From f890830b337f321f17ad36b45d7d63801a753554 Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Fri, 11 Dec 2020 16:31:14 +0100
Subject: [PATCH] primops/fromJSON: add error position in case of parse error

This makes it easier to track down where invalid JSON was passed to
`builtins.fromJSON`.
---
 src/libexpr/primops.cc | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 41f06c219..63624c520 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1621,7 +1621,12 @@ static RegisterPrimOp primop_toJSON({
 static void prim_fromJSON(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     string s = state.forceStringNoCtx(*args[0], pos);
-    parseJSON(state, s, v);
+    try {
+        parseJSON(state, s, v);
+    } catch (JSONParseError &e) {
+        e.addTrace(pos, "while decoding a JSON string");
+        throw e;
+    }
 }
 
 static RegisterPrimOp primop_fromJSON({