身份 Monad

這是在 JavaScript 中實現身份 monad 的示例，可以作為建立其他 monad 的起點。

基於 Douglas Crockford 關於 monad 和性腺的會議

使用這種方法重用你的函式會更容易，因為這個 monad 提供的靈活性和構圖噩夢：

f(g(h(i(j(k(value), j1), i2), h1, h2), g1, g2), f1, f2)

可讀，漂亮和乾淨：

identityMonad(value)
    .bind(k)
    .bind(j, j1, j2)
    .bind(i, i2)
    .bind(h, h1, h2)
    .bind(g, g1, g2)
    .bind(f, f1, f2);

function identityMonad(value) {
    var monad = Object.create(null);
    
    // func should return a monad
    monad.bind = function (func, ...args) {
        return func(value, ...args);
    };

    // whatever func does, we get our monad back
    monad.call = function (func, ...args) {
        func(value, ...args);

        return identityMonad(value);
    };
    
    // func doesn't have to know anything about monads
    monad.apply = function (func, ...args) {
        return identityMonad(func(value, ...args));
    };

    // Get the value wrapped in this monad
    monad.value = function () {
        return value;
    };
    
    return monad;
};

它適用於原始值

var value = 'foo',
    f = x => x + ' changed',
    g = x => x + ' again';

identityMonad(value)
    .apply(f)
    .apply(g)
    .bind(alert); // Alerts 'foo changed again'

還有物件

var value = { foo: 'foo' },
    f = x => identityMonad(Object.assign(x, { foo: 'bar' })),
    g = x => Object.assign(x, { bar: 'foo' }),
    h = x => console.log('foo: ' + x.foo + ', bar: ' + x.bar);

identityMonad(value)
    .bind(f)
    .apply(g)
    .bind(h); // Logs 'foo: bar, bar: foo'

讓我們嘗試一切：

var add = (x, ...args) => x + args.reduce((r, n) => r + n, 0),
    multiply = (x, ...args) => x * args.reduce((r, n) => r * n, 1),
    divideMonad = (x, ...args) => identityMonad(x / multiply(...args)),
    log = x => console.log(x),
    substract = (x, ...args) => x - add(...args);

identityMonad(100)
    .apply(add, 10, 29, 13)
    .apply(multiply, 2)
    .bind(divideMonad, 2)
    .apply(substract, 67, 34)
    .apply(multiply, 1239)
    .bind(divideMonad, 20, 54, 2)
    .apply(Math.round)
    .call(log); // Logs 29