[swift-evolution] A path forward on rationalizing unicode identifiers and operators

[Mike Sanderson]

After reading the original proposal and the Unicode Annex #31 document that
underlies it (https://unicode.org/reports/tr31/) I think that  the existing
work as an underlying layer could help frame the discussion and push it
forward.

Although I do see the concerns about defining things too strictly in
Unicode terms, the proposal brings in some really helpful work without
really tethering Swift to Unicode categories now or in the future. If
nothing else, throwing out the Unicode framework to start from scratch was
always likely to send the discussion into a massive detailed, unstructured
discussion of how to handle individual characters, which seems to be what
happened.

Particularly regarding identifiers, taking advantage of the Unicode work on
Unicode characters seems to be more promising to make progress than
starting from scratch.

My suggestion is to revive the proposal in part, keeping identifier work
and leaving operator work at the structural level:

1) Identifiers: Swift valid identifier characters will be aligned with UAX#31,
with exceptions.
2) Operators: Swift valid operator characters will be defined as an
arbitrary list, subject to community discussion within the Unicode
structure.

Identifiers:

The existing Unicode work in document UAX#31 is entirely about identifiers
and like the proposal authors, I think Swift can use this, with exceptions,
as the proposal specifies. In particular, the work in UAX#31 for
identifiers seems well-thought out and worthy of inclusion, and specifies
that individual programming languages can define their syntax *relative* to
these defaults, which is probably what Swift wants to do. 1) Swift
definitely will have "_" as a valid identifier-head. 2) Swift may need
additional rules for using Emoji as identifiers.

This is described more clearly later in the proposal, under
details-identifiers https://github.com/xwu/swift-evolution/blob/master/
proposals/NNNN-refining-identifier-and-operator-symbology.md#identifiers It
also has some guidelines on identifier equivalence that would be worth
pulling in.

Not sure if there is objection to the identifier part of the proposal. It
may have been phrased too prescriptively in terms of Unicode, but that
doesn't seem to be the intent.

Operators:

Unicode doesn't provide any guidance on operators, so what Swift includes
is arbitrary regardless. The rule in the proposal was solid but, first, too
complex: Default operator characters would be all unicode characters 1)
tagged by Unicode as 1a) "Pattern Syntax" characters and 1b) "Mathematical"
but 2) excluding characters in the blocks for 2a) "Geometric Shapes", 2b)
"Miscellaneous Symbols" and 2c) "Miscellaneous Technical". I can see why
this was considered too complex, but as an attempt for a non-arbitrary
definition it was a great start.

So the Swit list of valid operatior characters is arbitrary: embrace it.

Right now Swift's arbitrary list of Unicode ranges that are valid
operator-head or operator-character is unreadable and therefore mostly
useless for looking up which characters are valid (
https://developer.apple.com/library/content/documentation/
Swift/Conceptual/Swift_Programming_Language/LexicalStructure
.html#//apple_ref/doc/uid/TP40014097-CH30-ID418). It's introduced in text
as:

Custom operators can begin with one of the ASCII characters /, =, -, +, !,
> *, %, <, >, &, |, ^, ?, or ~, or one of the Unicode characters defined in
> the grammar below (which include characters from the Mathematical
> Operators, Miscellaneous Symbols, and Dingbats Unicode blocks, among
> others). After the first character, combining Unicode characters are also
> allowed.


This description is more helpful. Discussions of characters should make
reference to which code blocks they belong to, and to attempt to align with
Unicode groupings or categories when it makes sense to do so.

The online "UnicodeSet" functionality is very useful for seeing what
characters are included (https://unicode.org/cldr/utility/list-unicodeset.
jsp). For the current Swift 4 Language Reference, I made a list to see what
was included, which for this tool required manually expanding some ranges
and making a few other inferences, i.e., Swift allows the "dingbats" code
block with the exception of "Dingbat circled digits."

You can see the unicode-set list of existing Swift http://bit.ly/2yYzduM
(warning: may not be perfect).

The UnicodeSet tool doesn't scale very well for discussions. Regardless, it
would be very helpful if the final lists were accordingly grouped by
Unicode block or subhead, together with at least some descriptions /
examples:

*Ascii*
/­  =­  -­  +­  !­  *­  %­  <­  >­  &­  |­  ^­  ~­  ?

*Latin 1 Supplement*
Latin-1 punctuation and symbols: (INVERTED EXCLAMATION MARK) U+00A1–U+00A7,
U+00A9, U+00AB, U+00AC, U+00AE, U+00B0–U+00B1, U+00B6, U+00BB
(RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK)
Punctuation: U+00BF (INVERTED QUESTION MARK)
Mathematical operator: U+00D7 (MULTIPLICATION SIGN), U+00F7 (DIVISION SIGN)

*General Punctuation*
General punctuation: U+2016 (DOUBLE VERTICAL LINE)-U+2057 (QUADRUPLE PRIME)
Quotation marks: U+2039, U+203A
Double punctuation for vertical text: U+203C, U+2047-U+2049
Archaic punctuation: U+2056 (THREE DOT PUNCTUATION) - U+205E (VERTICAL FOUR
DOTS)


 ...


The included names of characters are just to help illustrate what kind of
marks are in each block, which with the titles helps to stay oriented. If
some kind of organization like this made it into Swift documentation, it
would be hugely helpful.

Of course it does raise the question of why certain characters made it in
and others didn't--did someone affirmatively decide Ancient Greek Textual
Symbols should be valid operators? It appears operators can start General
Punctuation, Arrows, Miscellaneous Technical, Box Drawing, Dingbats
(except Dingbat Circled Digits), Supplemental Punctuation and some CJK
Symbols and Punctuation.

Some criteria on how to decide what to include would be helpful -- I would
suggest that criteria go into this proposal, and the final list be
assembled in the future, perhaps subject to regular (annual?) review.

And for the mailing list (as long as we're still on it) can we have a tag
for discussions of specific operators, something like
[operator-[X]-discussion]? Probably many people would like to follow the
overall discussion but not most discussions of specific operators. I think
the forum-based discussion will help, and my point is mainly that
discussion of the criteria or broad scope is different from the discussion
of any particular character's inclusion or exclusion.

Does this direction seem like it could preserve flexibility while taking
advantage of Unicode standards for identifiers as a starting point for
Unicode characters as identifiers, while creating a criteria and structure
for future discussion of operators?

Mike Sand


On Sat, Sep 30, 2017 at 6:59 PM, Chris Lattner via swift-evolution <
swift-evolution at swift.org> wrote:

>
> The core team recently met to discuss PR609 - Refining identifier and
> operator symbology:
> https://github.com/xwu/swift-evolution/blob/7c2c4df63b1d92a1677461f41bc638
> f31926c9c3/proposals/NNNN-refining-identifier-and-operator-symbology.md
>
> The proposal correctly observes that the partitioning of unicode
> codepoints into identifiers and operators is a mess in some cases.  It
> really is an outright bug for 🙂 to be an identifier, but ☹️ to be an
> operator.  That said, the proposal itself is complicated and is defined in
> terms of a bunch of unicode classes that may evolve in the “wrong way for
> Swift” in the future.
>
> The core team would really like to get this sorted out for Swift 5, and
> sooner is better than later :-).  Because it seems that this is a really
> hard problem and that perfection is becoming the enemy of good
> <https://en.wikipedia.org/wiki/Perfect_is_the_enemy_of_good>, the core
> team requests the creation of a new proposal with a different approach.
> The general observation is that there are three kinds of characters: things
> that are obviously identifiers, things that are obviously math operators,
> and things that are non-obvious.  Things that are non-obvious can be made
> into invalid code points, and legislated later in follow-up proposals
> if/when someone cares to argue for them.
>
>
> To make progress on this, we suggest a few separable steps:
>
> First, please split out the changes to the ASCII characters (e.g. . and \
> operator parsing rules) to its own (small) proposal, since it is unrelated
> to the unicode changes, and can make progress on that proposal
> independently.
>
>
> Second, someone should take a look at the concrete set of unicode
> identifiers that are accepted by Swift 4 and write a new proposal that
> splits them into the three groups: those that are clearly identifiers
> (which become identifiers), those that are clearly operators (which become
> operators), and those that are unclear or don’t matter (these become
> invalid code points).
>
> I suggest that the criteria be based on *utility for Swift code*, not on
> the underlying unicode classification.  For example, the discussion thread
> for PR609 mentions that the T character in “  xᵀ  ” is defined in unicode
> as a latin “letter”.  Despite that, its use is Swift would clearly be as a
> postfix operator, so we should classify it as an operator.
>
> Other suggestions:
>  - Math symbols are operators excepting those primarily used as
> identifiers like “alpha”.  If there are any characters that are used for
> both, this proposal should make them invalid.
>  - While there may be useful ranges for some identifiers (e.g. to handle
> european accented characters), the Emoji range should probably have each
> codepoint independently judged, and currently unassigned codepoints should
> not get a meaning defined for them.
>  - Unicode “faces”, “people”, “animals” etc are all identifiers.
>  - In order to reduce the scope of the proposal, it is a safe default to
> exclude characters that are unlikely to be used by Swift code today,
> including Braille, weird currency symbols, or any set of characters that
> are so broken and useless in Swift 4 that it isn’t worth worrying about.
>  - The proposal is likely to turn a large number of code points into
> rejected characters.  In the discussions, some people will be tempted to
> argue endlessly about individual rejections.  To control that, we can
> require that people point out an example where the character is already in
> use, or where it has a clear application to a domain that is known today:
> the discussion needs to be grounded and practical, not theoretical.
>
>
> Third, if there is interest sometime in the future, we can have subsequent
> proposals that expand the range of accepted code points, motivated by the
> specific application domain that cares about them.  These proposals will
> not be source breaking, so they can happen at any time.
>
>
> Is anyone interested in helping to push this effort forward?
>
> -Chris
>
>
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> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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