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RE: <Further LC Followup from IE> RE: Potential bugs identified in XHR LC Test Suite

From: Ian Hickson <ian@hixie.ch>
Date: Tue, 17 Jun 2008 08:51:02 +0000 (UTC)
To: Zhenbin Xu <Zhenbin.Xu@microsoft.com>
Cc: Sunava Dutta <sunavad@windows.microsoft.com>, Web API public <public-webapi@w3.org>, IE8 Core AJAX SWAT Team <ieajax@microsoft.com>, "public-webapps@w3.org" <public-webapps@w3.org>
Message-ID: <Pine.LNX.4.62.0806170835100.6527@hixie.dreamhostps.com>

On Tue, 17 Jun 2008, Zhenbin Xu wrote:
> > >
> > > I am not sure if I understand your question. responseXML.parseError 
> > > has the error information 
> > > http://msdn.microsoft.com/en-us/library/aa926483.aspx
> >
> > Oh, I assumed Sunava meant a conforming Document object was returned. 
> > A parseError-type object would be what I had in mind, yes. However, if 
> > we do this, then we should specify it. If we don't specify it, I'd 
> > rather have an exception.
> 
> The spec can simply state that a conforming document object is returned, 
> which includes out-of-band error information. This is what IE does today 
> and is a very reasonable approach that allows rich error information for 
> debugging.

I don't believe it is conforming for Document objects to have parseError 
attributes, but I could be mistaken -- is there a spec for parseError? 
Even if there isn't, though, I agree that it is a generally good solution 
to the problem, I'm just saying that we should specify it, so that UAs 
can be standards-compliant and support it interoperably.


> > Not really; if the script is expecting an exception, and receives null 
> > instead, then they'll just get an exception as soon as they 
> > dereference the object, which in almost all cases will be straight 
> > away.
> 
> [Zhenbin] I should explain the scenario I talked about. For instance, if 
> I am to write a wrapper object myXHR, it makes a difference for me when 
> I do the following
> 
>     myXHR.responseXML
>         if (!_innerResponseXML)
>       {
>                 try
>                 {
>                    _innerResponseXML = _innerXHR.responseXML;
>                 }
>                 catch (e)
>             {
>                 _myexception = e;
>                 return _dummpyResponseXML;
>             }
>         }
>         return _innerResponseXML;
> 
> My try catch would not catch null. And the exception would be passed on 
> to my callers, which is not what I wanted.

Indeed.


> > > If we are going to spec it to accommodate all existing browsers, we 
> > > would want to make it "return null or INVALID_STATE_ERR exception".
> >
> > We want interoperable behaviour, so defining it in this way would be a 
> > bad idea. (I don't really have an opinion either way about exception 
> > vs null, but it seems that we should just pick whatever is most 
> > commonly implemented, which I'm guessing is what Anne did here.)
> 
> Fair enough. So let's pick one.
> 
> What is "commonly implemented"? Is it largest browser market share?

Since the cost to implementations for fixing the problem is independent of 
the size of the user base, it would be based just on the number of 
independent implementations.


> Is it number of enterprise applications written on top of particular 
> browser?

All the browsers want to be compatible with the Web, so if there are mroe 
Web sites depending on the exception behaviour than the null behaviour, 
then we clearly should do the exception behaviour. And vice versa. Do we 
have any good numbers on this? (That there are widely deployed browsers 
that return null instead of throwing an exception tends to suggest that 
Web pages don't depend on either behaviour; we'd probably need evidence 
to the contrary to decide one way or the other based on compatibility.)


> Is it the number of browers, in which case I hope my fictional 
> home grown personal browser gets a vote :-)

Obviously fictional browsers aren't relevant, since the cost of fixing a 
fictional browser is zero.


> From a pure technical point of view, predictably throw exception on 
> state violations is easier to understand.  I hope you would agree there 
> is value to change spec for the sake of consistent programming model 
> (which happens to be the IE model).

Indeed.


> Did the spec call out that responseXML returned from XHR should have 
> equivalent DOM support as UA's object?  If it is, that would be a good 
> topic for us to debate about.

I believe the spec just says that you return a Document object; it is the 
lack of a distinction between different Document objects that requires the 
level of support to be the same. As you said, a consistent programming 
model has value.


> I disagree that because DOM Level 3 is 3+ yr old spec that every UA has 
> to support it in order to be XHR compliant, if that is what you implied. 

I didn't mean to imply that. I don't think XHR should require any 
particular level of support.

-- 
Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
Received on Tuesday, 17 June 2008 08:51:42 GMT

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