CODEWITHSUNDEEP
swift
Fitsyu
Fitsyu

Reputation: 900

What does `_?` mean in Swift?

Few days ago, I converted my old Xcode 8 project to Swift 4 in Xcode 9. I noticed additional Swift codes generated along with explanation just above the code.

Here what it looks like:

// FIXME: comparison operators with optionals were removed from the Swift Standard Libary.
// Consider refactoring the code to use the non-optional operators.
fileprivate func < <T : Comparable>(lhs: T?, rhs: T?) -> Bool {
  switch (lhs, rhs) {
  case let (l?, r?):
    return l < r
  case (nil, _?):
    return true
  default:
    return false
  }
}

I tried to understand what the code does and find what I think is kind of unusual _? in the code.

I guess it is unused optional because _ means we are not going to use a particular variable so we don't care about a variable's name and ? is optional syntax.

Thank you for your help!

Upvotes: 4

Views: 1453

Answers (1)

Martin R
Martin R

Reputation: 540075

_ is the wildcard pattern which matches anything, see for example

And x? is the optional pattern, a shortcut for .some(x), it matches an optional which is not nil.

Here we have the combination of both: _? matches anything that is not nil.

case (nil, _?) matches if the left operand is nil and the right operand is not. You could also write it as case (.none, .some).

Xcode can insert this function during migration from older Swift versions, compare Strange generic function appear in view controller after converting to swift 3.

Upvotes: 5

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