On 2/3/14 12:35 PM, Anne van Kesteren wrote:
Could you maybe give an explicit example? I think that would help in
understanding the bug in the promises specification. I suspect not
everyone here is familiar with what you are referring to.
Sure. Given a promise named "somePromise", something needs to define
the behavior of the following two cases:
1) somePromise.then(window.postMessage.bind(window, 5, "*"));
2) var set = Object.getOwnPropertyDescriptor(location, "href").set;
var navigate = set.bind(location, "relativeURI");
somePromise.then(navigate);
The incumbent settings object (aka "the script you were called from")
defines the behavior of case #1: it determines the .origin property of
the resulting MessageEvent.
The entry settings object (aka "that weird backwards-compat thing
Location needs, which is kinda where you entered JS execution from
browser code, whatever that means") defines the behavior of case #2: it
determines the base URI used for resolving the "relativeURI" string.
My real point was that we don't want to make every single specification
writer worry about edge cases like this and various other ones scattered
about the web platform unless we absolutely have to. Especially since
the "sane" behavior in the cases above is not trivially obvious.
-Boris
P.S. For the record, if Promise.prototype.then took a WebIDL callback
as an argument the behavior would be as follows:
In case 1 the origin is the origin of the incumbent settings object at
the time the "then()" call was made. If that happened from a script,
it's the origin of that script; if it happened via more bind/callback
games then it depends on what those callbacks pushed on the script
settings stack.
In case 2 the base URI is the base URI of the global associated with the
"navigate" Function object.
P.P.S. You can add some curliques to case #2 by grabbing the setter from
one global, the location object from a second global, calling
Function.prototype.bind from a third global, having the promise come
from a fourth global, having the "then" method come from a fifth global
and binding it to the promise using Function.prototype.bind from a sixth
global. Just to keep people on their toes and all.
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