lib/eval-config.nix: expose defaults to config

It's quite likely that somebody doing a custom build of Spectrum might
want to use a custom overlay without changing the underlying version
of Nixpkgs.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

1 files changed, 9 insertions, 11 deletions

diff --git a/Documentation/development/build-configuration.adoc b/Documentation/development/build-configuration.adoc
index c9a8c99..4f7d8e9 100644
--- a/Documentation/development/build-configuration.adoc
+++ b/Documentation/development/build-configuration.adoc
@@ -23,17 +23,15 @@ custom Nixpkgs to evaluate Spectrum.
 [example]
 [source,nix]
 ----
+{ default, ... }:
+
 {
-  pkgs = import <nixpkgs> {
-    overlays = [
-      (final: super: {
-        weston = super.weston.overrideAttrs ({ patches ? [], ... }: {
-          patches = patches ++ [
-            path/to/weston.patch
-          ];
-        });
-      })
-    ];
-  };
+  pkgs = default.pkgs.extend (final: super: {
+    weston = super.weston.overrideAttrs ({ patches ? [], ... }: {
+      patches = patches ++ [
+        path/to/weston.patch
+      ];
+    });
+  });
 }
 ----