<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-option-definitions"> <title>Option Definitions</title> <para> Option definitions are generally straight-forward bindings of values to option names, like </para> <programlisting language="bash"> config = { services.httpd.enable = true; }; </programlisting> <para> However, sometimes you need to wrap an option definition or set of option definitions in a <emphasis>property</emphasis> to achieve certain effects: </para> <section xml:id="sec-option-definitions-delaying-conditionals"> <title>Delaying Conditionals</title> <para> If a set of option definitions is conditional on the value of another option, you may need to use <literal>mkIf</literal>. Consider, for instance: </para> <programlisting language="bash"> config = if config.services.httpd.enable then { environment.systemPackages = [ ... ]; ... } else {}; </programlisting> <para> This definition will cause Nix to fail with an <quote>infinite recursion</quote> error. Why? Because the value of <literal>config.services.httpd.enable</literal> depends on the value being constructed here. After all, you could also write the clearly circular and contradictory: </para> <programlisting language="bash"> config = if config.services.httpd.enable then { services.httpd.enable = false; } else { services.httpd.enable = true; }; </programlisting> <para> The solution is to write: </para> <programlisting language="bash"> config = mkIf config.services.httpd.enable { environment.systemPackages = [ ... ]; ... }; </programlisting> <para> The special function <literal>mkIf</literal> causes the evaluation of the conditional to be <quote>pushed down</quote> into the individual definitions, as if you had written: </para> <programlisting language="bash"> config = { environment.systemPackages = if config.services.httpd.enable then [ ... ] else []; ... }; </programlisting> </section> <section xml:id="sec-option-definitions-setting-priorities"> <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 <literal>mkOverride</literal>, e.g. </para> <programlisting language="bash"> services.openssh.enable = mkOverride 10 false; </programlisting> <para> This definition causes all other definitions with priorities above 10 to be discarded. The function <literal>mkForce</literal> is equal to <literal>mkOverride 50</literal>. </para> </section> <section xml:id="sec-option-definitions-merging"> <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 <literal>mkMerge</literal>: </para> <programlisting language="bash"> config = mkMerge [ # Unconditional stuff. { environment.systemPackages = [ ... ]; } # Conditional stuff. (mkIf config.services.bla.enable { environment.systemPackages = [ ... ]; }) ]; </programlisting> </section> </section>