Re: WebIDL overload resolution, arrays and Nullable

On Fri, Jun 29, 2012 at 9:25 AM, Joshua Bell <jsbell@chromium.org> wrote:

> Over in WebKit-land there's some disagreement about WebIDL method overload
> resolution, specifically around passing null, arrays (T[]) and the concept
> of Nullable.
>
> Here's an example where we're just not sure what the WebIDL spec dictates:
>
> void f(float[] x); // overload A
> void f(DOMString x); // overload B
>
> WebIDL itself, of course, doesn't dictate how matching and dispatching
> should be implemented; it instead defines whether types are
> distinguishable. The implication is that an IDL that defines methods with
> signatures that are not distinguishable is invalid, so it's a non-issue in
> terms of the spec. So rephrasing the question: are the above types
> distinguishable? And if so, which would be expected to handle the call:
>
> f(null);
>
> Several interpretations and hence outcomes occur to us, hopefully
> presented without indicating my particular bias:
>
> (1) T[] is inherently Nullable (i.e. T[] === T[]?), DOMString is not,
> overload A would be invoked with a null argument and the implementation is
> expected to handle this case
> (2) T[] accepts null but the IDL type to ECMAScript conversion rules
> produce an empty array; overload A is invoked with an empty float array
> (3) T[] does not match null, but as null is an ECMAScript primitive value
> it is run through ToString() and hence overload B is invoked with DOMString
> "null"
> (4) Either T[] or DOMString could match null, so types of the arguments
> are not distinguishable and hence the above is invalid WebIDL
> (5) Neither T[] nor DOMString is inherently Nullable, so a TypeError is
> thrown
>
> Anyone? (Cameron?)
>

#5.  Neither T[] nor DOMString admits null as a value.

"Note also that null is not a value of type DOMString. To allow null, a
nullable DOMString, written as DOMString? in IDL, needs to be used. "

http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString

"The T[] type is a parameterized type. The values of this type are non-null
references to arrays of values of type T."

http://dev.w3.org/2006/webapi/WebIDL/#idl-array

- Kyle