infix operator <*> {
    associativity left
    precedence 100
}

infix operator <?> {
    associativity left
    precedence 100
}

infix operator <~~ {
    associativity left
    precedence 110
}

infix operator <<~ {
    associativity left
    precedence 110
}

func <*><A, B>(f: (A -> B)?, x: A?) -> B? {
    guard let f = f, x = x else {
        return nil
    }
    return f(x)
}

func <?><A, B>(f: (A? -> B)?, x: A?) -> B? {
    guard let f = f else {
        return nil
    }
    return f(x)
}

func <~~<A, B>(x: A?, f: (A -> B?)) -> B? {
    guard let x = x else {
        return nil
    }
    return f(x)
}

func <<~<A, B>(x: [A]?, f: (A -> B?)) -> [B]? {
    guard let x = x else {
        return nil
    }
    
    return x.map(f).flatMap{ $0 }
}