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Re: Overload of Dictionary and Callback - why is this disallowed?

From: Boris Zbarsky <bzbarsky@mit.edu>
Date: Tue, 1 Dec 2015 10:43:45 -0500
To: Harald Alvestrand <harald@alvestrand.no>, "public-script-coord@w3.org" <public-script-coord@w3.org>
Message-ID: <565DC031.5000205@mit.edu>
On 12/1/15 9:56 AM, Harald Alvestrand wrote:
> But the table at http://heycam.github.io/webidl/#idl-overloading says
> that dictionary, callback functions and objects aren't distinguishable
> from each other.
>
> Can someone point me to an explanation of why?

Historically, because IDL aimed to not make things distinguishable that 
might be hard to distinguish.

In particular, any object can really be used as a dictionary.  JS 
Functions are objects.  If a Function is passed to an overload of 
dictionary and callback function, which one should it be used for?

It's even worse for the generic "object", which is literally "any object 
at all".

That said, I think we could, in fact, make callback functions and 
dictionaries distinguishable.  It wouldn't even require any changes to 
the overload resolution algorithm, as far as I can tell: 
http://heycam.github.io/webidl/#dfn-overload-resolution-algorithm step 
12 already checks IsCallable(V), so we would end up with overloads 
between dictionary and callback function distinguished by what 
IsCallable() returns.  Dictionaries would still not be distinguishable 
from callback _interfaces_, of course, and none of this stuff would be 
distinguishable from "object".

That said, what is the actual use case here?  If this is a new API, you 
should probably just use a dictionary and not create the two overloads.

-Boris
Received on Tuesday, 1 December 2015 15:44:17 UTC

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