From 1bd171ef6bf50f5ab98d4c726cc9ca986a295938 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Wed, 18 May 2022 11:03:42 +0200
Subject: [PATCH] Add pkgs parameter to perSystem

---
 all-modules.nix                              |  1 +
 dev/flake-module.nix                         |  2 +-
 modules/nixpkgs.nix                          | 26 ++++++++++++++++++++
 template/default/flake.nix                   |  5 ++--
 template/multi-module/hello/flake-module.nix |  4 +--
 5 files changed, 33 insertions(+), 5 deletions(-)
 create mode 100644 modules/nixpkgs.nix

diff --git a/all-modules.nix b/all-modules.nix
index 104553c..e1e4cf9 100644
--- a/all-modules.nix
+++ b/all-modules.nix
@@ -9,6 +9,7 @@
     ./modules/legacyPackages.nix
     ./modules/nixosConfigurations.nix
     ./modules/nixosModules.nix
+    ./modules/nixpkgs.nix
     ./modules/overlay.nix
     ./modules/packages.nix
     ./modules/perSystem.nix
diff --git a/dev/flake-module.nix b/dev/flake-module.nix
index c1f8146..f4fb450 100644
--- a/dev/flake-module.nix
+++ b/dev/flake-module.nix
@@ -6,7 +6,6 @@ flakeModuleArgs@{ config, lib, inputs, ... }:
   ];
   systems = [ "x86_64-linux" "aarch64-darwin" ];
   perSystem = system: { config, self', inputs', pkgs, ... }: {
-    _module.args.pkgs = inputs'.nixpkgs.legacyPackages;
 
     devShells.default = pkgs.mkShell {
       nativeBuildInputs = [
@@ -17,6 +16,7 @@ flakeModuleArgs@{ config, lib, inputs, ... }:
         ${config.pre-commit.installationScript}
       '';
     };
+
     pre-commit = {
       inherit pkgs; # should make this default to the one it can get via follows
       settings = {
diff --git a/modules/nixpkgs.nix b/modules/nixpkgs.nix
new file mode 100644
index 0000000..76eaf21
--- /dev/null
+++ b/modules/nixpkgs.nix
@@ -0,0 +1,26 @@
+#
+# Nixpkgs module. The only exception to the rule.
+#
+# Provides a `pkgs` argument in `perSystem`.
+#
+# Arguably, this shouldn't be in flake-modules-core, but in nixpkgs.
+# Nixpkgs could define its own module that does this, which would be
+# a more consistent UX, but for now this will do.
+#
+# The existence of this module does not mean that other flakes' logic
+# will be accepted into flake-modules-core, because it's against the
+# spirit of Flakes.
+#
+{
+  config = {
+    perSystem = _: { inputs', lib, ... }: {
+      config = {
+        _module.args.pkgs = lib.mkOptionDefault (
+          builtins.seq
+            (inputs'.nixpkgs or (throw "flake-modules-core: The flake does not have a `nixpkgs` input. Please add it, or set `perSystem._module.args.pkgs` yourself."))
+            inputs'.nixpkgs.legacyPackages
+        );
+      };
+    };
+  };
+}
diff --git a/template/default/flake.nix b/template/default/flake.nix
index 7a6275a..1cb62a3 100644
--- a/template/default/flake.nix
+++ b/template/default/flake.nix
@@ -19,12 +19,13 @@
 
         ];
         systems = [ "x86_64-linux" "aarch64-darwin" ];
-        perSystem = system: { config, self', inputs', ... }: {
+        perSystem = system: { config, self', inputs', pkgs, ... }: {
           # Per-system attributes can be defined here. The self' and inputs'
           # module parameters provide easy access to attributes of the same
           # system.
 
-          packages.hello = inputs'.nixpkgs.legacyPackages.hello;
+          # Equivalent to  inputs'.nixpkgs.legacyPackages.hello;
+          packages.hello = pkgs.hello;
         };
         flake = {
           # The usual flake attributes can be defined here, including system-
diff --git a/template/multi-module/hello/flake-module.nix b/template/multi-module/hello/flake-module.nix
index 595ece3..8629bfe 100644
--- a/template/multi-module/hello/flake-module.nix
+++ b/template/multi-module/hello/flake-module.nix
@@ -1,10 +1,10 @@
 # Definitions can be imported from a separate file like this one
 
 { self, ... }: {
-  perSystem = system: { config, self', inputs', ... }: {
+  perSystem = system: { config, self', inputs', pkgs, ... }: {
     # Definitions like this are entirely equivalent to the ones
     # you may have directly in flake.nix.
-    packages.hello = inputs'.nixpkgs.legacyPackages.hello;
+    packages.hello = pkgs.hello;
   };
   flake = {
     nixosModules.hello = { pkgs, ... }: {