91 lines
2.5 KiB
Markdown
91 lines
2.5 KiB
Markdown
# Option Definitions {#sec-option-definitions}
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Option definitions are generally straight-forward bindings of values to
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option names, like
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```nix
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config = {
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services.httpd.enable = true;
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};
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```
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However, sometimes you need to wrap an option definition or set of
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option definitions in a *property* to achieve certain effects:
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## Delaying Conditionals {#sec-option-definitions-delaying-conditionals .unnumbered}
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If a set of option definitions is conditional on the value of another
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option, you may need to use `mkIf`. Consider, for instance:
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```nix
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config = if config.services.httpd.enable then {
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environment.systemPackages = [ ... ];
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...
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} else {};
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```
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This definition will cause Nix to fail with an "infinite recursion"
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error. Why? Because the value of `config.services.httpd.enable` depends
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on the value being constructed here. After all, you could also write the
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clearly circular and contradictory:
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```nix
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config = if config.services.httpd.enable then {
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services.httpd.enable = false;
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} else {
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services.httpd.enable = true;
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};
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```
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The solution is to write:
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```nix
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config = mkIf config.services.httpd.enable {
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environment.systemPackages = [ ... ];
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...
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};
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```
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The special function `mkIf` causes the evaluation of the conditional to
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be "pushed down" into the individual definitions, as if you had written:
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```nix
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config = {
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environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];
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...
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};
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```
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## Setting Priorities {#sec-option-definitions-setting-priorities .unnumbered}
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A module can override the definitions of an option in other modules by
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setting a *priority*. All option definitions that do not have the lowest
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priority value are discarded. By default, option definitions have
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priority 1000. You can specify an explicit priority by using
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`mkOverride`, e.g.
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```nix
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services.openssh.enable = mkOverride 10 false;
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```
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This definition causes all other definitions with priorities above 10 to
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be discarded. The function `mkForce` is equal to `mkOverride 50`.
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## Merging Configurations {#sec-option-definitions-merging .unnumbered}
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In conjunction with `mkIf`, it is sometimes useful for a module to
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return multiple sets of option definitions, to be merged together as if
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they were declared in separate modules. This can be done using
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`mkMerge`:
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```nix
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config = mkMerge
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[ # Unconditional stuff.
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{ environment.systemPackages = [ ... ];
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}
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# Conditional stuff.
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(mkIf config.services.bla.enable {
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environment.systemPackages = [ ... ];
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})
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];
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```
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