From 0e2aa97f3aa1341693885511fd78394b828c6477 Mon Sep 17 00:00:00 2001
From: Profpatsch <mail@profpatsch.de>
Date: Thu, 26 Jul 2018 20:45:55 +0200
Subject: [PATCH] lib/trivial: add assertMsg

---
 lib/default.nix |  3 ++-
 lib/trivial.nix | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/default.nix b/lib/default.nix
index dd6fcec75e21..fd3be3c6f4bc 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -59,7 +59,8 @@ let
     inherit (trivial) id const concat or and bitAnd bitOr bitXor bitNot
       boolToString mergeAttrs flip mapNullable inNixShell min max
       importJSON warn info nixpkgsVersion version mod compare
-      splitByAndCompare functionArgs setFunctionArgs isFunction;
+      splitByAndCompare functionArgs setFunctionArgs isFunction
+      assertMsg;
 
     inherit (fixedPoints) fix fix' extends composeExtensions
       makeExtensible makeExtensibleWithCustomName;
diff --git a/lib/trivial.nix b/lib/trivial.nix
index e702b8cdcc9f..bba284548d98 100644
--- a/lib/trivial.nix
+++ b/lib/trivial.nix
@@ -188,6 +188,26 @@ rec {
   warn = msg: builtins.trace "WARNING: ${msg}";
   info = msg: builtins.trace "INFO: ${msg}";
 
+  /* Print a trace message if pred is false.
+     Intended to be used to augment asserts with helpful error messages.
+
+     Example:
+       assertMsg false "nope"
+       => false
+       stderr> trace: nope
+
+       assert (assertMsg ("foo" == "bar") "foo is not bar, silly"); ""
+       stderr> trace: foo is not bar, silly
+       stderr> assert failed at …
+
+     Type:
+       assertMsg :: Bool -> String -> Bool
+  */
+  assertMsg = pred: msg:
+    if pred
+    then true
+    else builtins.trace msg false;
+
 
   ## Function annotations