Command.swift

public protocol Command {
    associatedtype Input
    associatedtype Output
    func execute(input:Input) -> Output
}

public final class Signal<Input, Output> : Command {

    private let block:(Input) -> Output

    public init(_ block:(Input) -> Output) {
        self.block = block
    }

    public func execute(input:Input) -> Output {
        return block(input)
    }
}