Implement optional top-level system

src/core/compose.nix

@@ -56,11 +56,13 @@
 let
   l = builtins;
   core = import ./mod.nix;
+
 in
 {
   src,
   root,
   config,
+  system ? null,
   extern ? { },
   features ? [ ],
   # internal features of the composer function
@@ -78,6 +80,8 @@ let
     inherit __internal__test;
   } (../. + "/std@.toml");
 
+  systemIsDefined = system != null;
+
   coreFeatures' = core.features.resolve core.coreToml.features coreFeatures;
   stdFeatures' = core.features.resolve core.stdToml.features stdFeatures;
 
@@ -134,6 +138,14 @@ let
               _if = __isStd__;
               std = l.removeAttrs (extern // atom) [ "std" ];
             }
+            {
+              _if = !__isStd__ && systemIsDefined;
+              inherit system;
+            }
+            {
+              _if = !__isStd__;
+              use = l.removeAttrs extern [ "std" ];
+            }
             {
               _if = __internal__test;
               # information about the internal module system itself
@@ -197,7 +209,7 @@ let
       # Base case: no module
       { };
 
-  atomScope = l.removeAttrs (extern // atom // { inherit extern; }) [
+  atomScope = l.removeAttrs atom [
     "atom"
     (baseNameOf par)
   ];

src/core/errors.nix

@@ -44,7 +44,7 @@ in
     modFromDir (stripParentDir par path);
   import = abort "Importing arbitrary Nix files is forbidden. Declare your dependencies via the module system instead.";
   fetch = abort "Ad hoc fetching is illegal. Declare dependencies statically in the manifest instead.";
-  system = abort "Accessing the current system is impure. Declare supported systems in the manifest.";
+  system = abort "`system` has been accessed and is undefined";
   time = _: warn "currentTime: Ignoring request for current time, returning: 0";
   nixPath = _: warn "nixPath: ignoring impure NIX_PATH request, returning: []";
   storePath = abort "Making explicit dependencies on store paths is illegal.";

src/core/importAtom.nix

@@ -16,6 +16,7 @@
   valid input (and the CLI should type check on it's end)
 */
 {
+  system ? null,
   features ? null,
   __internal__test ? false,
 }:
@@ -96,11 +97,12 @@ let
 in
 mod.compose {
   inherit
+    src
+    root
+    config
+    system
     extern
     __internal__test
-    config
-    root
-    src
     ;
   features = features';
   coreFeatures =

src/dev/shell.nix

@@ -1,5 +1,5 @@
 {
-  pkgs ? atom.pkgs,
+  pkgs ? use.pkgs,
 }:
 pkgs.mkShell {
   packages = with pkgs; [