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Re: [WebIDL] Handling undefined in Overload Resolution Algorithm

From: Cameron McCormack <cam@mcc.id.au>
Date: Thu, 4 Aug 2011 23:01:39 +1200
To: Lachlan Hunt <lachlan.hunt@lachy.id.au>
Cc: public-script-coord@w3.org, jonas@sicking.cc, bzbarsky@mit.edu, allen@wirfs-brock.com
Message-ID: <20110804110139.GB14448@wok.mcc.id.au>
Lachlan Hunt:
> No, this is not always true for all methods.  Consider:

I was a bit worried about something like this, but didn’t think it
through.

> document.write() // writes nothing
> document.write(undefined) // writes "undefined"
> 
> Although this particular case isn't overloaded, so the overload
> resolution algorithm won't be run.

Actually, I did recently remove the different behaviour between
overloaded and non-overloaded operation invocation.  So it does look
like treating undefined the same as omitting the argument entirely does
not work across the board.

> >* Anywhere an undefined would help select a nullable primitive type
> >during overload resolution, it should help select nullable types
> >regardless of the inner type. (This would still matter in cases
> >where explicit undefined is passed in the middle of the argument list,
> >and later arguments are not undefined.)
> 
> Is that intended to handle this case?
> 
> e.g. consider a function
> 
> void f(in DOMString x, in DOMString y, in DOMString z);
> 
> f("a", undefined, "c")
> 
> This should not throw a type error, even if the function is overloaded.

Looking at how the overload resolution algorithm currently works, it
looks at each argument position across all of the overloads for a given
argument list length, and if it’s not the case that all the types are
the same at that position, then it uses the ECMAScript value passed in
to discriminate between them.  So in the case where you just have that
single ‘f’ operation above, it wouldn’t throw due to not finding an
overload, and undefined would get converted to a DOMString as you
expect.

If however you had

  void f(in DOMString x, in DOMString y, in DOMString z);
  void f(in DOMString x, in float y, in DOMString z);

and you called `f("a", undefined, "b")`, then it would use undefined to
help distinguish between those two, and it would choose the second one.

> >And finally for the API at hand:
> >
> >Element querySelector(in DOMString a, in optional Element b);
> >Element querySelector(in DOMString a, in sequence<Node>? b);
> >
> >Overloads are:
> >(DOMString)
> >(DOMString, Element)
> >(DOMString, sequence<Node>?)
> >
> >querySelector(undefined) === throw TypeError
> 
> No, I don't think that one should throw a type error.  Right now, it
> stringifies to "undefined" in implementations and doesn't throw.

OK.  So treating querySelector(undefined) as a zero argument call
doesn’t work here (in conjunction with throwing for too few arguments).

Will think about it more tomorrow…

-- 
Cameron McCormack ≝ http://mcc.id.au/
Received on Thursday, 4 August 2011 11:02:20 UTC

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