Implement ES6 symbol registry and predefined symbols

src/symbol.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 45 matching lines @@
 
 function SymbolValueOf() {
   if (!(IS_SYMBOL(this) || IS_SYMBOL_WRAPPER(this))) {
     throw MakeTypeError(
       'incompatible_method_receiver', ["Symbol.prototype.valueOf", this]);
   }
   return %_ValueOf(this);
 }
 
 
+function SymbolRegistry() {

[Michael Starzinger, 2014/03/19 10:15:25]

nit: This naming makes it look like this is the implementation of
Symbol.registry, which doesn't exist. Could we instead rename it to
GetSymbolRegistry and move it up to around line 37 to clarify that it's an
internal helper?

[rossberg, 2014/03/20 10:40:13]

Done.

+  var registry = %SymbolRegistry();
+  if (!('hook' in registry)) {
+    registry.hook = {__proto__: null};
+    registry.for = {__proto__: null};
+    registry.keyFor = {__proto__: null};
+  }
+  return registry;
+}
+
+
+function SymbolHook(key) {

[Michael Starzinger, 2014/03/19 10:15:25]

nit: Likewise for GetSymbolHook here.

[rossberg, 2014/03/20 10:40:13]

Renamed to InternalSymbol.

+  var registry = SymbolRegistry();
+  if (!(key in registry.for)) {
+    registry.hook[key] = %CreateSymbol(key);
+  }
+  return registry.hook[key];
+}
+
+
+function SymbolFor(key) {
+  key = TO_STRING_INLINE(key);
+  var registry = SymbolRegistry();
+  if (!(key in registry.for)) {
+    var symbol = %CreateSymbol(key);
+    registry.for[key] = symbol;
+    registry.keyFor[symbol] = key;
+  }
+  return registry.for[key];
+}
+
+
+function SymbolKeyFor(symbol) {

[Michael Starzinger, 2014/03/19 10:15:25]

Is it specified what "Symbol.keyFor(Symbol.create)" should return?

[rossberg, 2014/03/20 10:40:13]

Simply undefined, since it's not in the (public) registry. I added a test.

+  if (!IS_SYMBOL(symbol)) {
+    throw MakeTypeError("not_a_symbol", ["Symbol.keyFor", symbol]);

[sof, 2014/03/19 09:06:47]

Looks like 'symbol' (%1) isn't used in the not_a_symbol format string.

[rossberg, 2014/03/20 10:40:13]

Done.

+  }
+  return SymbolRegistry().keyFor[symbol];
+}
+
+
 // ES6 19.1.2.8
 function ObjectGetOwnPropertySymbols(obj) {
   if (!IS_SPEC_OBJECT(obj)) {
     throw MakeTypeError("called_on_non_object",
                         ["Object.getOwnPropertySymbols"]);
   }
 
   // TODO(arv): Proxies use a shared trap for String and Symbol keys.
 
   return ObjectGetOwnPropertyKeys(obj, true);
 }
 
 
 //-------------------------------------------------------------------
 
+var symbolCreate = SymbolHook("@@create");

[sof, 2014/03/19 09:06:47]

Just trying to understand.. where is the implicit addition to the global
registry for the well-known symbols given?

[rossberg, 2014/03/20 10:40:13]

Hm, I'm not sure what you are asking. The well-known symbols are not supposed to
be in the (public) registry. Or did I miss something?

[sof, 2014/03/20 10:49:06]

ok, good - thought not. But the lookup in InternalSymbol (nee SymbolHook) is
confusing to me -- why is registry.for being consulted?

[rossberg, 2014/03/20 11:05:20]

Because that's a bug. :) Thanks for pointing out! Fixed

+var symbolHasInstance = SymbolHook("@@hasInstance");
+var symbolIsConcatSpreadable = SymbolHook("@@isConcatSpreadable");
+var symbolIsRegExp = SymbolHook("@@isRegExp");
+var symbolIterator = SymbolHook("@@iterator");
+var symbolToStringTag = SymbolHook("@@toStringTag");
+var symbolUnscopables = SymbolHook("@@unscopables");
+
+
+//-------------------------------------------------------------------
+
 function SetUpSymbol() {
   %CheckIsBootstrapping();
 
   %SetCode($Symbol, SymbolConstructor);
   %FunctionSetPrototype($Symbol, new $Object());
 
+  %SetProperty($Symbol, "create", symbolCreate, DONT_ENUM);
+  %SetProperty($Symbol, "hasInstance", symbolHasInstance, DONT_ENUM);
+  %SetProperty($Symbol, "isConcatSpreadable",
+      symbolIsConcatSpreadable, DONT_ENUM);
+  %SetProperty($Symbol, "isRegExp", symbolIsRegExp, DONT_ENUM);
+  %SetProperty($Symbol, "iterator", symbolIterator, DONT_ENUM);
+  %SetProperty($Symbol, "toStringTag", symbolToStringTag, DONT_ENUM);
+  %SetProperty($Symbol, "unscopables", symbolUnscopables, DONT_ENUM);
+  InstallFunctions($Symbol, DONT_ENUM, $Array(
+    "for", SymbolFor,
+    "keyFor", SymbolKeyFor
+  ));
+
   %SetProperty($Symbol.prototype, "constructor", $Symbol, DONT_ENUM);
   InstallFunctions($Symbol.prototype, DONT_ENUM, $Array(
     "toString", SymbolToString,
     "valueOf", SymbolValueOf
   ));
 }
 
 SetUpSymbol();
 
 
 function ExtendObject() {
   %CheckIsBootstrapping();
 
   InstallFunctions($Object, DONT_ENUM, $Array(
     "getOwnPropertySymbols", ObjectGetOwnPropertySymbols
   ));
 }
 
 ExtendObject();