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

[L Mihalkovic]

> On May 20, 2016, at 8:01 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On May 20, 2016, at 11:53 AM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> I am less certain about allowing simple boolean expressions that are not introduced by the `where` keyword (excepting the first one which is introduced with the `guard` keyword).  I think this is a separate question that should receive independent consideration.  I think a reasonable argument can be made both ways.  
> 
> That's kind of the heart of the matter.
> 
> There's an assumption that guard statements are:
> 
> `guard` (conjoined boolean tests) | (conjoined boolean tests,)? (pattern | binding (where boolean)?, )+ else {...}
> 
> Right? (Except I couldn't figure out how to grammar out the final comma,) I'm suggesting:
> 
> `guard` ((boolean | (pattern | binding (where boolean)?)),)+ else {...}
> 
> instead. (And ditto about final comma) This promotes boolean statements to the same standing, so they're not limited to the start of the guard statement or syntactically tied to a pattern/binding where there may be no semantic basis.
> 
> -- E
> 
> 
> 

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

it is all shared at the moment. 

///   stmt-guard:
///     'guard' condition 'else' stmt-brace
///
ParserResult<Stmt> Parser::parseStmtGuard() {
  SourceLoc GuardLoc = consumeToken(tok::kw_guard);
  
...


I guess it means that stepping out of what it currently does is straightforward to isolate from side effects, but likely a significant piece of work if it is not make a superset (meaning that bits and parts or the way the current grammar is structured can be reused in the manner they are today) of what it is today.


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