From cf0aad391c10473fa7613dcc41b1f6a366d03148 Mon Sep 17 00:00:00 2001
From: Jonas Holst Damtoft <jonasdamtoft@hotmail.com>
Date: Sat, 29 Dec 2018 21:21:11 +0100
Subject: [PATCH] emacs: fix merging of `extraPackages` and `overrides`

Because `extraPackages` and `overrides` expect functions as values it
has not been possible to perform merges. This adds suitable types for
these options that allow reasonable merging.
---
 modules/lib/types.nix      | 28 ++++++++++++++++++++++++++++
 modules/programs/emacs.nix |  4 ++++
 2 files changed, 32 insertions(+)
 create mode 100644 modules/lib/types.nix

diff --git a/modules/lib/types.nix b/modules/lib/types.nix
new file mode 100644
index 00000000..1b514d20
--- /dev/null
+++ b/modules/lib/types.nix
@@ -0,0 +1,28 @@
+{ lib }:
+
+with lib;
+
+{
+
+  selectorFunction = mkOptionType {
+    name = "selectorFunction";
+    description =
+      "Function that takes an attribute set and returns a list"
+      + " containing a selection of the values of the input set";
+    check = isFunction;
+    merge = _loc: defs:
+      as: concatMap (select: select as) (getValues defs);
+  };
+
+  overlayFunction = mkOptionType {
+    name = "overlayFunction";
+    description =
+      "An overlay function, takes self and super and returns"
+      + " an attribute set overriding the desired attributes.";
+    check = isFunction;
+    merge = _loc: defs:
+      self: super:
+        foldl' (res: def: mergeAttrs res (def.value self super)) {} defs;
+  };
+
+}
diff --git a/modules/programs/emacs.nix b/modules/programs/emacs.nix
index a7f8b94c..fc796af5 100644
--- a/modules/programs/emacs.nix
+++ b/modules/programs/emacs.nix
@@ -4,6 +4,8 @@ with lib;
 
 let
 
+  hmTypes = import ../lib/types.nix { inherit lib; };
+
   cfg = config.programs.emacs;
 
   # Copied from all-packages.nix, with modifications to support
@@ -34,6 +36,7 @@ in
 
       extraPackages = mkOption {
         default = self: [];
+        type = hmTypes.selectorFunction;
         defaultText = "epkgs: []";
         example = literalExample "epkgs: [ epkgs.emms epkgs.magit ]";
         description = "Extra packages available to Emacs.";
@@ -41,6 +44,7 @@ in
 
       overrides = mkOption {
         default = self: super: {};
+        type = hmTypes.overlayFunction;
         defaultText = "self: super: {}";
         example = literalExample ''
           self: super: rec {