79 lines
2.6 KiB
Markdown
79 lines
2.6 KiB
Markdown
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---
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title: Y combinator
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id: 594810f028c0303b75339ad5
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challengeType: 5
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videoUrl: ''
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localeTitle: 和组合
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---
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## Description
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<section id="description"><p>在严格的<a href="https://en.wikipedia.org/wiki/Functional programming" title="wp：函数式编程">函数编程</a>和<a href="https://en.wikipedia.org/wiki/lambda calculus" title="wp：lambda演算">lambda演算中</a> ，函数（lambda表达式）没有状态，只允许引用封闭函数的参数。这排除了递归函数的通常定义，其中函数与变量的状态相关联，并且该变量的状态在函数体中使用。 </p><p> <a href="http://mvanier.livejournal.com/2897.html">Y组合</a>器本身是一个无状态函数，当应用于另一个无状态函数时，它返回函数的递归版本。 Y组合器是这类函数中最简单的一种，称为<a href="https://en.wikipedia.org/wiki/Fixed-point combinator" title="wp：定点组合器">定点组合器</a> 。 </p>任务： <pre> <code>Define the stateless Y combinator function and use it to compute <a href="https://en.wikipedia.org/wiki/Factorial" title="wp: factorial">factorial</a>.</code> </pre><p> <code>factorial(N)</code>功能已经给你了。另见<a href="http://vimeo.com/45140590">Jim Weirich：功能编程中的冒险</a> 。 </p></section>
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## Instructions
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<section id="instructions">
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: Y必须返回一个函数
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testString: 'assert.equal(typeof Y(f => n => n), "function", "Y must return a function");'
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- text: factorial（1）必须返回1。
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testString: 'assert.equal(factorial(1), 1, "factorial(1) must return 1.");'
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- text: factorial（2）必须返回2。
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testString: 'assert.equal(factorial(2), 2, "factorial(2) must return 2.");'
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- text: factorial（3）必须返回6。
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testString: 'assert.equal(factorial(3), 6, "factorial(3) must return 6.");'
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- text: factorial（4）必须返回24。
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testString: 'assert.equal(factorial(4), 24, "factorial(4) must return 24.");'
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- text: factorial（10）必须返回3628800。
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testString: 'assert.equal(factorial(10), 3628800, "factorial(10) must return 3628800.");'
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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function Y(f) {
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return function() {
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// Good luck!
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};
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}
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var factorial = Y(function(f) {
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return function (n) {
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return n > 1 ? n * f(n - 1) : 1;
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};
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});
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```
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</div>
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### After Test
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<div id='js-teardown'>
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```js
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console.info('after the test');
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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// solution required
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```
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</section>
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