Configuration.nix config

olaf · August 5, 2022, 4:45pm

a configuration.nix is a module

modules have this structure
Example: Structure of NixOS Modules

{ config, pkgs, ... }:

{
  imports =
    [ paths of other modules
    ];

  options = {
    option declarations
  };

  config = {
    option definitions
  };
}

but why do we usually write

{ config, pkgs, ... }: {
  users.users.alice.isNormalUser = true;
}

instead of

{ config, pkgs, ... }: {
  config = { users.users.alice.isNormalUser = true; };
}

Is this syntactic sugar, backwards compatibility, both, neither or something completely different?

nrdxp · August 5, 2022, 4:55pm

syntactic sugar to avoid redundancy when one is simply wishing to set existing options instead of defining new ones.

olaf · August 5, 2022, 4:58pm

@nrdxp do you know where it is implemented?

nrdxp · August 5, 2022, 5:06pm

Not exactly, but likely somewhere in this beast:

github.com

NixOS/nixpkgs/blob/cd6e5884c74d6dd6394d5a8697929621f7220552/lib/modules.nix#L104


      
          
          
    ‘_module’: A portion of the configuration tree which is elided from
                    ‘config’. It contains some values that are mostly internal to the
                    module system implementation.
          
          
   !!! Please think twice before adding to this argument list! The more
             that is specified here instead of in the modules themselves the harder
             it is to transparently move a set of modules to be a submodule of another
             config (as the proper arguments need to be replicated at each call to
             evalModules) and the less declarative the module set is. */
          evalModules = evalModulesArgs@
                        { modules
                        , prefix ? []
                        , # This should only be used for special arguments that need to be evaluated
                          # when resolving module structure (like in imports). For everything else,
                          # there's _module.args. If specialArgs.modulesPath is defined it will be
                          # used as the base path for disabledModules.
                          specialArgs ? {}
                        , # This would be remove in the future, Prefer _module.args option instead.
                          args ? {}
                        , # This would be remove in the future, Prefer _module.check option instead.

tejing · August 5, 2022, 8:19pm

More specifically,

github.com

NixOS/nixpkgs/blob/cd6e5884c74d6dd6394d5a8697929621f7220552/lib/modules.nix#L452-L472


      
          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 {}));
              }
          else
            lib.throwIfNot (isAttrs m) "module ${file} (${key}) does not look like a module."
            { _file = toString m._file or file;
              key = toString m.key or key;
              disabledModules = m.disabledModules or [];
              imports = m.require or [] ++ m.imports or [];
              options = {};
              config = addFreeformType (addMeta (removeAttrs m ["_file" "key" "disabledModules" "require" "imports" "freeformType"]));
            };

dramforever · August 6, 2022, 3:04am

It’s documented right above the example you gave:

This is actually an abbreviated form of module that only defines options, but does not declare any. The structure of full NixOS modules is shown in Example: Structure of NixOS Modules.