[swift-evolution] [Proposal] Refining Identifier and Operator Symbology

David Sweeris davesweeris at mac.com
Sat Oct 22 11:03:44 CDT 2016

> On Oct 20, 2016, at 9:37 AM, Jonathan S. Shapiro <jonathan.s.shapiro at gmail.com> wrote:
> On Thu, Oct 20, 2016 at 7:30 AM, David Sweeris <davesweeris at mac.com <mailto:davesweeris at mac.com>> wrote:
> Sent from my iPhone
> On Oct 20, 2016, at 09:03, Jonathan S. Shapiro via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> On Thu, Oct 20, 2016 at 12:12 AM, Austin Zheng via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Freeze the set of allowed emoji to whatever the current version of the Unicode spec defines...
>> UAX31 won't include emojis in either space, because there is no clear consensus about where they belong (identifiers or operators). Individual languages can certainly add them to one space or the other, but should take care not to cross-contaminate. So if we add them to operators, we need to exclude any that are already part of normal identifiers and vice versa. That sanity restriction is technically necessary, but it shouldn't be an inconvenience in practical terms.
> My understanding (which is admittedly fuzzy) is that the distinction between operators and identifiers is only "technically necessary" because allowing characters to be both causes the parsing algorithm lose its virtual mind, and it takes a century for it to figure out what's going on. What I don't recall being discussed before is whether that's a blanket penalty or if the compile times increases are proportional to the amount overlap between the two character sets.
> The hard requirements are:
> Nothing in identifier start can be in operator start or operator continue. [*]
> Nothing in operator start can be in identifier start or identifier continue. [*]
> Nothing in syntactic punctuation (period, brackets, parens, and so forth) can be in either type of identifier without creating a lot of serious hair. You can see one example of hair in the "double dots" rule.
> If these requirements are not preserved, the consequence is that white space becomes required between identifiers and operators. So, for example, without these rules:
> a+b    // gets broken
> a + b  // works
> The presence of dots in operators is actually causing a whole bunch of constraints to get introduced that I'm going to talk about in a moment.

Ah, ok. Your explanation sounds very familiar… Clearly I’ve read it before and forgot. I must be going senile in my old age (37).

Would this whitespace rule affect pre/postfix operators, or just the infix ones? Personally, I’m fine with requiring whitespace around infix operators, but pre/postfix operators would completely lose readability if they needed it, too. Have we previously discussed this on the mailing list?

Given my apparent forgetfulness, I have no doubt that we discussed it at length two weeks ago, and someone will probably reply to this, quoting some forgotten 3-page email I sent on the topic :-)

- Dave Sweeris
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