Domenic Denicola wrote:
Would it suffice to allow cross-frame sharing of symbols via postMessage and
its structured clone algorithm? They're immutable, right?
They are immutable but you'd still have to pass your @iterator to
another same-origin frame, and then have to use it carefully when
iterating objects from the first frame. This is unusable.
Making @iterator a singleton (to the limits of observability:
same-origin, CORS, out-of-process-via-DOM window.open in IE9+
notwithstanding!) can be done. That wins, no need to pass and use the
other frame's @iterator symbol.
But how to let users create such singletons?
/be
On Oct 3, 2012, at 15:01, "Brendan Eich"<[email protected]> 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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