最近我花了一些时间研究转换器(函数式编程中的工具,旨在提高性能并保持代码的可读性和灵活性),但当我测试它们的实际速度时,得到了一些令人失望的结果。比如:
const inc = x => x + 1;
const isEven = x => x % 2 === 0;
// simplest, shortest way I would be comfortable with if performance wasn't an issue
const mapFilter = xs => xs.filter(isEven).map(inc);
// transducers way
// function composition
const compose = (...fns) => x => fns.reduceRight((y, f) => f(y), x);
const map = f => step => (a, c) => step(a, f(c));
const filter = p => step => (a, c) => (p(c) ? step(a, c) : a);
// basic reducer for building array
const build = (acc, x) => {
acc.push(x);
return acc;
};
// transducer, it doesn't create intermediate arrays hence should theoretically be faster
const transducers = xs =>
xs.reduce(compose(filter(isEven), map(inc))(build), []);
// native loop for comparison
const nativeLoop = data => {
const result = [];
const l = data.length;
for (let i = 0; i < l; i++) {
const x = data[i];
if (isEven(x)) result.push(inc(x));
}
return result;
};
const data = Array(1000).fill(1);
const base = ["simplest, chained map and filter", () => mapFilter(data)];
const alternative = ["composed transducers", () => transducers(data)];
const alternative2 = ["native loop", () => nativeLoop(data)];
/* console.log(Benchmark) */
console.log("Running benchmarks....");
const suite = new Benchmark.Suite();
suite
.add(...base)
.add(...alternative)
.add(...alternative2)
.on("cycle", function(event) {
console.log(String(event.target));
})
.on("complete", function() {
console.log("Fastest is " + this.filter("fastest").map("name").join(", "));
})
// run async
.run({ async: true });
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/benchmark/2.1.4/benchmark.min.js"></script>
我预计性能顺序将是:
本地循环 > 转换器 > 链式map/filter
同时,除了本地方法比其他方法要快得多之外,令我惊讶的是,结果表明 reduce/transduce 方法比使用 map/filter 和创建中间数组要慢得多(在 Chrome 中慢一个数量级)。能否有人解释一下这个结果的起源?
Array#map
和Array#filter
等函数都是高度优化的本地代码,但转换器会受到许多函数调用开销的影响。必须在符号表中查找函数,创建堆栈帧,推送参数,跳转,返回结果...如果性能很重要,我同意Bob的观点-坚持使用本地和朴素的代码。即使性能不重要,转换器对我来说也不太可读,尽管我不是Scheme语言专家。const mapFilter = xs => xs.filter(isEven).map(inc);
是惯用的、直接的和非巧妙的写法。 - ggorlenbuild
替换为const build = (acc, x) => { return [...acc, x]; };
以使其成为一个纯函数。 - Jawi