How can I share arbitrary values between local flake-modules? #279

BenChand · 2025-03-13T11:55:55Z

BenChand
Mar 13, 2025

Really basic, say I have two local flake-modules a.nix and b.nix. In a.nix I'd like to do something like this:

{
  perSystem =
    { pkgs, config, ... }:
    {
      hello = "world";
    };
}

But that will give an error like this:

error: The option `perSystem.aarch64-darwin.hello' does not exist. Definition values:
       - In `/nix/store/a.nix, via option perSystem': "world"

Is there a lib option I can put things in or something? The docs don't seem particularly clear for this use-case

Answered by roberth

Mar 13, 2025

The docs assume some familiarity with the module system.
The thing in perSystem is a module, which means that you can do module things like declaring more options.

config (and therefore the shorthand for config.perSystem) is ignored while generating docs, so if you wish to render option docs from the new options, you may refer to this example, placing everything under options:

flake-parts/modules/formatter.nix

Line 16 in f4330d2

options = {

flake-parts/modules/formatter.nix

Lines 27 to 38 in f4330d2

     perSystem = mkPerSystemOption {  
   _file = ./formatter.nix;  
   options = {  
   formatter = mkOption {  
   type = types.nullOr types.package;  
   default = n…

View full answer

roberth · 2025-03-13T12:06:52Z

roberth
Mar 13, 2025
Maintainer

The docs assume some familiarity with the module system.
The thing in perSystem is a module, which means that you can do module things like declaring more options.

config (and therefore the shorthand for config.perSystem) is ignored while generating docs, so if you wish to render option docs from the new options, you may refer to this example, placing everything under options:

flake-parts/modules/formatter.nix

Line 16 in f4330d2

options = {

flake-parts/modules/formatter.nix

Lines 27 to 38 in f4330d2

    
               perSystem = mkPerSystemOption { 
        
                 _file = ./formatter.nix; 
        
                 options = { 
        
                   formatter = mkOption { 
        
                     type = types.nullOr types.package; 
        
                     default = null; 
        
                     description = '' 
        
                       A package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html). 
        
                     ''; 
        
                   }; 
        
                 }; 
        
               };

That module does a bit more, like exposing a flake output attribute, which you may or may not need, but at least the referenced part seems to be your missing ingredient.

1 reply

BenChand Mar 13, 2025
Author

Ah okay so modules can define options which they also then set themselves? I think I understand, thanks 😄

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	perSystem = mkPerSystemOption {
	_file = ./formatter.nix;
	options = {
	formatter = mkOption {
	type = types.nullOr types.package;
	default = n…

How can I share arbitrary values between local flake-modules? #279

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BenChand Mar 13, 2025

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roberth Mar 13, 2025 Maintainer

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roberth
Mar 13, 2025
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