[swift-evolution] [Core team] Addressing the SE-0110 usability regression in Swift 4

[Douglas Gregor]

Hello Swift community,

Swift 3’s SE-0110 <https://github.com/apple/swift-evolution/blob/master/proposals/0110-distingish-single-tuple-arg.md> eliminated the equivalence between function types that accept a single type and function types that take multiple arguments. However, for various implementation reasons <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170612/037560.html>, the implementation of SE-0110 (as well as the elimination of tuple “splat” behavior in SE-0029 <https://github.com/apple/swift-evolution/blob/master/proposals/0029-remove-implicit-tuple-splat.md>) 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

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