Don't stackoverflow on recursive lists and maps. Fix #3.

src/vm/wren_core.c

@@ -141,6 +141,34 @@ static const char* coreLibSource =
 "}\n"
 "\n"
 "class String is Sequence {\n"
+"  // Avoids recursively calling [toString] on [object] and overflowing the\n"
+"  // stack.\n"
+"  //\n"
+"  // If we are already within a call to [safeToString_] on [object] in this\n"
+"  // fiber, then this returns \"...\". Otherwise, it returns the result of\n"
+"  // calling [ifUnseen].\n"
+"  static safeToString_(object, ifUnseen) {\n"
+"    if (__seenByFiber == null) __seenByFiber = new Map\n"
+"    var seen = __seenByFiber[Fiber.current]\n"
+"    if (seen == null) {\n"
+"      __seenByFiber[Fiber.current] = seen = new List\n"
+"    }\n"
+"\n"
+"    // See if we are recursing on it.\n"
+"    for (outer in seen) {\n"
+"      if (Object.same(outer, object)) return \"...\"\n"
+"    }\n"
+"\n"
+"    seen.add(object)\n"
+"\n"
+"    var result = ifUnseen.call()\n"
+"\n"
+"    seen.removeAt(-1)\n"
+"    if (seen.count == 0) __seenByFiber.remove(Fiber.current)\n"
+"\n"
+"    return result\n"
+"  }\n"
+"\n"
 "  bytes { new StringByteSequence(this) }\n"
 "}\n"
 "\n"
@@ -162,7 +190,7 @@ static const char* coreLibSource =
 "    return other\n"
 "  }\n"
 "\n"
-"  toString { \"[\" + join(\", \") + \"]\" }\n"
+"  toString { String.safeToString_(this) { \"[\" + join(\", \") + \"]\" } }\n"
 "\n"
 "  +(other) {\n"
 "    var result = this[0..-1]\n"
@@ -178,16 +206,18 @@ static const char* coreLibSource =
 "  values { new MapValueSequence(this) }\n"
 "\n"
 "  toString {\n"
-"    var first = true\n"
-"    var result = \"{\"\n"
+"    return String.safeToString_(this) {\n"
+"      var first = true\n"
+"      var result = \"{\"\n"
 "\n"
-"    for (key in keys) {\n"
-"      if (!first) result = result + \", \"\n"
-"      first = false\n"
-"      result = result + key.toString + \": \" + this[key].toString\n"
+"      for (key in keys) {\n"
+"        if (!first) result = result + \", \"\n"
+"        first = false\n"
+"        result = result + key.toString + \": \" + this[key].toString\n"
+"      }\n"
+"\n"
+"      return result + \"}\"\n"
 "    }\n"
-"\n"
-"    return result + \"}\"\n"
 "  }\n"
 "}\n"
 "\n"