On Tue, Feb 17, 2009 at 6:38 PM, Brendan Eich <[email protected]> wrote:
> On Feb 17, 2009, at 6:31 PM, Mark Miller wrote: > > Now that I think I understand "current" and how weak the legacy >> constraints are, why not simply spec that your WindowProxy is the object to >> treated as the ECMAScript global object? The consequence would be that both >> f() and g() in your original example would return 2. >> > > That either opens a huge security hole, or imposes costly runtime checks on > all lexical references to global variables. > If Ian's Window object were provided as the ES global, then we'd have the security problem you're concerned about. But, IIUC, with the WindowProxy we don't. As for performance, wouldn't a shallow binding implementation[1] make switching which page is current be more expensive, but variable access within a page, even after a switch, would have no added expense? [1] The two primary methods of implementing dynamic scoping were known as "shallow binding" and "deep binding". Although "current" is not dynamic scope, some of the same issues and tradeoffs apply. In a shallow bound implementation. the WindowProxy's properties are direct properties -- not forwarded. On page navigation, these properties are copied into the Window object corresponding to the old page, and the properties on the Window object corresponding to the new page are copied onto the WindowProxy. Another analogy is with the register store in thread control blocks in an OS implementation. One can describe the semantics of computation in a given OS as directly updating the "current" per-thread register set. But instead of paying the indirection on register access, we'd rather pay the copying overhead on context switch. -- Text by me above is hereby placed in the public domain Cheers, --MarkM
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