Tuesday, August 9, 2022
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 buddy. Think about the next code:

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

It received’t compile.

While you bind an emblem for one sample, it’s essential to bind that image for each sample in a case. This prevents you, for instance, from binding str in a single sample after which making an attempt 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 {
    change foo {
    case .a: return "a"
    case .b(let str) the place str.hasPrefix("c"), .c:
        // Utilizing `str` right here is dangerous!
        return "c"
    case .b: return "b"

Regardless of my first knee-jerk refactoring, shifting 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 might be used within the successive case physique:

Nonetheless, as Greg Titus identified, in the event you change the order to make use of the binding case first with fallthrough, Swift is aware of at compile time that the binding received’t keep it up past that scope. This resolves the error, since str is simply used within the the place clause to slender the sample matching:

Additional, when utilizing bindings in case assessments, a waterfall strategy the place the certain objects are used earlier than fallthrough can lengthen 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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