From e1fc9f66900c1ea2b835bafbdc82beb7a10e88ff Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 2 Apr 2020 19:03:32 +0200
Subject: [PATCH] Improve error message

---
 src/libexpr/primops/fetchTree.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 8a4970726..0838416df 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -81,7 +81,7 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
         input = lookupInRegistries(state.store, input).first;
 
     if (evalSettings.pureEval && !input->isImmutable())
-        throw Error("in pure evaluation mode, 'fetchTree' requires an immutable input");
+        throw Error("in pure evaluation mode, 'fetchTree' requires an immutable input, at %s", pos);
 
     // FIXME: use fetchOrSubstituteTree
     auto [tree, input2] = input->fetchTree(state.store);