nixos/oh-my-zsh: add documentation

In the last year `programs.oh-my-zsh` gained more complexity and since
the introduction of features like `customPkgs` which builds a
`ZSH_CUSTOM` path from a sequence of derivation a documentation may be
fairly helpful to make the knowledge how to use the module and how to
package new ZSH plugins visible.

See https://github.com/NixOS/nixpkgs/pull/43282#issuecomment-410770432

2 files changed, 127 insertions, 0 deletions

diff --git a/nixos/modules/programs/zsh/oh-my-zsh.nix b/nixos/modules/programs/zsh/oh-my-zsh.nix
index 8435e0eb9f0..f4df4e983e4 100644
--- a/nixos/modules/programs/zsh/oh-my-zsh.nix
+++ b/nixos/modules/programs/zsh/oh-my-zsh.nix
@@ -133,4 +133,6 @@ in
       ];
 
     };
+
+    meta.doc = ./oh-my-zsh.xml;
   }
diff --git a/nixos/modules/programs/zsh/oh-my-zsh.xml b/nixos/modules/programs/zsh/oh-my-zsh.xml
new file mode 100644
index 00000000000..b74da8630ee
--- /dev/null
+++ b/nixos/modules/programs/zsh/oh-my-zsh.xml
@@ -0,0 +1,125 @@
+<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-programs-zsh-ohmyzsh">
+
+<title>Oh my ZSH</title>
+
+<para><literal><link xlink:href="https://ohmyz.sh/">oh-my-zsh</link></literal> is a framework
+to manage your <link xlink:href="https://www.zsh.org/">ZSH</link> configuration
+including completion scripts for several CLI tools or custom prompt themes.</para>
+
+<section><title>Basic usage</title>
+<para>The module uses the <literal>oh-my-zsh</literal> package with all available features.  The
+initial setup using Nix expressions is fairly similar to the configuration format
+of <literal>oh-my-zsh</literal>.
+
+<programlisting>
+{
+  programs.ohMyZsh = {
+    enable = true;
+    plugins = [ "git" "python" "man" ];
+    theme = "agnoster";
+  };
+}
+</programlisting>
+
+For a detailed explanation of these arguments please refer to the
+<link xlink:href="https://github.com/robbyrussell/oh-my-zsh/wiki"><literal>oh-my-zsh</literal> docs</link>.
+</para>
+<para>The expression generates the needed
+configuration and writes it into your <literal>/etc/zshrc</literal>.
+</para></section>
+
+<section><title>Custom additions</title>
+
+<para>Sometimes third-party or custom scripts such as a modified theme may be needed.
+<literal>oh-my-zsh</literal> provides the
+<link xlink:href="https://github.com/robbyrussell/oh-my-zsh/wiki/Customization#overriding-internals"><literal>ZSH_CUSTOM</literal></link> 
+environment variable for this which points to a directory with additional scripts.</para>
+
+<para>The module can do this as well:
+
+<programlisting>
+{
+  programs.ohMyZsh.custom = "~/path/to/custom/scripts";
+}
+</programlisting>
+</para></section>
+
+<section><title>Custom environments</title>
+
+<para>There are several extensions for <literal>oh-my-zsh</literal> packaged in <literal>nixpkgs</literal>.
+One of them is <link xlink:href="https://github.com/spwhitt/nix-zsh-completions">nix-zsh-completions</link>
+which bundles completion scripts and a plugin for <literal>oh-my-zsh</literal>.</para>
+
+<para>Rather than using a single mutable path for <literal>ZSH_CUSTOM</literal>, it's also possible to
+generate this path from a list of Nix packages:
+
+<programlisting>
+{ pkgs, ... }:
+{
+  programs.ohMyZsh.customPkgs = with pkgs; [
+    pkgs.nix-zsh-completions
+    # and even more...
+  ];
+}
+</programlisting>
+
+Internally a single store path will be created using <literal>buildEnv</literal>.
+Please refer to the docs of
+<link xlink:href="https://nixos.org/nixpkgs/manual/#sec-building-environment"><literal>buildEnv</literal></link>
+for further reference.</para>
+
+<para><emphasis>Please keep in mind that this is not compatible with <literal>programs.ohMyZsh.custom</literal>
+as it requires an immutable store path while <literal>custom</literal> shall remain mutable! An evaluation failure
+will be thrown if both <literal>custom</literal> and <literal>customPkgs</literal> are set.</emphasis>
+</para></section>
+
+<section><title>Package your own customizations</title>
+
+<para>If third-party customizations (e.g. new themes) are supposed to be added to <literal>oh-my-zsh</literal>
+there are several pitfalls to keep in mind:</para>
+
+<itemizedlist>
+  <listitem>
+    <para>To comply with the default structure of <literal>ZSH</literal> the entire output needs to be written to
+    <literal>$out/share/zsh.</literal></para>
+  </listitem>
+  <listitem>
+    <para>Completion scripts are supposed to be stored at <literal>$out/share/zsh/site-functions</literal>. This directory
+    is part of the <literal><link xlink:href="http://zsh.sourceforge.net/Doc/Release/Functions.html">fpath</link></literal>
+    and the package should be compatible with pure <literal>ZSH</literal> setups. The module will automatically link
+    the contents of <literal>site-functions</literal> to completions directory in the proper store path.</para>
+  </listitem>
+  <listitem>
+    <para>The <literal>plugins</literal> directory needs the structure <literal>pluginname/pluginname.plugin.zsh</literal>
+    as structured in the <link xlink:href="https://github.com/robbyrussell/oh-my-zsh/tree/91b771914bc7c43dd7c7a43b586c5de2c225ceb7/plugins">upstream repo.</link>
+    </para>
+  </listitem>
+</itemizedlist>
+
+<para>
+A derivation for <literal>oh-my-zsh</literal> may look like this:
+<programlisting>
+{ stdenv, fetchFromGitHub }:
+
+stdenv.mkDerivation rec {
+  name = "exemplary-zsh-customization-${version}";
+  version = "1.0.0";
+  src = fetchFromGitHub {
+    # path to the upstream repository
+  };
+
+  dontBuild = true;
+  installPhase = ''
+    mkdir -p $out/share/zsh/site-functions
+    cp {themes,plugins} $out/share/zsh
+    cp completions $out/share/zsh/site-functions
+  '';
+}
+</programlisting>
+</para>
+</section>
+</chapter>