[swift-evolution] [Pitch] making where and , interchangeable in guard conditions.

[Erica Sadun]

> 
> GRAMMAR OF A GUARD STATEMENT
> 
>  <>guard-statement → guard­condition-clause <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/condition-clause>­else­code-block <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block>­
>  <>condition-clause → expression <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression>­
>  <>condition-clause → expression <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression>­,­condition-list <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/condition-list>­
>  <>condition-clause → condition-list <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/condition-list>­
>  <>condition-clause → availability-condition <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/availability-condition>­,­expression <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression>­
>  <>condition-list → condition <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/condition>­  condition <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/condition>­,­condition-list <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/condition-list>­
>  <>condition → availability-condition <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/availability-condition>­  case-condition <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/case-condition>­  optional-binding-condition <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/optional-binding-condition>­
>  <>case-condition → case­pattern <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html#//apple_ref/swift/grammar/pattern>­initializer <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/initializer>­where-clause <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/where-clause>­opt­

While the easiest solution allows commas interchangeably with `where`, it would allow construction of statements like this and
it wouldn't introduce the ability to mix and match Boolean expressions, availability conditions, case conditions, and optional binding conditions.

// where-clause → (where | ,) where-expression
for i in 0...10, i % 2 == 0 { print(i) }

To get that you need:

‌condition-list → condition | expression | condition , condition-list | expression, condition-list

Which means that `guard` would be redefined to:

guard condition-list else code-block

This would affect `while`, `repeat-while`, `if`, and `guard`.

-- E

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