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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