(ab)using enum pattern matching to lift generic types to specific types and vice-versa in a type-safe way

CrossModuleGenericLifting.swift

// MARK: Interface Module
struct Request<Output> {
    let kind: Kind

    enum Kind {
        case one((Int) -> Output)
        case two((String) -> Output)
    }
}

extension Request where Output == Int {
    static func exampleOne() -> Self {
        .init(kind: .one({ $0 }))
    }
}

extension Request where Output == String {
    static func exampleTwo() -> Self {
        .init(kind: .two({ $0 }))
    }
}

protocol Thing {
    func doThing<Output>(
        _ request: Request<Output>
    ) -> Output
}


// MARK: Implementation Module
struct Concrete: Thing {
    func doThing<Output>(
        _ request: Request<Output>
    ) -> Output {

        switch request.kind {
        case .one(let lift):
            return lift(42)
        case .two(let lift):
            return lift("hello, world!")
        }
    }
}

// MARK: Callsite
let thing: Thing = Concrete()
thing.doThing(.exampleOne())
thing.doThing(.exampleTwo())