From f34d07dd57bf0df4d0695a1067b3c12307f2bd5e Mon Sep 17 00:00:00 2001
From: Archit Gupta <archit@accelbread.com>
Date: Sat, 29 Jun 2024 16:49:38 -0700
Subject: [PATCH] Document defining packages for a subset of systems

---
 API_GUIDE.md | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/API_GUIDE.md b/API_GUIDE.md
index 94ffcfb..3b24fb6 100644
--- a/API_GUIDE.md
+++ b/API_GUIDE.md
@@ -365,7 +365,9 @@ and have build checks in `checks.${system}`.
 
 `package` can be set to a package definition, and will set `packages.default`.
 
-`packages` can be set to attrs of package definitions.
+`packages` can be set to attrs of package definitions. If it is a function, it
+will additionally get a `system` arg in addition to module args, to allow
+conditionally including package definitions depending on the system.
 
 By default, the `packages.default` package's name (its attribute name in
 the package set and overlay) is automatically determined from the derivation's
@@ -451,6 +453,26 @@ To use the first example, but manually specify the package name:
 }
 ```
 
+To add a package only for certain systems, you can take `system` as an arg as
+follows:
+
+```nix
+{
+  inputs.flakelight.url = "github:nix-community/flakelight";
+  outputs = { flakelight, ... }:
+    flakelight ./. {
+      packages = { system, ... }: if (system == "x86_64-linux") then {
+        pkg1 = { stdenv }:
+          stdenv.mkDerivation {
+            name = "pkg1";
+            src = ./.;
+            installPhase = "make DESTDIR=$out install";
+          };
+      } else { };
+    };
+}
+```
+
 ### devShell
 
 ```