nixpkgs/nixos/doc/manual/development/option-def.section.md

# 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}

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}

A module can override the definitions of an option in other modules by
setting an *override priority*. All option definitions that do not have the lowest
priority value are discarded. By default, option definitions have
priority 100 and option defaults have priority 1500.
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`, and
`mkDefault` is equal to `mkOverride 1000`.

## Ordering Definitions {#sec-option-definitions-ordering}

It is also possible to influence the order in which the definitions for an option are
merged by setting an *order priority* with `mkOrder`. The default order priority is 1000.
The functions `mkBefore` and `mkAfter` are equal to `mkOrder 500` and `mkOrder 1500`, respectively.
As an example,

```nix
hardware.firmware = mkBefore [ myFirmware ];
```

This definition ensures that `myFirmware` comes before other unordered
definitions in the final list value of `hardware.firmware`.

Note that this is different from [override priorities](#sec-option-definitions-setting-priorities):
setting an order does not affect whether the definition is included or not.

## Merging Configurations {#sec-option-definitions-merging}

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 = [ ... ];
    })
  ];
```