[0006] Auto call manyVariant packages to separately

eeps/0006-auto-call-many-variants.md

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+---
+Title: Auto call manyVariant packages to separate variant and dependency concerns
+Author: jonringer
+Discussions-To: https://github.com/jonringer/multiple-package-versions-proposal?
+Status: Draft
+Type: Standards Track
+Topic: Packaging
+Created: 2024-9-22
+---
+
+# Summary
+
+Argument passing in multiple-package-versions-proposal was awkward, this creates
+a mkAutoCalledPolyPkgDir function which creates an overlay in which the directories
+in a specified directory will be automatically called with mkPolyPkg, and that
+partially applied function then passed to callPackage.
+
+This separates the creation of a package which contains multiple variants and
+the callPackage call which should only be passing arguments needed to conduct
+a build.
+
+## Detailed Implementation
+
+Example PR: https://github.com/jonringer/core-pkgs/pull/6
+
+```nix
+let
+  mkAutoCalledPolyPackageDir = baseDirectory:
+  let
+    namesForShard = lib.packageSets.mkNamesForDirectory baseDirectory;
+    # This is defined up here in order to allow reuse of the value (it's kind of expensive to compute)
+    # if the overlay has to be applied multiple times
+    packageFiles = mergeAttrsList (mapAttrsToList namesForShard (builtins.readDir baseDirectory));
+  in
+  # TODO: Consider optimising this using `builtins.deepSeq packageFiles`,
+  # which could free up the above thunks and reduce GC times.
+  # Currently this would be hard to measure until we have more packages
+  # and ideally https://github.com/NixOS/nix/pull/8895
+  self: _:
+    {
+      _internalCallPolyFile = file: self.callPackage (import file { inherit (self) mkPolyPkg; }) { };
+    }
+    // builtins.mapAttrs
+      (name: value: self._internalCallPolyFile value)
+      packageFiles;
+
+  polyPkgOverlay = mkAutoCalledPolyPackageDir ./polyPkgs;
+in
+
+import stdenvRepo {
+  overlays = [
+    polyPkgOverlay
+    ...
+  ];
+}
+
+```
+
+## Openssl example
+
+Previously, arguments for mkPolyPkg needed to be passed along with the arguments
+for the underlying package, this now separates the two concerns.
+
+```nix
+# before changes
+{ callPackage
+, mkPolyPkg
+, ...
+}@args:
+
+callPackage (mkPolyPkg {
+  versions = ./versions.nix;
+  aliases = ./aliases.nix;
+  defaultSelector = (p: p.v3_3);
+  genericBuilder = ./generic.nix;
+}) args
+```
+
+```nix
+# post changes
+{ mkPolyPkg }:
+
+mkPolyPkg {
+  versions = ./versions.nix;
+  aliases = ./aliases.nix;
+  defaultSelector = (p: p.v3_3);
+  genericBuilder = ./generic.nix;
+}
+```
+