Swift 3: Nested Generic Type

Question

I've got some weird behavior from the compiler I can't figure out (Xcode 8, Swift 3) as I'm trying to migrate my code from Swift 2. I think it has to do with tuples, but I'm not entirely certain.

I've got a generic class that defines a couple generics. In those generics, I've also got a couple type alias's setup and a function that uses them:

open class GuardPool <Key: Hashable, Resource> {



  public typealias ResourceCallback = ([Resource]) -> Void

  public typealias Request = (keys: Set<Key>, cb: ResourceCallback)

  fileprivate var pendingRequests: [Request] = []



  open func request(_ keys: Set<Key>, cb: ([Resource]) -> Void) {

    let pendingRequest: Request = (keys: keys, cb: cb)

    pendingRequests.append(pendingRequest)

  }

}

On the assignment (

let pendingRequest: Request = ...

), I get this error:

Cannot convert value of type 'Set<Key>' to specified type 'Set<_>'

I cannot figure out how to fix this. It seems like the compiler can't recognize the type information for

Set

.

Note: Obviously, the class is much larger that this. I copied and pasted out the relevant code instead of inserting 300 lines.

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Answer 1

The error message is misleading. If you remove the (unnecessary) explicit type annotation in the second line

    let pendingRequest = (keys: keys, cb: cb)

then the compiler states that

error: non-escaping parameter 'cb' may only be called
note: parameter 'cb' is implicitly non-escaping

which reveals the actual problem: The cb parameter must be marked as @escaping because it is stored in a property and might be called at a later time, after returning from the request method:

open func request(_ keys: Set<Key>, cb: @escaping ([Resource]) -> Void) {
    let pendingRequest = (keys: keys, cb: cb)
    pendingRequests.append(pendingRequest)
}

In Swift 2, closures were escaping by default and could be marked with @noescape. In Swift 3, closures are non-escaping by default, compare SE-0103: Make non-escaping closures the default.

Swift 3: Nested Generic Type

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