函数复合

//f(g(x))   m(x) = f(g(x))
val add5 = {i: Int -> i + 5} // g(x)
val multiplyBy2 = {i : Int -> i * 2} // f(x)
fun main(args: Array<String>) {
    println(multiplyBy2(add5(8))) // (5 + 8) * 2
    val add5AndMultiplyBy2 = add5 andThen multiplyBy2
    val add5ComposeMultiplyBy2 = add5 compose  multiplyBy2
      // P1 = 8; P2:add5的返回值
    println(add5AndMultiplyBy2(8)) // m(x) = f(g(x))
    println(add5ComposeMultiplyBy2(8)) // m(x) = g(f(x))
}
// infix 中缀表达式,Function1一个参数
// Function1<P1, P2>.andThen()扩展方法，P1参数类型，P2返回值类型
infix fun <P1, P2, R> Function1<P1, P2>.andThen(function: Function1<P2, R>): Function1<P1,R>{
    return fun(p1: P1): R{
        return function.invoke(this.invoke(p1))
    }
}
infix fun <P1,P2, R> Function1<P2, R>.compose(function: Function1<P1, P2>): Function1<P1, R>{
    return fun(p1: P1): R{
        return this.invoke(function.invoke(p1))
    }
}