diff --git a/lib/customisation.nix b/lib/customisation.nix
index 6d6c0234087..98a46ca6c61 100644
--- a/lib/customisation.nix
+++ b/lib/customisation.nix
@@ -51,6 +51,24 @@ rec {
        else { }));
 
 
+  /* `makeOverridable` takes a function from attribute set to attribute set and
+     injects `override` attibute which can be used to override arguments of
+     the function.
+
+       nix-repl> x = {a, b}: { result = a + b; }
+
+       nix-repl> y = lib.makeOverridable x { a = 1; b = 2; }
+
+       nix-repl> y
+       { override = «lambda»; overrideDerivation = «lambda»; result = 3; }
+
+       nix-repl> y.override { a = 10; }
+       { override = «lambda»; overrideDerivation = «lambda»; result = 12; }
+
+     Please refer to "Nixpkgs Contributors Guide" section
+     "<pkg>.overrideDerivation" to learn about `overrideDerivation` and caveats
+     related to its use.
+  */
   makeOverridable = f: origArgs:
     let
       ff = f origArgs;
diff --git a/pkgs/development/haskell-modules/make-package-set.nix b/pkgs/development/haskell-modules/make-package-set.nix
index cc161e3ca2d..6ed8d0864ca 100644
--- a/pkgs/development/haskell-modules/make-package-set.nix
+++ b/pkgs/development/haskell-modules/make-package-set.nix
@@ -130,7 +130,7 @@ in package-set { inherit pkgs stdenv callPackage; } self // {
         in generateExprs name src {}) overrides;
 
     # : { root : Path
-    #   , source-overrides : Defaulted (Either Path VersionNumber
+    #   , source-overrides : Defaulted (Either Path VersionNumber)
     #   , overrides : Defaulted (HaskellPackageOverrideSet)
     #   } -> NixShellAwareDerivation
     # Given a path to a haskell package directory whose cabal file is