> On 14 Jul 2016, at 05:47, 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 > > <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).
During the review, Michael Ilseman suggested using SE-0021 for declaration names. <http://thread.gmane.org/gmane.comp.lang.swift.evolution/22488/focus=22580> Your example "op(lhs:,rhs:)" would be "op(lhs:rhs:)" instead (i.e. without the comma). -- Ben > 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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