NixOS:config argument: Difference between revisions

The following code is used to support the explanation.

{config, pkgs, ...}:

   };

}

This snippet of code is a module which declare the option "<code>foo</code>" and define the option "<code>bar</code>".  The option "<code>bar</code>" is defined with the value <code>config.foo</code>.  This attribute <code>foo</code> of the <code>config</code> argument, is the result of the [[NixOS:Declaration#Evaluation|evaluation]] of the definitions and the declarations of the option "<code>foo</code>".  Definitions of the option "<code>foo</code>" may exists in other module used by the system.

The process of module computation is highly recursive and may cause trouble when you want to add control flow statements.  A common mistake is to use "<code>if</code>" or "<code>assert</code>" statements in the computation of a module.

{config, pkgs, ...}:

       {};

}

The previous module cause an infinite recursion:

To avoid such infinite recursion, [[NixOS:Properties|properties]] have been introduced, thus the previous code should be rewritten in:

{config, pkgs, ...}:

   };

}

== Common Pattern ==

Often, the module declare options embedded inside an attribute set.  To access these options, we add an attribute <code>cfg</code> as a shortcut notation.

{config, pkgs, ...}:

   };

}

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