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Remove the Boolean protocol

Introduction

For legacy and historical reasons Swift has supported a protocol named Boolean for abstracting over different concrete Boolean types. This causes problems primarily because it is pointless and very confusing to newcomers to Swift: is quite different than Bool, but shows up right next to it in documentation and code completion. Once you know that it is something you don't want, you constantly ignore it. Boolean values are simple enough that we don't need a protocol to abstract over multiple concrete implementations.

From a historical perspective, it was a very early solution to the bridging challenge of BOOL (which comes in as ObjCBool). In the time since then, Swift has developed a number of more advanced ways to solve these sorts of bridging problems, and BOOL is already bridged in automatically as Bool in almost all cases.

Another pragmatic problem with the Boolean protocol is that it isn't used consistently in APIs that take Boolean parameters: almost everything takes Bool concretely. This means that its supposed abstraction isn't useful. The only significant users are the unary !, and binary && and || operators.

Discussion thread

Proposal

Remove Boolean protocol, and change the three operators that take it to operate on Bool directly.

Impact on existing code

This change is backwards incompatible, but extremely unlikely to break code in practice. As already mentioned, APIs that have parameters or return values of type BOOL are automatically bridged in as Bool, so they will not see any change at all. The only significant place they appear in APIs are for the STOP out parameters on certain Objective-C collection types, e.g.:

class NSArray : NSObject ... {
  func enumerateObjects(_ block: @noescape (AnyObject, Int, UnsafeMutablePointer<ObjCBool>) -> Void)

which is used like this:

x.enumerateObjects { value, index, STOP in
  STOP.pointee = true
}

This continues to work even without the Boolean protocol because ObjCBool still conforms to BooleanLiteralConvertible.