Symbols, Protocols, Frames, and Versioning

Domenic Denicola domenic at domenicdenicola.com
Wed Oct 3 12:39:18 PDT 2012


Would it suffice to allow cross-frame sharing of symbols via postMessage and its structured clone algorithm? They're immutable, right?

On Oct 3, 2012, at 15:01, "Brendan Eich" <brendan at mozilla.org> wrote:

> Thanks for pointing this out. Python's dunder-prefixing or anything like it in JS has that advantage: you can spell the magic property name with a string that works in any frame or global object. Of course strings can collide.
> 
> Symbols are useful in spite of this, but it is telling that we want @iterator to be a singleton across all potentially connected frames.
> 
> So should there be a way in the language to create singleton symbols? If so, how?
> 
> /be
> 
> Kevin Smith wrote:
>> One of the main use cases for symbols is for defining object "protocols" that don't suffer from property name conflicts.  The recently discussed `iterator` and `toStringTag` method names fall into this category.  The idea is that we can implement the protocol by defining methods using symbols, and thus avoid namespacing considerations.
>> 
>> Designing and maintaining a global namespace is, well, no fun.
>> 
>> But consider the multiple-global case in which we have scripts running in more than one frame.  It seems like protocols should be transferrable across frames.  For built-in protocols like `iterator`, this has to work:
>> 
>>    function f(iterable) {
>>      for (x of iterable) {
>>        // This must work regardless of which frame `iterable` comes from
>>      }
>>    }
>> 
>> But what about user-defined protocols?  Let's say we have a "Persistable" protocol:
>> 
>>    export var persistName = new Symbol; // unique, not "private"
>> 
>> And a function which makes use of this protocol:
>> 
>>    import persistName from "Persistable.js";
>> 
>>    function usePersistable(obj) {
>> 
>>      if (obj[persistName])
>>        obj[persistName]();
>>    }
>> 
>> It seems like `usePersistable` should be able to work as expected even if `obj` comes from a different frame (in which "Persistable.js" was separately loaded).
>> 
>> Another expression of the same problem occurs with versioning.
>> 
>> Suppose that in a fairly complex module dependency graph, "Persistable-0.1.js" and "Persistable-0.2.js" are simultaneously loaded. ("Persistable" is on github and therefore in perpetual version-zero purgatory.)  It seems reasonable to expect that objects implementing the protocol defined by "Persistable-0.2.js" should be able to work with functions consuming the "Persistable-0.1.js" protocol.  But that is not possible with unique symbols.
>> 
>> In summary, I don't think that we can really avoid global namespacing issues using system-generated unique symbols as we currently conceive of them.  Built-in protocols like `iterator` are a special "cheating" case, but we need to have an equally consistent story for user-defined protocols.
>> 
>> Kevin
>> 
>> 
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