lib/trivial: add assertOneOf

1 files changed, 18 insertions, 0 deletions

diff --git a/lib/trivial.nix b/lib/trivial.nix
index bba284548d9..f1001ee10ca 100644
--- a/lib/trivial.nix
+++ b/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