libexpr/json-to-value: Migrate parseJSON to return a Value

Part of #1136

Change-Id: I5ac23ce461a106360b150e64a6bc0f2f6a6a6964

lix/libexpr/json-to-value.cc

@@ -190,13 +190,13 @@ public:
     }
 };
 
-void parseJSON(EvalState & state, const std::string_view & s_, Value & v)
+Value parseJSON(EvalState & state, const std::string_view & s_)
 {
     JSONSax parser(state);
     bool res = JSON::sax_parse(s_, &parser);
     if (!res)
         throw JSONParseError("Invalid JSON Value");
-    v = parser.result();
+    return parser.result();
 }
 
 }

lix/libexpr/json-to-value.hh

@@ -12,6 +12,5 @@ struct Value;
 
 MakeError(JSONParseError, Error);
 
-void parseJSON(EvalState & state, const std::string_view & s, Value & v);
-
+Value parseJSON(EvalState & state, const std::string_view & s);
 }

lix/libexpr/primops.cc

@@ -1622,7 +1622,7 @@ static void prim_fromJSON(EvalState & state, Value * * args, Value & v)
 {
     auto s = state.forceStringNoCtx(*args[0], noPos, "while evaluating the first argument passed to builtins.fromJSON");
     try {
-        parseJSON(state, s, v);
+        v = parseJSON(state, s);
     } catch (JSONParseError &e) {
         e.addTrace(nullptr, "while decoding a JSON string");
         throw;