monad 例子

参考自：

Translation from Haskell to JavaScript of selected portions of the best introduction to monads I've ever read – The If Works

haskell - What is a monad? - Stack Overflow

但是我依然不知道monad有什么用处^v^

/**
 * number => number
 * number => [number, String] //添加调试信息
 * [number, String] => [number, String] //变成对称性，也就是可结合的
 * 将对称性的函数使用compose进行链接
 * 函数名不重要，重要的是抽象
 * ===================abstraction===================
 */


//第一步，原始函数
var f = (x) => x + 2;
var g = (x) => x * x;
/**
 * 第二步，将原始函数(x) => x * 2
 * 转变为 (x) => [x*2, "f被调用"]
 * 为了完成这一步，添加帮助函数unit
 * var unit = (x) => [f(x), "f 被调用"];
 * 有点像恒等函数 x => x
 */
var unit = (x) => [x, ""];
var shiftF = (f) => x => [f(x), "f被调用"];
var shiftG = (g) => (x) => [g(x), "g被调用"];
/**
 * 第三步
 * 将函数修改为对称的，也就是可结合的
 * 即直接将入参变成出参的类型
 */
var bind = (f) => (tuple) => {
    // console.log("tuple is " + tuple)
    var inputVal = tuple[0];
    var inputMsg = tuple[1];
    var val = f(inputVal);
    // console.log("val is " + val)
    if(inputMsg == ""){
        return [val[0], val[1]];
    } else {
        return [val[0], inputMsg + ", " + val[1]];
    }
    
}

/**
 * 将函数结合起来
 */
var compose = (f, g) => x => f(g(x));

/**
 * 需要的是满足对称性的函数
 */
 var reslut = compose(bind(shiftF(f)), bind(shiftG(g)))(unit(10));
console.log(reslut)

// var reslut = compose(bind(shiftF(f)), bind(shiftG(g)))(10);
// var reslut = compose(bind(shiftF), bind(shiftG))(10);
// var show = console.log
// var sg = shiftG(g)(10)
// show(sg)
// var bsf = bind(shiftF(f))([10, "test "])
// show(bsf)