Tuesday, July 5, 2022
HomeiOS DevelopmentThe (Change) Case of the Lacking Binding — Erica Sadun

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


Right here’s a cool little problem introduced up this morning by a pal. Think about 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 received’t compile.

Once you bind an emblem for one sample, you could 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 {
    swap foo {
    case .a: return "a"
    case .b(let str) the place str.hasPrefix("c"), .c:
        // Utilizing `str` right here is dangerous!
        print(str)
        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’s because str is just not certain for .c and might be used within the successive case physique:

Nevertheless, as Greg Titus identified, in the event you swap 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 on past that scope. This resolves the error, since str is barely used within the the place clause to slender the sample matching:

Additional, when utilizing bindings in case checks, a waterfall strategy the place the certain gadgets are used earlier than fallthrough can prolong by 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
    fallthrough
case .medium(let second) the place second.satisfiesAnotherCondition():
    // can use `second` right here even when it was certain 
    // by way of `widest` above by way of fallthrough
    fallthrough
case .narrowest: return someValue

My because of Greg Titus for figuring this all out!



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