On Aug 24, 2013, at 5:42 , Mathias Bynens <math...@qiwi.be> wrote:
> To clarify: consider what the Identifier Identification strawman[1] or any
> scripts that emulate similar behavior should do if Allen’s suggestion would
> be implemented:
>
> String.isIdentifierStart('\uD87E\uDC00'); // should be `false`
> String.isIdentifierStart('\u{2F800}'); // should be `true`
> // this is impossible, since `'\uD87E\uDC00' === '\u{2F800}'` and there is
> no way to distinguish these strings
>
> [1] http://wiki.ecmascript.org/doku.php?id=strawman:identifier_identification
On Aug 24, 2013, at 14:19 , Mathias Bynens <math...@qiwi.be> wrote:
> I just want to make sure it’s possible to write a polyfill (in ES5) for the
> `String.isIdentifier{Start,Part}` strawman. As long as
> `String.isIdentifierStart('\uD87E\uDC00')` and
> `String.isIdentifierStart('\u{2F800}')` are expected to return different
> results (as Allen suggests), this is impossible.
Allen didn't discuss these functions - the strawman didn't exist during the
previous round of this discussion. Your code uses string literals, and in ES6
string literals '\uD87E\uDC00' === '\u{2F800}'. This means the functions
proposed in my Identifier Identification strawman cannot tell the difference,
but then the specification doesn't require them to.
What Allen suggested, and the current ES6 spec says, is that identifiers in
source text using different Unicode escape forms behave differently:
var \uD87E\uDC00;
throws an exception, while
var \u{2F800};
declares a variable.
I don't think that's a technical problem. String.isIdentifier{Start,Part}, as I
proposed them, don't deal with actual identifiers in source text; they check
individual identifier characters. The functions are intended to be called by a
parser, and it's up to the parser to deal with escaping rules, throwing
exceptions or unescaping as specified before passing code points to
String.isIdentifier{Start,Part}. Calling the functions with string literals
doesn't seem like a useful use case.
I do think it's a problem in learning and understanding the language. Having
different rules for \uD87E\uDC00 in string literals and identifiers, and
therefore also for identifiers embedded in strings passed to eval(), adds yet
another of those random inconsistencies that already litter ECMAScript, and
ensures a "wat" moment for everybody who comes across them.
On a side note, the strawman hasn't been discussed by TC39 and hasn't been
accepted for either ES6 or ES7, so it may be a bit premature to polyfill it.
Informal feedback from some members indicated that they'd rather discuss it in
the context of a complete proposal for Unicode character properties support.
Norbert
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