Opaque parameter declarations

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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Other changes in Swift 5.7…

• if let shorthand for unwrapping optionals
• Multi-statement closure type inference
• Clock, Instant, and Duration
• Regular expressions
• Type inference from default expressions
• Concurrency in top-level code
• Structural opaque result types
• Unlock existentials for all protocols
• Lightweight same-type requirements for primary associated types
• Constrained existential types
• Distributed actor isolation
• buildPartialBlock for result builders
• Implicitly opened existentials
• Unavailable from async attribute

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