Can someone explain the magic of `makeOverridable` to me?

winston0410 · September 21, 2021, 10:24am

I am trying to understand the makeOverridable that being used in nixpkgs, and recreated one for learning purpose:

The original code:

NixOS/nixpkgs/blob/857c478cbd7758ef7d985900b37ceb3ec3fe2a2d/lib/customisation.nix#L67-L93

    
      
          makeOverridable = f: origArgs:
            let
              result = f origArgs;
          
          
    # Creates a functor with the same arguments as f
              copyArgs = g: lib.setFunctionArgs g (lib.functionArgs f);
              # Changes the original arguments with (potentially a function that returns) a set of new attributes
              overrideWith = newArgs: origArgs // (if lib.isFunction newArgs then newArgs origArgs else newArgs);
          
          
    # Re-call the function but with different arguments
              overrideArgs = copyArgs (newArgs: makeOverridable f (overrideWith newArgs));
              # Change the result of the function call by applying g to it
              overrideResult = g: makeOverridable (copyArgs (args: g (f args))) origArgs;
            in
              if builtins.isAttrs result then
                result // {
                  override = overrideArgs;
                  overrideDerivation = fdrv: overrideResult (x: overrideDerivation x fdrv);
                  ${if result ? overrideAttrs then "overrideAttrs" else null} = fdrv:
                    overrideResult (x: x.overrideAttrs fdrv);

This file has been truncated. show original

My attempts:

      mkExtendable = f: origArgs:
        ((f origArgs) // {
          extend = newArgs:
            (mkExtendable f
              (nixpkgs.lib.attrsets.recursiveUpdate origArgs newArgs));
        });

And I am trying to use this to create nixpkgs.lib.nixosSystem:

      mkSystem = mkExtendable ({ system, inputs, modules, extraModules ? [ ] }:
        inputs.nixpkgs.lib.nixosSystem {
          inherit system;
          modules = modules ++ extraModules;
          specialArgs = { inherit inputs; };
        });

But when I run this from my flake, I got an error from here:

github.com

NixOS/nixpkgs/blob/c7d2925d5b04735368401e5098924c7a3b29a29e/lib/modules.nix#L268

    
      
          unifyModuleSyntax = file: key: m:
            let
              addMeta = config: if m ? meta
                then mkMerge [ config { meta = m.meta; } ]
                else config;
              addFreeformType = config: if m ? freeformType
                then mkMerge [ config { _module.freeformType = m.freeformType; } ]
                else config;
            in
            if m ? config || m ? options then
              let badAttrs = removeAttrs m ["_file" "key" "disabledModules" "imports" "options" "config" "meta" "freeformType"]; in
              if badAttrs != {} then
                throw "Module `${key}' has an unsupported attribute `${head (attrNames badAttrs)}'. This is caused by introducing a top-level `config' or `options' attribute. Add configuration attributes immediately on the top level instead, or move all of them (namely: ${toString (attrNames badAttrs)}) into the explicit `config' attribute."
              else
                { _file = toString m._file or file;
                  key = toString m.key or key;
                  disabledModules = m.disabledModules or [];
                  imports = m.imports or [];
                  options = m.options or {};
                  config = addFreeformType (addMeta (m.config or {}));
                }

Which obviously the extend I have there is a bad attrs for a proper Nix Module. My question is, how can makeOverridable get away with override? It should be considered as bad attrs as well isn’t it?

DamienCassou · September 21, 2021, 6:50pm

I suggest you read the Nix Pills and particularly Chapter 14. Override Design Pattern.