From e28e6b96bb671309218e5b9de56aeac22b2bf7be Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Wed, 26 Jun 2024 02:03:21 +0200
Subject: [PATCH] flake.nix: Tidy `packages` build matrix code

flatMapAttrs is easier to read because it introduces the values
before using them, kind of like a `let` bindings with multiple
values.

The repeated comments remind the reader of the purpose of the
innermost attrsets, which is actually very simple.
Knowing that they go right into the result should help a lot
with building a mental model for this pattern.
---
 flake.nix | 62 +++++++++++++++++++++++++++++--------------------------
 1 file changed, 33 insertions(+), 29 deletions(-)

diff --git a/flake.nix b/flake.nix
index 35280f038..dfae6ce2d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -351,36 +351,40 @@
       // devFlake.checks.${system} or {}
       );
 
-      packages = forAllSystems (system: {
-        inherit (nixpkgsFor.${system}.native)
-          changelog-d;
-        default = self.packages.${system}.nix;
-        nix-internal-api-docs = nixpkgsFor.${system}.native.nix-internal-api-docs;
-        nix-external-api-docs = nixpkgsFor.${system}.native.nix-external-api-docs;
-      } // lib.concatMapAttrs
-        # We need to flatten recursive attribute sets of derivations to pass `flake check`.
-        (pkgName: {}: {
-          "${pkgName}" = nixpkgsFor.${system}.native.${pkgName};
-          "${pkgName}-static" = nixpkgsFor.${system}.static.${pkgName};
-        } // lib.concatMapAttrs
-          (crossSystem: {}: {
-            "${pkgName}-${crossSystem}" = nixpkgsFor.${system}.cross.${crossSystem}.${pkgName};
-          })
-          (lib.genAttrs crossSystems (_: { }))
-        // lib.concatMapAttrs
-          (stdenvName: {}: {
-            "${pkgName}-${stdenvName}" = nixpkgsFor.${system}.stdenvs."${stdenvName}Packages".${pkgName};
-          })
-          (lib.genAttrs stdenvs (_: { })))
-        {
-          "nix" = { };
-          # Temporarily disabled because GitHub Actions OOM issues. Once
-          # the old build system is gone and we are back to one build
-          # system, we should reenable these.
-          #"nix-util" = { };
-          #"nix-store" = { };
-          #"nix-fetchers" = { };
+      packages = forAllSystems (system:
+        { # Here we put attributes that map 1:1 into packages.<system>, ie
+          # for which we don't apply the full build matrix such as cross or static.
+          inherit (nixpkgsFor.${system}.native)
+            changelog-d;
+          default = self.packages.${system}.nix;
+          nix-internal-api-docs = nixpkgsFor.${system}.native.nix-internal-api-docs;
+          nix-external-api-docs = nixpkgsFor.${system}.native.nix-external-api-docs;
         }
+        # We need to flatten recursive attribute sets of derivations to pass `flake check`.
+        // flatMapAttrs
+          { # Components we'll iterate over in the upcoming lambda
+            "nix" = { };
+            # Temporarily disabled because GitHub Actions OOM issues. Once
+            # the old build system is gone and we are back to one build
+            # system, we should reenable these.
+            #"nix-util" = { };
+            #"nix-store" = { };
+            #"nix-fetchers" = { };
+          }
+          (pkgName: {}: {
+              # These attributes go right into `packages.<system>`.
+              "${pkgName}" = nixpkgsFor.${system}.native.${pkgName};
+              "${pkgName}-static" = nixpkgsFor.${system}.static.${pkgName};
+            }
+            // flatMapAttrs (lib.genAttrs crossSystems (_: { })) (crossSystem: {}: {
+              # These attributes go right into `packages.<system>`.
+              "${pkgName}-${crossSystem}" = nixpkgsFor.${system}.cross.${crossSystem}.${pkgName};
+            })
+            // flatMapAttrs (lib.genAttrs stdenvs (_: { })) (stdenvName: {}: {
+              # These attributes go right into `packages.<system>`.
+              "${pkgName}-${stdenvName}" = nixpkgsFor.${system}.stdenvs."${stdenvName}Packages".${pkgName};
+            })
+          )
         // lib.optionalAttrs (builtins.elem system linux64BitSystems) {
         dockerImage =
           let