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 = [ ... ];
})
];