Swift Protocols Wish List
If you’re adopting a protocol (especially one from the standard library), the tools don’t provide any help whatsoever in knowing what you actually have to implement.
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Basically any time you find yourself using a PAT, you know that you’re either going to have to create a type eraser, or you’re going to have to make things generic that have no business being generic.
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I wish the compiler generated type erasers for me.
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I wish I could provide default implementations of protocol methods that assign to self.
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I wish I could extend a protocol to conform to another protocol.
Previously: Swift Protocols With Associated Types, Swift Type Erasure, Patterns for Working With Associated Types.
Update (2018-03-05): See also: Swift Unwrapped.
Update (2018-07-27): Dave DeLong (tweet):
What I really want to do is this:
protocol Predicate<Element> { func contains(_ other: Element) -> Bool func union(_ other: Predicate<Element>) -> Predicate<Element> }Of course, the compiler doesn't let me do this.
I want to express the idea that
union()can take anyPredicate-conforming value with the same element type, and you're going to get back some sort ofPredicate-conforming value that contains elements of that type.Then in specific situations where I can provide a much more efficient implementation (like the
IndexSet+IndexSet=IndexSetscenario), I want to provide those explicit overrides for that method, and otherwise default back to the implementations I provide in my protocol extension (which happen to return anOrPredicate<Int>)
What I think you actually want here is the generalized existential feature. “Generalized existential” is a ten-dollar term for a ten-cent idea: “Swift should have built-in type-erasing wrappers for PATs, much like it has them for ordinary protocols.” Their syntax might be slightly different—they’d probably use a
whereclause instead of a generic parameter list—but they would do the thing you want to do here.
