javascript - 在所有可迭代对象上懒惰地调用过滤器、查找(数组方法)

Question

<分区>

Answer 1

不幸的是，像find 这样的迭代器方法是使用序列协议(protocol)(.length + Get)实现的，而不是迭代器协议(protocol)。你可以尝试用一个代理来愚弄他们，让可迭代对象模拟序列，例如

let asArray = iterable => new Proxy(iterable, {

    get(target, prop, receiver) {
        if(prop === 'length')
            return Number.MAX_SAFE_INTEGER;
        return target.next().value;
    }
});


function *fib() {
    let [a, b] = [1, 1];

    while (1) {
        yield b;
        [a, b] = [b, a + b];
    }
}

found = [].find.call(
    asArray(fib()),
    x => x > 500);

console.log(found);

需要更多的工作，但您明白了。

另一种(IMO 更简洁的方法)是重新实现迭代器方法以支持可迭代对象(并成为生成器本身)。幸运的是，这非常简单:

function *lazyMap(iter, fn) {
    for (let x of iter)
        yield fn(x);
}


for (let x of lazyMap(fib(), x => x + ' hey'))...

下面是如何使用可链接的方法创建惰性迭代器对象:

let iter = function (it) {
    return new _iter(it);
};

let _iter = function(it) {
    this.it = it;
};

_iter.prototype[Symbol.iterator] = function *() {
    for (let x of this.it) {
        yield x;
    }
};

_iter.prototype.map = function (fn) {
    let _it = this.it;
    return iter((function *() {
        for (let x of _it) {
            yield fn(x)
        }
    })())
};

_iter.prototype.take = function (n) {
    let _it = this.it;
    return iter((function *() {
        for (let x of _it) {
            yield x;
            if (!--n)
                break;
        }
    })())
};

// @TODO: filter, find, takeWhile, dropWhile etc

// example:


// endless fibonacci generator
function *fib() {
    let [a, b] = [1, 1];

    while (1) {
        yield b;
        [a, b] = [b, a + b];
    }
}

// get first 10 fibs, multiplied by 11
a =  iter(fib())
     .map(x => x * 11)
     .take(10)

console.log([...a])