lib: Add composeExtensions for composing extension functions

(cherry picked from commit 0a15af2)
NixOS · Apr 11, 2017 · 2d02be0 · 2d02be0
1 parent 32491fc
commit 2d02be0
Showing 1 changed file with 9 additions and 0 deletions.
diff --git a/lib/trivial.nix b/lib/trivial.nix
@@ -53,6 +53,15 @@ rec {
   # argument, but it's nice this way if several uses of `extends` are cascaded.
   extends = f: rattrs: self: let super = rattrs self; in super // f self super;
 
+  # Compose two extending functions of the type expected by 'extends'
+  # into one where changes made in the first are available in the
+  # 'super' of the second
+  composeExtensions =
+    f: g: self: super:
+      let fApplied = f self super;
+          super' = super // fApplied;
+      in fApplied // g self super';
+
   # Create an overridable, recursive attribute set. For example:
   #
   #     nix-repl> obj = makeExtensible (self: { })