lib: eachDefaultSystemPassThrough/eachSystemPassThrough: init

Expose the eachDefaultSystemPassThrough and eachSystemPassThrough
functions to handle cases where the system key should not be injected by
eachDefaultSystem and eachSystem:

    inputs.flake-utils.lib.eachDefaultSystem (system: {
      checks./*<SYSTEM>.*/"<CHECK>" = /* ... */;
      devShells./*<SYSTEM>.*/"<DEV_SHELL>" = /* ... */;
      packages./*<SYSTEM>.*/"<PACKAGE>" = /* ... */;
    })
    // inputs.flake-utils.lib.eachDefaultSystemPassThrough (system: {
      homeConfigurations."<HOME_CONFIGURATION>" = /* ... */;
      nixosConfigurations."<NIXOS_CONFIGURATION>" = /* ... */;
    })

These functions prevent users from re-implementing simplified
eachDefaultSystem and eachSystem versions to avoid system key
injections, while benefiting from current and future complex logic, like
handling the '--impure' flag.

This addresses flake-utils' arguably biggest issue. [1]

[1]: https://ayats.org/blog/no-flake-utils

Author: trueNAHO

README.md

@@ -83,6 +83,11 @@ eachSystem allSystems (system: { hello = 42; })
 }
 ```
 
+### `eachSystemPassThrough :: [<system>] -> (<system> -> attrs)`
+
+Unlike `eachSystem`, this function does not inject the `${system}` key by merely
+providing the system argument to the function.
+
 ### `eachDefaultSystem :: (<system> -> attrs)`
 
 `eachSystem` pre-populated with `defaultSystems`.
@@ -113,6 +118,24 @@ eachSystem allSystems (system: { hello = 42; })
 }
 ```
 
+### `eachDefaultSystemPassThrough :: (<system> -> attrs)`
+
+`eachSystemPassThrough` pre-populated with `defaultSystems`.
+
+#### Example
+
+```nix
+inputs.flake-utils.lib.eachDefaultSystem (system: {
+  checks./*<SYSTEM>.*/"<CHECK>" = /* ... */;
+  devShells./*<SYSTEM>.*/"<DEV_SHELL>" = /* ... */;
+  packages./*<SYSTEM>.*/"<PACKAGE>" = /* ... */;
+})
+// inputs.flake-utils.lib.eachDefaultSystemPassThrough (system: {
+  homeConfigurations."<HOME_CONFIGURATION>" = /* ... */;
+  nixosConfigurations."<NIXOS_CONFIGURATION>" = /* ... */;
+})
+```
+
 ### `meld :: attrs -> [ path ] -> attrs`
 
 Meld merges subflakes using common inputs.  Useful when you want to

lib.nix

@@ -26,36 +26,47 @@ let
   # eachSystem using defaultSystems
   eachDefaultSystem = eachSystem defaultSystems;
 
+  # eachSystemPassThrough using defaultSystems
+  eachDefaultSystemPassThrough = eachSystemPassThrough defaultSystems;
+
   # Builds a map from <attr>=value to <attr>.<system>=value for each system.
-  eachSystem =
-    systems: f:
-    builtins.foldl'
-      (
-        # Merge outputs for each system.
-        attrs: system:
-        let
-          ret = f system;
-        in
-        builtins.foldl' (
-          attrs: key:
-          attrs
-          // {
-            ${key} = (attrs.${key} or { }) // {
-              ${system} = ret.${key};
-            };
-          }
-        ) attrs (builtins.attrNames ret)
-      )
-      { }
-      (
-        if
-          !builtins ? currentSystem || builtins.elem builtins.currentSystem systems
-        then
-          systems
-        else
-          # Add the current system if the --impure flag is used.
-          systems ++ [ builtins.currentSystem ]
-      );
+  eachSystem = eachSystemOp (
+    # Merge outputs for each system.
+    f: attrs: system:
+    let
+      ret = f system;
+    in
+    builtins.foldl' (
+      attrs: key:
+      attrs
+      // {
+        ${key} = (attrs.${key} or { }) // {
+          ${system} = ret.${key};
+        };
+      }
+    ) attrs (builtins.attrNames ret)
+  );
+
+  # Applies a merge operation accross systems.
+  eachSystemOp =
+    op: systems: f:
+    builtins.foldl' (op f) { } (
+      if
+        !builtins ? currentSystem || builtins.elem builtins.currentSystem systems
+      then
+        systems
+      else
+        # Add the current system if the --impure flag is used.
+        systems ++ [ builtins.currentSystem ]
+    );
+
+  # Merely provides the system argument to the function.
+  #
+  # Unlike eachSystem, this function does not inject the `${system}` key.
+  eachSystemPassThrough = eachSystemOp (
+    f: attrs: system:
+    attrs // (f system)
+  );
 
   # eachSystemMap using defaultSystems
   eachDefaultSystemMap = eachSystemMap defaultSystems;
@@ -202,8 +213,10 @@ let
       defaultSystems
       eachDefaultSystem
       eachDefaultSystemMap
+      eachDefaultSystemPassThrough
       eachSystem
       eachSystemMap
+      eachSystemPassThrough
       filterPackages
       flattenTree
       meld