thing.swift

public protocol Thing {
    associatedtype T
    func work() throws ->  T
}

public struct AnyThing<T> : Thing {
    let doWork: () throws -> T
    public func work() throws -> T {
        return try doWork()
    }
}

public struct CompositeThing<T> : Thing {
    public enum Error : Swift.Error {
        case noap
    }
    public func work() throws -> T {
        for thing in things {
            do {
                return try thing.work()
            } catch {

            }
        }
        throw CompositeThing.Error.noap
    }
    public var things: [AnyThing<T>] = []
    public mutating func addThing<U : Thing>(_ thing: U) where U.T == T {
        things.append(AnyThing(doWork: thing.work))
    }
}

public struct ThingOne<T> : Thing {
    public func work() -> T {
        fatalError("one")
    }
}

public struct ThingTwo<T> : Thing {
    public func work() -> T {
        fatalError("two")
    }
}

var composite = CompositeThing<Int>()
composite.addThing(ThingOne())
composite.addThing(ThingTwo())
_ = try composite.work()