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Opaque parameter declarations

Available from Swift 5.7

Paul Hudson      @twostraws

SE-0341 unlocks the ability to use some with parameter declarations in places where simpler generics were being used.

As an example, if we wanted to write a function that checks whether an array is sorted, Swift 5.7 and later allow us to write this:

func isSorted(array: [some Comparable]) -> Bool {
    array == array.sorted()
}

The [some Comparable] parameter type means this function works with an array containing elements of one type that conforms to the Comparable protocol, which is syntactic sugar for the equivalent generic code:

func isSortedOld<T: Comparable>(array: [T]) -> Bool {
    array == array.sorted()
}

Of course, we could also write the even longer constrained extension:

extension Array where Element: Comparable {
    func isSorted() -> Bool {
        self == self.sorted()
    }
}

This simplified generic syntax does mean we no longer have the ability to add more complex constraints our types, because there is no specific name for the synthesized generic parameter.

Important: You can switch between explicit generic parameters and this new simpler syntax without breaking your API.

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