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[Bug 24959] "Exposed=Window,Worker" will be parsed to 2 extended attributes

From: <bugzilla@jessica.w3.org>
Date: Fri, 02 May 2014 02:30:03 +0000
To: public-script-coord@w3.org
Message-ID: <bug-24959-3890-zBzn7r7zOe@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24959

Nils Barth <nbarth+w3bugzilla@google.com> changed:

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--- Comment #1 from Nils Barth <nbarth+w3bugzilla@google.com> ---
I believe that the issue is that extended attribute phrase grammar is
*context-sensitive*, specifically due to collision of , as a extended attribute
separator  (*between* extended attributes) and as an identifier separator
(*within* an extended attribute).

Cameron, is this intended?

This makes parsing extended attributes *much* more difficult, because we need
to do it during semantic analysis, not during parsing.

Consider supporting both [Exposed=Window,Worker] and [Global]
(grammar-wise: these are both valid interface extended attributes).

Writing this as:
[Exposed=Window,Worker,Global]
...is ambiguous, and requires complex semantic analysis just to *parse*.

Writing this as:
[Exposed=Window,Worker, Global]
...seems to disambiguate, but requires white space to be significant.

Using a different separator fixes this:
[Exposed=Window;Worker,Global]

In Blink we've been using | for extended attribute value separators, so we'd
write this as:
[Exposed=Window|Worker,Global]

...or actually (style-wise):
[
    Exposed=Window|Worker,
    Global,
] interface Foo { ... };

Specifically, the problem is with the production rule
ExtendedAttributeIdentList,
which allows one to write [Exposed=Window,Worker]
http://heycam.github.io/webidl/#prod-ExtendedAttributeIdentList

It's currently used for 3 extended attributes:
[Exposed], [Global], and [PrimaryGlobal]


The underlying issue is that the extended attribute phrase grammar is
*context-sensitive*, and thus needs to be parsed during semantic analysis.

This is stated at:
http://heycam.github.io/webidl/#idl-extended-attributes
"""
Any extended attribute encountered in an IDL fragment is matched against the
following six grammar symbols to determine which form (or forms) it is in:
ExtendedAttributeNoArgs
ExtendedAttributeArgList
ExtendedAttributeNamedArgList
ExtendedAttributeIdent
ExtendedAttributeIdentList
ExtendedAttributeTypePair
...
Each extended attribute definition will state which of the above six forms are
allowed.
"""
I.e., the phrase grammar depends on the ExtendedAttribute (the value of the
key).

This is *not* a serious issue for argument lists, like [Constructor(foo x, bar
y)], since the () delimiters set this off, allowing context-free parsing.

However, for bare identifier lists, this causes parsing problems.

Could we change this?

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Received on Friday, 2 May 2014 02:30:05 UTC

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