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# Option Definitions {#sec-option-definitions}
Option definitions are generally straight-forward bindings of values to
option names, like
```nix
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 {#sec-option-definitions-delaying-conditionals .unnumbered}
If a set of option definitions is conditional on the value of another
option, you may need to use `mkIf`. Consider, for instance:
```nix
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:
```nix
config = if config.services.httpd.enable then {
services.httpd.enable = false;
} else {
services.httpd.enable = true;
};
```
The solution is to write:
```nix
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:
```nix
config = {
environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];
...
};
```
## Setting Priorities {#sec-option-definitions-setting-priorities .unnumbered}
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.
```nix
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 {#sec-option-definitions-merging .unnumbered}
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`:
```nix
config = mkMerge
[ # Unconditional stuff.
{ environment.systemPackages = [ ... ];
}
# Conditional stuff.
(mkIf config.services.bla.enable {
environment.systemPackages = [ ... ];
})
];
```
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