lib/trivial: add assertOneOf

lib/default.nix

@@ -60,7 +60,7 @@ let
       boolToString mergeAttrs flip mapNullable inNixShell min max
       importJSON warn info nixpkgsVersion version mod compare
       splitByAndCompare functionArgs setFunctionArgs isFunction
-      assertMsg;
+      assertMsg assertOneOf;
 
     inherit (fixedPoints) fix fix' extends composeExtensions
       makeExtensible makeExtensibleWithCustomName;

lib/trivial.nix

@@ -203,11 +203,29 @@ rec {
      Type:
        assertMsg :: Bool -> String -> Bool
   */
+  # TODO(Profpatsch): add tests that check stderr
   assertMsg = pred: msg:
     if pred
     then true
     else builtins.trace msg false;
 
+  /* Specialized `assertMsg` for checking if val is one of the elements
+     of a list. Useful for checking enums.
+
+     Example:
+       let sslLibrary = "libressl"
+       in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
+       => false
+       stderr> trace: sslLibrary must be one of "openssl", "bearssl", but is: "libressl"
+
+     Type:
+       assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool
+  */
+  assertOneOf = name: val: xs: assertMsg
+    (lib.elem val xs)
+    "${name} must be one of ${
+      lib.generators.toPretty {} xs}, but is: ${
+        lib.generators.toPretty {} val}";
 
   ## Function annotations