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

[Chris Lattner]

The core team recently met to discuss PR609 - Refining identifier and operator symbology:
https://github.com/xwu/swift-evolution/blob/7c2c4df63b1d92a1677461f41bc638f31926c9c3/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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