startup nil)
(package-initialize)
]]></programlisting>
   </example>
  </para>

  <para>
   After the declarative emacs package configuration has been tested,
   previously downloaded packages can be cleaned up by removing
   <filename>~/.emacs.d/elpa</filename> (do make a backup first, in case you
   forgot a package).
  </para>

<!--
      todo: is it worth documenting customizations for
      server-switch-hook, server-done-hook?
  -->

  <section xml:id="module-services-emacs-major-mode">
   <title>A Major Mode for Nix Expressions</title>

   <para>
    Of interest may be <varname>melpaPackages.nix-mode</varname>, which
    provides syntax highlighting for the Nix language. This is particularly
    convenient if you regularly edit Nix files.
   </para>
  </section>

  <section xml:id="module-services-emacs-man-pages">
   <title>Accessing man pages</title>

   <para>
    You can use <function>woman</function> to get completion of all available
    man pages. For example, type <literal>M-x woman &lt;RET&gt; nixos-rebuild
    &lt;RET&gt;.</literal>
   </para>
  </section>

  <section xml:id="sec-emacs-docbook-xml">
   <title>Editing DocBook 5 XML Documents</title>

   <para>
    Emacs includes
    <link
      xlink:href="https://www.gnu.org/software/emacs/manual/html_node/nxml-mode/Introduction.html">nXML</link>,
    a major-mode for validating and editing XML documents. When editing DocBook
    5.0 documents, such as <link linkend="book-nixos-manual">this one</link>,
    nXML needs to be configured with the relevant schema, which is not
    included.
   </para>

   <para>
    To install the DocBook 5.0 schemas, either add
    <varname>pkgs.docbook5</varname> to
    <xref linkend="opt-environment.systemPackages"/>
    (<link
      linkend="sec-declarative-package-mgmt">NixOS</link>), or run
    <literal>nix-env -f '&lt;nixpkgs&gt;' -iA docbook5</literal>
    (<link linkend="sec-ad-hoc-packages">Nix</link>).
   </para>

   <para>
    Then customize the variable <varname>rng-schema-locating-files</varname> to
    include <filename>~/.emacs.d/schemas.xml</filename> and put the following
    text into that file:
    <example xml:id="ex-emacs-docbook-xml">
     <title>nXML Schema Configuration (<filename>~/.emacs.d/schemas.xml</filename>)</title>
<programlisting language="xml"><![CDATA[
<?xml version="1.0"?>
<!--
  To let emacs find this file, evaluate:
  (add-to-list 'rng-schema-locating-files "~/.emacs.d/schemas.xml")
-->
<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <!--
    Use this variation if pkgs.docbook5 is added to environment.systemPackages
  -->
  <namespace ns="http://docbook.org/ns/docbook"
             uri="/run/current-system/sw/share/xml/docbook-5.0/rng/docbookxi.rnc"/>
  <!--
    Use this variation if installing schema with "nix-env -iA pkgs.docbook5".
  <namespace ns="http://docbook.org/ns/docbook"
             uri="../.nix-profile/share/xml/docbook-5.0/rng/docbookxi.rnc"/>
  -->
</locatingRules>
]]></programlisting>
    </example>
   </para>
  </section>
 </section>
</chapter>
