modules/nixpkgs: Make customizable & support multiple evaluations

dev/README.md

@@ -1,6 +1,10 @@
-
 # Separate `dev` flake
 
 Wouldn't recommend this pattern normally, but I'm trying to keep
 deps low for `flake-parts` until we have split dev inputs
 that don't carry over to dependent lock files.
+
+```sh
+nix develop --impure -f './dev' 'mySystem.devShells.default'
+nix repl -f './dev'
+```

dev/default.nix

@@ -1,16 +1,36 @@
 let
+  flake-parts = builtins.getFlake (toString ../.);
+  lib = flake-parts.inputs.nixpkgs-lib.lib;
+  sourceInfo = inputs.flake-parts.sourceInfo; # used by pre-commit module, etc
   flake = builtins.getFlake (toString ./.);
-  fmc-lib = (builtins.getFlake (toString ../.)).lib;
-  args = {
-    inherit self;
-  } // flake.inputs;
-  self = {
-    inherit (flake) inputs;
-    outPath = ../.; # used by pre-commit module, etc
-    outputs = self.config.flake;
-  } //
-  fmc-lib.mkFlake
-    { inputs = args; }
-    ./flake-module.nix;
+  inputs = flake.inputs // { inherit flake-parts; };
+  makeResult = specialArgs: flakeModule: result:
+    let
+      outputs = flake.outputs // flake-parts.lib.mkFlake
+        {
+          inputs = inputs // { self = result; };
+          # debugging tool
+          specialArgs = {
+            replaceSpecialArgs = newSpecialArgs:
+              let
+                newSpecialArgs' =
+                  if lib.isFunction newSpecialArgs
+                  then newSpecialArgs specialArgs
+                  else newSpecialArgs;
+                newResult = makeResult newSpecialArgs' flakeModule newResult;
+              in
+              newResult;
+          } // specialArgs;
+        }
+        flakeModule;
+    in
+    outputs // sourceInfo // {
+      inherit inputs outputs sourceInfo;
+      _type = "flake";
+    };
 in
-self.config.flake // { inherit (flake) inputs; }
+let
+  # eagerly import to reproduce inline evaluation
+  result = makeResult { } (import ./flake-module.nix) result;
+in
+result

dev/flake-module.nix

@@ -1,19 +1,25 @@
-{ config, lib, inputs, withSystem, ... }:
-
-{
+{ config, inputs, lib, options, specialArgs, withSystem, ... } @ args:
+let
+  rootArgs = args;
+  rootConfig = config;
+  rootOptions = options;
+  rootSpecialArgs = specialArgs;
+in
+# debugging tool
+specialArgs.flakeModuleTransformer or (args: flakeModule: flakeModule) args {
   imports = [
     inputs.pre-commit-hooks-nix.flakeModule
     inputs.hercules-ci-effects.flakeModule # herculesCI attr
   ];
-  systems = [ "x86_64-linux" "aarch64-darwin" ];
+  config.systems = [ "x86_64-linux" "aarch64-darwin" ];
 
-  hercules-ci.flake-update = {
+  config.hercules-ci.flake-update = {
     enable = true;
     autoMergeMethod = "merge";
     when.dayOfMonth = 1;
   };
 
-  perSystem = { config, pkgs, ... }: {
+  config.perSystem = { config, pkgs, ... }: {
 
     devShells.default = pkgs.mkShell {
       nativeBuildInputs = [
@@ -38,11 +44,18 @@
       in tests.runTests pkgs.emptyFile // { internals = tests; };
 
   };
-  flake = {
-    # for repl exploration / debug
-    config.config = config;
-    options.mySystem = lib.mkOption { default = config.allSystems.${builtins.currentSystem}; };
-    config.effects = withSystem "x86_64-linux" ({ pkgs, hci-effects, ... }: {
+  config.flake = { config, options, specialArgs, ... } @ args: {
+    # for REPL exploration / debugging
+    config.allFlakeModuleArgs = args // config._module.args // specialArgs;
+    config.allRootModuleArgs = rootArgs // rootConfig._module.args // rootSpecialArgs;
+    config.transformFlakeModule = flakeModuleTransformer:
+      rootSpecialArgs.replaceSpecialArgs (prevSpecialArgs: prevSpecialArgs // {
+        inherit flakeModuleTransformer;
+      });
+    options.mySystem = lib.mkOption {
+      default = rootConfig.allSystems.${builtins.currentSystem};
+    };
+    config.effects = withSystem "x86_64-linux" ({ hci-effects, pkgs, ... }: {
       tests = {
         template = pkgs.callPackage ./tests/template.nix { inherit hci-effects; };
       };