Saturday, December 9, 2023
HomeIOS DevelopmentThe (Swap) Case of the Lacking Binding — Erica Sadun

The (Swap) Case of the Lacking Binding — Erica Sadun

Right here’s a cool little problem introduced up this morning by a pal. Take into account the next code:

swap foo {
  case .a: return "a"
  case .b(let str) the place str.hasPrefix("c"), .c: return "c"
  case .b: return "b"

It gained’t compile.

Whenever you bind a logo for one sample, you have to bind that image for each sample in a case. This prevents you, for instance, from binding str in a single sample after which trying to make use of str within the shared case physique. For instance, take into account this case. What would you anticipate to occur when foo is .c?

func switchTheFallthroughOrder(foo: Foo) -> String {
    swap foo {
    case .a: return "a"
    case .b(let str) the place str.hasPrefix("c"), .c:
        // Utilizing `str` right here is unhealthy!
        return "c"
    case .b: return "b"

Regardless of my first knee-jerk refactoring, transferring out the .c case to make use of fallthrough doesn’t work. Once more, it is because str shouldn’t be certain for .c and may be used within the successive case physique:

Nevertheless, as Greg Titus identified, when you swap the order to make use of the binding case first with fallthrough, Swift is aware of at compile time that the binding gained’t keep on past that scope. This resolves the error, since str is just used within the the place clause to slim the sample matching:

Additional, when utilizing bindings in case exams, a waterfall strategy the place the certain gadgets are used earlier than fallthrough can prolong via a number of steps with the blessing of the compiler:

case .widest(let first, let second) the place first.satisfiesACondition():
    // can use `first`, `second` right here
case .medium(let second) the place second.satisfiesAnotherCondition():
    // can use `second` right here even when it was certain 
    // through `widest` above through fallthrough
case .narrowest: return someValue

My due to Greg Titus for figuring this all out!



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