typeErasureClosure.swift

public class AnyCup<LiquidType: Liquid>: Cup {

    // inner mechanism to "remember" the behavior of the cup passed in the init function
    private let fillClosure: (LiquidType) -> Void
    private let liquidClosure: () -> LiquidType?

    init<CupType: Cup>(with cup: CupType) where CupType.LiquidType == LiquidType {
        self.fillClosure = cup.fill
        self.liquidClosure = { return cup.liquid }
    }

    // conformance to Cup protocol
    public var liquid: LiquidType? {
        return self.liquidClosure()
    }

    public func fill(with liquid: LiquidType) {
        self.fillClosure(liquid)
    }
}