Doug Gregor writes:

Hello Swift community,

Swift 3’s SE-0110 eliminated the equivalence between function types that accept a single type and function types that take multiple arguments. However, for various implementation reasons, the implementation of SE-0110 (as well as the elimination of tuple “splat” behavior in SE-0029) was not fully completed.

 

Swift 4 implemented more of SE-0110, which caused a fairly serious usability regression, particularly with closures. Here are a few simple examples involving closures that worked in Swift 3 but do not work in Swift 4:

// #1: Works in Swift 3, error in Swift 4

myDictionary.forEach {

  print(“\($0) -> \($1)”)

}

 

// #2: Works in Swift 3, error in Swift 4

myDictionary.forEach { key, value in

  print(“\(key) -> \(value)”)

}

 

// #3: Works in Swift 3, error in Swift 4

myDictionary.forEach { (key, value) in

  print(“\(key) -> \(value)”)

}

Similar issues occur with passing multi-argument functions where a tuple argument is expected:

// #4: Works in Swift 3, error in Swift 4

_ = zip(array1, array2).map(+)

In all of these cases, it is possible to write a closure that achieves the desired effect, but the result is more verbose and less intuitive:

// Works in both Swift 3 and Swift 4

myDictionary.forEach { element in

  let (key, value) = element

  print(“\(key) -> \(value)”)

}

 

The Swift core team feels that these usability regressions are unacceptable for Swift 4. There are a number of promising solutions that would provide a better model for closures and address the usability regression, but fully designing and implementing those are out of scope for Swift 4.  Therefore, we will “back out” the SE-0110 change regarding function arguments from Swift 4.

 

Specifically, when passing an argument value of function type (including closures) to a parameter of function type, a multi-parameter argument function can be passed to a parameter whose function type accepts a single tuple (whose tuple elements match the parameter types of the argument function). Practically speaking, all of the examples #1-#4 will be accepted in both Swift 3 and Swift 4.

 

We will revisit the design in this area post-Swift 4.

 

– Doug