Add comments about the module system interface

Ideally the module system could be configured pretty much completely by
the contents of the modules themselves, so add comments about avoiding
complicating it further and possibly removing now-redundant
configurability from the existing interface.

3 files changed, 37 insertions, 8 deletions

diff --git a/lib/modules.nix b/lib/modules.nix
index 1d7c7b22765e..26193b269410 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -9,8 +9,19 @@ rec {
 
   /* Evaluate a set of modules.  The result is a set of two
      attributes: ‘options’: the nested set of all option declarations,
-     and ‘config’: the nested set of all option values. */
-  evalModules = { modules, prefix ? [], args ? {}, check ? true }:
+     and ‘config’: the nested set of all option values.
+     !!! 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 = { modules
+                , prefix ? []
+                , # !!! This can be specified modularly now, can we remove it?
+                  args ? {}
+                , # !!! This can be specified modularly now, can we remove it?
+                  check ? true
+                }:
     let
       internalModule = rec {
         _file = ./modules.nix;
@@ -21,6 +32,7 @@ rec {
           __internal.args = mkOption {
             description = "Arguments passed to each module.";
 
+            # !!! Should this be types.uniq types.unspecified?
             type = types.attrsOf types.unspecified;
 
             internal = true;
diff --git a/lib/types.nix b/lib/types.nix
index 4d336c1d9466..32f332ed21bb 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -200,7 +200,12 @@ in rec {
           let
             coerce = def: if isFunction def then def else { config = def; };
             modules = opts' ++ map (def: { _file = def.file; imports = [(coerce def.value)]; }) defs;
-          in (evalModules { inherit modules; args.name = last loc; prefix = loc; }).config;
+          in (evalModules {
+            inherit modules;
+            # !!! See comment about args in lib/modules.nix
+            args.name = last loc;
+            prefix = loc;
+          }).config;
         getSubOptions = prefix: (evalModules
           { modules = opts'; inherit prefix;
             # FIXME: hack to get shit to evaluate.
diff --git a/nixos/lib/eval-config.nix b/nixos/lib/eval-config.nix
index 4ee1c61f54ff..b4b251d2581a 100644
--- a/nixos/lib/eval-config.nix
+++ b/nixos/lib/eval-config.nix
@@ -2,12 +2,24 @@
 # configuration object (`config') from which we can retrieve option
 # values.
 
-{ system ? builtins.currentSystem
-, pkgs ? null
-, baseModules ? import ../modules/module-list.nix
-, extraArgs ? {}
+# !!! Please think twice before adding to this argument list!
+# Ideally eval-config.nix would be an extremely thin wrapper
+# around lib.evalModules, so that modular systems that have nixos configs
+# as subcomponents (e.g. the container feature, or nixops if network
+# expressions are ever made modular at the top level) can just use
+# types.submodule instead of using eval-config.nix
+{ # !!! system can be set modularly, would be nice to remove
+  system ? builtins.currentSystem
+, # !!! is this argument needed any more? The pkgs argument can
+  # be set modularly anyway.
+  pkgs ? null
+, # !!! what do we gain by making this configurable?
+  baseModules ? import ../modules/module-list.nix
+, # !!! See comment about args in lib/modules.nix
+  extraArgs ? {}
 , modules
-, check ? true
+, # !!! See comment about check in lib/modules.nix
+  check ? true
 , prefix ? []
 , lib ? import ../../lib
 }: