# Option Definitions

Option definitions are generally straight-forward bindings of values to option names, like

    config = {
      services.httpd.enable = true;
    };

However, sometimes you need to wrap an option definition or set of option definitions in a property to achieve certain effects:

## Delaying Conditionals

If a set of option definitions is conditional on the value of another option, you may need to use `mkIf`. Consider, for instance:

    config = if config.services.httpd.enable then {
      environment.systemPackages = [ ... ];
      ...
    } else {};

This definition will cause Nix to fail with an “infinite recursion” error. Why? Because the value of `config.services.httpd.enable` depends on the value being constructed here. After all, you could also write the clearly circular and contradictory:

    config = if config.services.httpd.enable then {
      services.httpd.enable = false;
    } else {
      services.httpd.enable = true;
    };

The solution is to write:

    config = mkIf config.services.httpd.enable {
      environment.systemPackages = [ ... ];
      ...
    };

The special function `mkIf` causes the evaluation of the conditional to be “pushed down” into the individual definitions, as if you had written:

    config = {
      environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];
      ...
    };

## Setting Priorities

A module can override the definitions of an option in other modules by setting a priority. 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 `mkOverride`, e.g.

    services.openssh.enable = mkOverride 10 false;

This definition causes all other definitions with priorities above 10 to be discarded. The function `mkForce` is equal to `mkOverride 50`.

## Merging Configurations

In conjunction with `mkIf`, 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 `mkMerge`:

    config = mkMerge
      [ # Unconditional stuff.
        { environment.systemPackages = [ ... ];
        }
        # Conditional stuff.
        (mkIf config.services.bla.enable {
          environment.systemPackages = [ ... ];
        })
      ];