This is great news! Thank you!! On Thursday, 14 July 2016, Chris Lattner via swift-evolution < swift-evolution@swift.org> wrote:
> Proposal: > https://github.com/apple/swift-evolution/blob/master/proposals/0111-remove-arg-label-type-significance.md > > Shortly after SE-0111 was accepted last week, several people newly noticed > the proposal and started a discussion about how it appears to be a > regression for closure parameters (e.g. callbacks) that could formerly > carry labels, but are now not allowed to. These folks observed that it > would be more expressive (and consistent with the rest of Swift) to allow > parameter labels in function types, because the invocation site of a > closure “should" be required to provide those labels. The core team has > been following the discussion, agrees that this is a concern, and wants to > update the community with a path forward. > > The reality of the situation is that the current implementation of > parameter labels in function types is inherently broken. Specifically, as > one example, there is an implicit conversion from "(a: Int) -> Int” to “(Int) > -> Int”. However, there is also an implicit conversion from "(Int) -> Int” > to “(b : Int) -> Int”. This means that the compiler currently allows > converting from “(a: Int) -> Int” to “(b: Int) -> Int”, which doesn’t > make sense, introduces surprising behavior, introduces complexity into the > compiler implementation, and is generally a problem. We do have one > specific hack to prevent conversion of (e.g.) “(a : Int, b : Int) -> Void” > to “(b : Int, a : Int) -> Void”, but this only triggers in specific > cases. There are other more complex cases as well, e.g. when using > generics "T<(a : Int)->Int>” cannot be considered compatible with "T<(b : > Int)->Int>”. > > These problems are what initially motivated SE-0111. However, given the > feedback, the core team went back to the drawing board to determine > whether: a) SE-0111 by itself is the right long term answer, b) whether > there were alternate models that could solve the same problems in a > different way, or c) whether SE-0111 was the right first step to "ultimate > glory" in the field of closure parameter labels. After a long discussion, > and many alternatives considered, the core team believes in c), that > SE-0111 (with a minor modification) is the right step for Swift 3, because > it paves the way for the right model over the long term. > > ----8<---- > > The specific revision requested by the core team to SE-0111 is that all > “cosmetic” labels should be required to include an API name of _. For > example, this would not be allowed: > > var op : (lhs : Int, rhs : Int) -> Int > > instead, it should be spelled as: > > var op : (_ lhs : Int, _ rhs : Int) -> Int > > With this change, we believe that we have paved the way for a purely > additive proposal (and thus, post-Swift 3) that will restore the expressive > capability of closures with parameter labels. > > ----8<---- > > Here is a sketch of how that would work, in two steps: > > > First, we extend declaration names for variables, properties, and > parameters to allow *parameter names* as part of their declaration name. > For example: > > var op(lhs:,rhs:) : (Int, Int) -> Int // variable or property. > x = op(lhs: 1, rhs: 2) // use of the variable or property. > > // API name of parameter is “opToUse”, internal name is "op(lhs:,rhs:)”. > func foo(opToUse op(lhs:,rhs:) : (Int, Int) -> Int) { > x = op(lhs: 1, rhs: 2) // use of the parameter > } > foo(opToUse: +) // call of the function > > This will restore the ability to express the idea of a closure parameter > that carries labels as part of its declaration, without requiring parameter > labels to be part of the type system (allowing, e.g. the operator + to be > passed into something that requires parameter labels). > > > Second, extend the rules for function types to allow parameter API labels > *if and only if* they are used as the type of a declaration that allows > parameter labels, and interpret them as a sugar form for providing those > labels on the underlying declaration. This means that the example above > could be spelled as: > > var op : (lhs: Int, rhs: Int) -> Int // Nice declaration syntax > x = op(lhs: 1, rhs: 2) // Same as above > > // API name of parameter is “opToUse”, internal name is "op(lhs:,rhs:)”. > func foo(opToUse op : (lhs: Int, rhs: Int) -> Int) { > x = op(lhs: 1, rhs: 2) // Same as above. > } > foo(opToUse: +) // Same as above. > > > These two steps will provide the simple and expressive design approach > that we have now, without all of the problems that representing parameter > labels in the type system introduces. The core team believes that the > temporary regression in expressiveness is an acceptable loss for Swift 3, > particularly given that this will have no impact on Cocoa or the standard > library. In the case of Cocoa, recall that C and Objective-C don’t have > parameter labels on their corresponding concepts (Blocks and C function > pointers), and the higher order functions in the standard library should > not require parameter labels either. > > -Chris & the Core Team > > >
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