diff --git a/lib/types.nix b/lib/types.nix
index a7f9bf1946e6..7276f9af9fee 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -6,7 +6,7 @@ with import ./attrsets.nix;
 with import ./options.nix;
 with import ./trivial.nix;
 with import ./strings.nix;
-with {inherit (import ./modules.nix) mergeDefinitions; };
+with {inherit (import ./modules.nix) mergeDefinitions filterOverrides; };
 
 rec {
 
@@ -166,6 +166,23 @@ rec {
         substSubModules = m: loaOf (elemType.substSubModules m);
       };
 
+    # List or element of ...
+    loeOf = elemType: mkOptionType {
+      name = "element or list of ${elemType.name}s";
+      check = x: isList x || elemType.check x;
+      merge = loc: defs:
+        let
+          defs' = filterOverrides defs;
+          res = (head defs').value;
+        in
+        if isList res then concatLists (getValues defs')
+        else if lessThan 1 (length defs') then
+          throw "The option `${showOption loc}' is defined multiple times, in ${showFiles (getFiles defs)}."
+        else if !isString res then
+          throw "The option `${showOption loc}' does not have a string value, in ${showFiles (getFiles defs)}."
+        else res;
+    };
+
     uniq = elemType: mkOptionType {
       inherit (elemType) name check;
       merge = mergeOneOption;
diff --git a/nixos/modules/config/shells-environment.nix b/nixos/modules/config/shells-environment.nix
index bff0b2991323..533280890a70 100644
--- a/nixos/modules/config/shells-environment.nix
+++ b/nixos/modules/config/shells-environment.nix
@@ -41,20 +41,7 @@ in
         strings.  The latter is concatenated, interspersed with colon
         characters.
       '';
-      type = types.attrsOf (mkOptionType {
-        name = "a string or a list of strings";
-        merge = loc: defs:
-          let
-            defs' = filterOverrides defs;
-            res = (head defs').value;
-          in
-          if isList res then concatLists (getValues defs')
-          else if lessThan 1 (length defs') then
-            throw "The option `${showOption loc}' is defined multiple times, in ${showFiles (getFiles defs)}."
-          else if !isString res then
-            throw "The option `${showOption loc}' does not have a string value, in ${showFiles (getFiles defs)}."
-          else res;
-      });
+      type = types.attrsOf (types.loeOf types.str);
       apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);
     };
 
diff --git a/nixos/modules/config/system-environment.nix b/nixos/modules/config/system-environment.nix
index 3ab32f00fd1d..3362400326d2 100644
--- a/nixos/modules/config/system-environment.nix
+++ b/nixos/modules/config/system-environment.nix
@@ -23,20 +23,7 @@ in
         strings.  The latter is concatenated, interspersed with colon
         characters.
       '';
-      type = types.attrsOf (mkOptionType {
-        name = "a string or a list of strings";
-        merge = loc: defs:
-          let
-            defs' = filterOverrides defs;
-            res = (head defs').value;
-          in
-          if isList res then concatLists (getValues defs')
-          else if lessThan 1 (length defs') then
-            throw "The option `${showOption loc}' is defined multiple times, in ${showFiles (getFiles defs)}."
-          else if !isString res then
-            throw "The option `${showOption loc}' does not have a string value, in ${showFiles (getFiles defs)}."
-          else res;
-      });
+      type = types.attrsOf (types.loeOf types.str);
       apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);
     };