types.attrs: merge non-conflicting values only

lib/options.nix

@@ -27,6 +27,7 @@ let
     ;
   inherit (lib.attrsets)
     optionalAttrs
+    recursiveUpdateUntil
     ;
   inherit (lib.strings)
     concatMapStrings
@@ -147,6 +148,16 @@ rec {
       else
         first) (head defs) (tail defs)).value;
 
+  /* Merge attribute sets of non-conflicting values. */
+  mergeAttrSetOptions = loc: defs:
+    foldl' (val: def: recursiveUpdateUntil (path: lhs: rhs:
+      if isAttrs lhs && isAttrs rhs then
+        false
+      else if lhs != rhs then
+        throw ''The option `${showOption loc}' has conflicting definition values in '${concatStringsSep "." path}':${showFiles (getFiles defs)}.''
+      else
+        true) val def.value) {} defs;
+
   /* Extracts values of all "value" keys of the given list.
 
      Type: getValues :: [ { value :: a } ] -> [a]

lib/tests/modules.sh

@@ -279,6 +279,10 @@ checkConfigError 'A definition for option .fun.\[function body\]. is not of type
 checkConfigOutput '^"b a"$' config.result ./functionTo/list-order.nix
 checkConfigOutput '^"a c"$' config.result ./functionTo/merging-attrs.nix
 
+## types.attrs
+checkConfigError 'The option .* has conflicting definition values in .*foo.bar.*' config.value.foo.bar ./define-conflicting-attrs.nix
+checkConfigOutput '^42$' config.value.foo.baz ./define-merging-attrs.nix
+
 # moduleType
 checkConfigOutput '^"a b"$' config.resultFoo ./declare-variants.nix ./define-variant.nix
 checkConfigOutput '^"a y z"$' config.resultFooBar ./declare-variants.nix ./define-variant.nix

lib/tests/modules/declare-attrs.nix

@@ -0,0 +1,5 @@
+{ lib, ... }: {
+  options.value = lib.mkOption {
+    type = lib.types.attrs;
+  };
+}

lib/tests/modules/define-conflicting-attrs.nix

@@ -0,0 +1,7 @@
+{
+  imports = [
+    ./declare-attrs.nix
+    { value = { foo = { bar = false; }; }; }
+    { value = { foo = { bar = true; }; }; }
+  ];
+}

lib/tests/modules/define-merging-attrs.nix

@@ -0,0 +1,7 @@
+{
+  imports = [
+    ./declare-attrs.nix
+    { value = { foo = { bar = true; }; }; }
+    { value = { foo = { bar = true; baz = 42; }; }; }
+  ];
+}

lib/types.nix

@@ -48,6 +48,7 @@ let
     mergeDefaultOption
     mergeEqualOption
     mergeOneOption
+    mergeAttrSetOptions
     showFiles
     showOption
     ;
@@ -340,7 +341,7 @@ rec {
       name = "attrs";
       description = "attribute set";
       check = isAttrs;
-      merge = loc: foldl' (res: def: res // def.value) {};
+      merge = mergeAttrSetOptions;
       emptyValue = { value = {}; };
     };