From 320cdecd1697020cb367adc1f8408dbf689ca254 Mon Sep 17 00:00:00 2001
From: Profpatsch <mail@profpatsch.de>
Date: Mon, 6 Aug 2018 01:35:48 +0200
Subject: [PATCH] lib/trivial: add assertOneOf

---
 lib/default.nix |  2 +-
 lib/trivial.nix | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/default.nix b/lib/default.nix
index fd3be3c6f4bc..358c8ca0b8f9 100644
--- a/lib/default.nix
+++ b/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;
diff --git a/lib/trivial.nix b/lib/trivial.nix
index bba284548d98..f1001ee10ca3 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