The review of SE-0099 "Restructuring Condition Clauses" ran from January
13...18, 2016. The proposal has been accepted with revision for Swift 3. There
was near unanimous agreement that the Swift 2 grammar was inconsistent and
ambiguous and should be changed; most of the disagreement centered on how. Many
alternatives were discussed, including the following:
- The proposal as written suggests using ';' or newline as a separator. To many
people, this looked heavy, and it's also inconsistent with the rest of the
language, which never otherwise used semicolon as an intra-statement separator
(except in the defunct for;; loop).
- Introducing a keyword separator, such as using 'where' everywhere or
introducing a new 'and' keyword, is also bulky and either reads poorly or
requires stealing new keywords.
- Some commenters suggested using '&&' for consistency with simple boolean
conditions. This isn't workable due to precedence issues.
- The ambiguities arise from the fact that there are comma-separated lists
within comma-separated lists—within the list of conditions, each 'case' or
'let' condition can have multiple declarations. If we eliminated this feature,
so that every 'case' or 'let' condition had to start with 'case' or 'let', the
ambiguity is resolved, and comma can remain the condition separator. This does
break consistency with non-conditional 'let' declarations and case clauses in
'switch' but is otherwise workable.
Of these alternatives, the core team found the last one to be the best choice.
'case' and 'let' conditions should each specify a single declaration, comma
should remain the condition separator, and the 'where' keyword can be retired
from its purpose as a boolean condition introducer. Some code becomes more
verbose, but in common formatting patterns, it aligns more nicely, as in:
guard
let x = foo(),
let y = bar(),
let z = bas(),
x == y || y == z else {
}
and though it breaks commonality between 'let' conditions and 'let'
declarations, it's more important to preserve higher-level consistency
throughout the language in how components of expressions and statements are
separated. Thanks everyone for the discussion, and thanks Erica and Chris for
the proposal! Since, aside from the approved syntax, the fundamental thrust of
the proposal remains the same, Chris has volunteered to revise it to be in line
with the approved decision.
-Joe_______________________________________________
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