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

[Jacob Bandes-Storch]

On Wed, Oct 19, 2016 at 10:12 AM, Alex Martini <amartini at apple.com> wrote:

> Grammar changes
>
> operator → operator-head operator-characters[opt]
>
> operator-head → ! % & * + - / < = > ? ^ | ~
> operator-head → operator-dot operator-dots
> operator-character → operator-head
> operator-characters → operator-character operator-character[opt]
>
> operator-dot → .
> operator-dots → operator-dot operator-dots[opt]
>
>
> <https://github.com/jtbandes/swift-evolution/blob/unicode-id-op/proposals/NNNN-refining-identifier-and-operator-symbology.md#emoji>
>
>
> I think there's a mismatch between the English and grammar.  For example,
> is +..+ allowed or not?
>
> The English rule does allow +..+ because its dots appear in a run of two.
>
> The grammar allows a run of one or more dots as an operator head, but
> never allows dots as characters appearing in the middle of an operator,
> regardless of how many dots appear next to each other.  The grammar
> wouldn't allow +..+ because the dots don't come at the beginning.
>
>
>
> Here's an alternate version of the grammar that matches the "two or more"
> rule.  Because we no longer distinguish between which characters are
> allowed as the first character of an operator vs a character inside,
> there's no longer a need for a separate operator-head.
>
> operator --> operator-character operator-OPT
>
> operator-character --> ! % & * + - / < = > ? ^ | ~
> operator-character --> operator-dots
>
> operator-dots --> .. operator-additional-dots-OPT
> operator-additional-dots --> . operator-additional-dots-OPT
>

There is a typo in that operator-character[opt] should be
operator-characters[opt]. Aside from that, though, I believe the grammar as
written accepts +..+ already. Take a look at the following series of
substitutions based on the grammar rules:

operator
operator-head operator-characters
+ operator-characters
+ operator-character operator-characters[opt]
+ operator-head operator-head
+ operator-dot operator-dots operator-head
+ . . +
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