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diff --git a/nixos/doc/manual/development/option-def.xml b/nixos/doc/manual/development/option-def.xml new file mode 100644 index 000000000000..4e267ecfd1e3 --- /dev/null +++ b/nixos/doc/manual/development/option-def.xml @@ -0,0 +1,112 @@ +<section 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="sec-option-definitions"> + +<title>Option Definitions</title> + +<para>Option definitions are generally straight-forward bindings of values to option names, like + +<programlisting> +config = { + services.httpd.enable = true; +}; +</programlisting> + +However, sometimes you need to wrap an option definition or set of +option definitions in a <emphasis>property</emphasis> to achieve +certain effects:</para> + +<simplesect><title>Delaying Conditionals</title> + +<para>If a set of option definitions is conditional on the value of +another option, you may need to use <varname>mkIf</varname>. +Consider, for instance: + +<programlisting> +config = if config.services.httpd.enable then { + environment.systemPackages = [ <replaceable>...</replaceable> ]; + <replaceable>...</replaceable> +} else {}; +</programlisting> + +This definition will cause Nix to fail with an “infinite recursion” +error. Why? Because the value of +<option>config.services.httpd.enable</option> depends on the value +being constructed here. After all, you could also write the clearly +circular and contradictory: +<programlisting> +config = if config.services.httpd.enable then { + services.httpd.enable = false; +} else { + services.httpd.enable = true; +}; +</programlisting> + +The solution is to write: + +<programlisting> +config = mkIf config.services.httpd.enable { + environment.systemPackages = [ <replaceable>...</replaceable> ]; + <replaceable>...</replaceable> +}; +</programlisting> + +The special function <varname>mkIf</varname> causes the evaluation of +the conditional to be “pushed down” into the individual definitions, +as if you had written: + +<programlisting> +config = { + environment.systemPackages = if config.services.httpd.enable then [ <replaceable>...</replaceable> ] else []; + <replaceable>...</replaceable> +}; +</programlisting> + +</para> + +</simplesect> + +<simplesect><title>Setting Priorities</title> + +<para>A module can override the definitions of an option in other +modules by setting a <emphasis>priority</emphasis>. All option +definitions that do not have the lowest priority value are discarded. +By default, option definitions have priority 1000. You can specify an +explicit priority by using <varname>mkOverride</varname>, e.g. + +<programlisting> +services.openssh.enable = mkOverride 10 false; +</programlisting> + +This definition causes all other definitions with priorities above 10 +to be discarded. The function <varname>mkForce</varname> is +equal to <varname>mkOverride 50</varname>.</para> + +</simplesect> + +<simplesect><title>Merging Configurations</title> + +<para>In conjunction with <literal>mkIf</literal>, it is sometimes +useful for a module to return multiple sets of option definitions, to +be merged together as if they were declared in separate modules. This +can be done using <varname>mkMerge</varname>: + +<programlisting> +config = mkMerge + [ # Unconditional stuff. + { environment.systemPackages = [ <replaceable>...</replaceable> ]; + } + # Conditional stuff. + (mkIf config.services.bla.enable { + environment.systemPackages = [ <replaceable>...</replaceable> ]; + }) + ]; +</programlisting> + +</para> + +</simplesect> + +</section> \ No newline at end of file |