I was looking at D75 of 3.8 "Surrogates" My point is that there's no "legal" scenario for converting basically a UTF-32 input to an isolated surrogate pair. No valid Unicode string could contain that. So why support it?
-Shawn -----Original Message----- From: Norbert Lindenberg [mailto:[email protected]] Sent: Wednesday, December 12, 2012 2:40 PM To: Shawn Steele Cc: Norbert Lindenberg; Erik Arvidsson; [email protected] Subject: Re: String.fromCodePoint and surrogate pairs? The Unicode standard defines "code point" as any value in the range of integers from 0 to 0x10FFFF - see definitions D9 and D10 of chapter 3 [1]. Once you exclude surrogate code points, you have Unicode scalar values (definition D76), so you're basically proposing a String.fromScalarValue function. But then, why not also exclude code points that Unicode has defined as non-characters (chapter 16.7 [2])? It seems we're getting into policy-setting here, and so far ECMAScript has avoided setting policy for how you can use strings. Norbert [1] http://www.unicode.org/versions/Unicode6.2.0/ch03.pdf [2] http://www.unicode.org/versions/Unicode6.2.0/ch16.pdf On Dec 12, 2012, at 13:55 , Shawn Steele wrote: > IMO String.fromCodePoint should disallow U+D800-U+DFFF. > > There's already fromCharCode that does that, and a according to The Unicode > Standard, isolated surrogates have no meaning on their own and goes on to > compare them to illegal UTF-8 sequences. IMO "CodePoint" is a 21 Unicode > code point, and explicitly not UTF-16, so it shouldn't confuse things by > allowing UTF-16 (or other encoding) forms. > > -Shawn > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Norbert > Lindenberg > Sent: Wednesday, December 12, 2012 1:25 PM > To: Erik Arvidsson > Cc: [email protected] > Subject: Re: String.fromCodePoint and surrogate pairs? > > Do you know what the people who talked to you mean by "aware of UTF-16 code > units"? > > As specified, String.fromCodePoint, accepts all UTF-16 code units because > they use a subset of the integers allowed as code points (0 to 0xFFFF versus > 0 to 0x10FFFF). For non-surrogate values, you get exactly what you expect. > Surrogate values are interpreted as surrogate code points, which are valid > code points in Unicode (their use makes a string ill-formed in Unicode > terminology, but the proposed ECMAScript spec ignores issues of > well-formedness for compatibility with ES5). Since in conversion to UTF-16 a > surrogate code point just becomes the corresponding code unit, it can happen > that two surrogate code points (an ill-formed sequence) become a well-formed > surrogate pair: > String.fromCodePoint(0xD83D, 0xDE04) => "\uD83D\uDE04" = > "😄". > > The story for UTF-8 is very different: Of course all UTF-8 code units would > be accepted by String.fromCodePoint, but they would turn into a completely > different character sequence. E.g., the UTF-8 byte sequence for 😄: > String.fromCodePoint(0xF0, 0x9F, 0x98, 0x84) => "\u00F0\u009F\u0098\u0084" = > "ð\u009F\u0098\u0084" (the last three are control characters). > > Handling UTF-8 would require a way to identify the character encoding to > convert from, which indicates the beginning of an encoding conversion API, > and the internationalization ad-hoc decided not to work on one within > ECMAScript. There is an API being defined as part of the encoding standard > project at WhatWG. > > Norbert > > > On Dec 12, 2012, at 7:46 , Erik Arvidsson wrote: > >> It was suggested to me that we could probably extend String.fromCodePoint to >> be aware of UTF-16 code units too. It seems doable since the lead surrogate >> is not a valid code point. >> >> The question is if it is worth it? It seems like we are going down a >> slippery slope if we start to do things like this. Should we also handle >> UTF-8 code units. Maybe it is better not to do this and try to get people to >> move away from UTF-16 code units and move them towards code points. >> >> -- >> erik > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
