Swift泛型布尔不能转换为布尔

Question

I have written a protocol like:

public protocol Protocol1 {
    func execute<T, R>(req: T) -> Promise<R>
}

Implemented the protocol as Below:

struct Implemented1 : Protocol1 {
    func execute<String, Bool>(req : String) -> Promise<Bool> {
        return Promise<Bool>() { fulfill, reject in
           fulfill(true)
        }
    }
}

I am getting following error:

'Bool' is not convertible to 'Bool'

Please help me to understand what is the issue.

Answer 1

The problem is at the start of the method declaration.

func execute<String, Bool>

Who taught you to declare a generic method this way?

The generic parameters are supposed to go in the <> s, not actual types!

To implement the protocol, you need a generic method, not one that accepts a String and returns Bool .

So the compiler treats String and Bool as names of the generic parameters, instead of the actual swift types. So when the compiler says that Bool cannot be converted to Bool , it actually means that the swift Bool type cannot be converted to the generic parameter Bool .

I think what you need is associated types.

public protocol Protocol1 {
    associatedtype RequestType
    associatedtype ResultType
    func execute(req: RequestType) -> Promise<ResultType>
}

struct Implemented1 : Protocol1 {
    typealias ResultType = Bool
    typealias RequestType = String
    func execute(req : String) -> Promise<Bool> {
        return Promise { fulfill, reject in
            fulfill(true)
        }
    }
}

PS this:

Promise { fulfill, reject in
    fulfill(true)
}

can be simplified to:

Promise(value: true)