<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
  -->

  <para>
    <link xlink:href="http://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>
    <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>
      <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>.
      </para>

    </section>
    <section>
      <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>

      <note>
        <para>
          This documentation describes the new Emacs packages
          framework in NixOS 16.03
          (<varname>emacsPackagesNg</varname>) which should not be
          confused with the previous and deprecated framework
          (<varname>emacs24Packages</varname>).
        </para>
      </note>

      <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.emacsPackagesNgGen 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>
$ nix-build emacs.nix
$ ./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>
          <title>Querying Emacs packages</title>
          <programlisting><![CDATA[
nix-env -f "<nixpkgs>" -qaP -A emacsPackagesNg.elpaPackages
nix-env -f "<nixpkgs>" -qaP -A emacsPackagesNg.melpaPackages
nix-env -f "<nixpkgs>" -qaP -A emacsPackagesNg.melpaStablePackages
nix-env -f "<nixpkgs>" -qaP -A emacsPackagesNg.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>
          <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>
          <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>
      <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 GTK3-based Emacs binary pre-loaded with your
        favorite packages.
      </para>
    </section>
  </section>

<section>
  <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>
    <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><![CDATA[
services.emacs.enable = true;
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>
$ nixos-rebuild switch  # to activate the new configuration.nix
$ systemctl --user daemon-reload        # to force systemd reload
$ systemctl --user start emacs.service  # to start the Emacs daemon
</screen>

      The server should now be ready to serve Emacs clients.
    </para>

  </section>

  <section>
    <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>
    <title>Configuring the <varname>EDITOR</varname> variable</title>
    <!--<title><command>emacsclient</command> as the Default Editor</title>-->

    <para>
      If <varname>services.emacs.defaultEditor</varname> 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>
    <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><![CDATA[
services.emacs.enable = false;
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>
  <title>Configuring Emacs</title>

  <para>
    The Emacs init file should be changed to load the extension
    packages at startup:

    <example>
      <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>
    <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>
    <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
      <varname>environment.systemPackages</varname> (<link
      linkend="sec-declarative-package-mgmt">NixOS</link>), or run
      <literal>nix-env -i pkgs.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>
