blocks|key|70422|text|您应该使用null。在大多数情况下，这都无关紧要，但不管怎样，这都是一个好习惯，因为有时您可能想要检查不可比较的内容列表是否为空。下面是一个简短、简洁的示例，展示了这一区别：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|70423|>+null+[id]
False
>+[id]+==+[]
<interactive>:1:1:+error:
++++•+No+instance+for+(Eq+(a0+->+a0))+arising+from+a+use+of+‘==’
++++++++(maybe+you+haven't+applied+a+function+to+enough+arguments?)
++++•+In+the+expression:+[id]+==+[]
++++++In+an+equation+for+‘it’:+it+=+[id]+==+[]|code-block|syntax|javascript|70424|entityMap^0|5|4|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

You should use <code>null</code>. In most cases it doesn't matter, but it is a good habit to get into anyway, because occasionally you may want to check if a list of non-comparable things is empty. Here is a short, crisp example showing this difference:

<pre><code>&gt; null [id]
False
&gt; [id] == []
&lt;interactive&gt;:1:1: error:
 • No instance for (Eq (a0 -&gt; a0)) arising from a use of ‘==’
 (maybe you haven't applied a function to enough arguments?)
 • In the expression: [id] == []
 In an equation for ‘it’: it = [id] == []
</code></pre>

blocks|key|69061|text|除了到目前为止给出的很好的答案之外，null实际上还有类型|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|69062|null+::+Foldable+t+=>+t+a+->+Bool|code-block|syntax|javascript|69063|我不知道你是否已经了解了null中的类型类，但它的不足之处在于，null不仅可以用于列表，还可以用于任何实现LYAH的数据结构。|69064|这就是说，在Map或Set上使用null也是有效的。|69065|>+null+Map.empty
True
>+null+(Map.singleton+1)
False
>+null+Set.empty
True
>+null+(Set.singleton+1)
False
>+null+[]
True
>+null+[1]
False|69066|我认为编写需要如此通用的函数并不是特别常见，但默认编写更通用的代码也没什么坏处。|69067|附注|69068|在许多情况下，您最终会希望使用像null这样的函数来对列表(或其他数据结构)执行条件行为。如果您已经知道您的输入是一个特定的数据结构，那么只对它的空case进行模式匹配会更优雅。|69069|比较|69070|myMap+::+(a+->+b)+->+[a]+->+[b]
myMap+f+xs
++%7C+null+xs+=+[]
myMap+f+(x:xs)+=+f+x+:+myMap+f+xs|69071|至|69072|myMap'+::+(a+->+b)+->+[a]+->+[b]
myMap'+f+[]+=+[]
myMap'+f+(x:xs)+=+f+x+:+myMap'+f+xs|69073|通常，如果有意义，您应该尝试使用模式匹配。|69074|entityMap|0|LINK|mutability|MUTABLE|url|https://hackage.haskell.org/package/containers-0.6.2.1/docs/Data-Map-Lazy.html|1|https://hackage.haskell.org/package/containers-0.6.2.1/docs/Data-Set.html^0|I|4|0|0|C|4|W|4|0|6|3|A|3|G|4|6|3|0|A|3|1|0|0|0|0|G|4|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1G|8|@$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1J|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1K|8|@$9|1L|A|1M|B|C]|$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1P|8|@$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]|$9|1U|A|1V|B|C]]|D|@$9|1W|A|1X|1|1Y]|$9|1Z|A|20|1|21]]|E|$]]|$1|O|3|P|5|H|7|22|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|23|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|24|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|25|8|@$9|26|A|27|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|28|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|H|7|29|8|@]|D|@]|E|$I|J]]|$1|10|3|11|5|6|7|2A|8|@]|D|@]|E|$]]|$1|12|3|13|5|H|7|2B|8|@]|D|@]|E|$I|J]]|$1|14|3|15|5|6|7|2C|8|@]|D|@]|E|$]]|$1|16|3|-4|5|6|7|2D|8|@]|D|@]|E|$]]]|17|$18|$5|19|1A|1B|E|$1C|1D]]|1E|$5|19|1A|1B|E|$1C|1F]]]]

In addition to the good answers given so far, <code>null</code> actually has type

<pre class="lang-hs prettyprint-override"><code>null :: Foldable t =&gt; t a -&gt; Bool
</code></pre>

I don't know if you've gotten to typeclasses in LYAH, but the short of it is that <code>null</code> can be used not just for lists, but for any data structure that implements <code>null</code>.

This is to say that using <code>null</code> on a <a href="https://hackage.haskell.org/package/containers-0.6.2.1/docs/Data-Map-Lazy.html" rel="noreferrer"><code>Map</code></a> or a <a href="https://hackage.haskell.org/package/containers-0.6.2.1/docs/Data-Set.html" rel="noreferrer"><code>Set</code></a> is valid, too.

<pre><code>&gt; null Map.empty
True
&gt; null (Map.singleton 1)
False
&gt; null Set.empty
True
&gt; null (Set.singleton 1)
False
&gt; null []
True
&gt; null [1]
False
</code></pre>

I don't think it's especially common to write functions that need to be this general, but it doesn't hurt to default to writing more general code.

<h1>A side note</h1>

In many cases, you'll end up wanting to use a function like <code>null</code> to do conditional behavior on a list (or other data structure). If you already know that your input is a specific data structure, it's more elegant to just pattern match on its empty case.

Compare

<pre class="lang-hs prettyprint-override"><code>myMap :: (a -&gt; b) -&gt; [a] -&gt; [b]
myMap f xs
 | null xs = []
myMap f (x:xs) = f x : myMap f xs
</code></pre>

to

<pre class="lang-hs prettyprint-override"><code>myMap' :: (a -&gt; b) -&gt; [a] -&gt; [b]
myMap' f [] = []
myMap' f (x:xs) = f x : myMap' f xs
</code></pre>

In general, you should try to prefer pattern matching if it makes sense.

blocks|key|69106|text|同样，一个过滤所有空列表的简单函数也会失败：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|69107|withoutEmpty+=+filter+(==+[])|code-block|syntax|javascript|69108|那就是：|69109|withoutEmpty+=+filter+null|69110|请注意：|69111|withoutEmpty+ls+=+filter+(==[])+ls|69112|将工作得很好，但重要的一点是，在某些情况下，例如另一个可能会失败。|69113|还可以看看@cole+answer，它补充了这里的所有答案，类型类Foldable在那里有要实现的null函数：|offset|length|style|CODE|69114|查看有关Foldable+here的更多信息|69115|entityMap|0|LINK|mutability|MUTABLE|url|http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Foldable.html^0|0|0|0|0|0|0|0|1D|4|0|4|D|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|18|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|19|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1D|8|@$S|1E|T|1F|U|V]]|9|@]|A|$]]|$1|W|3|X|5|6|7|1G|8|@]|9|@$S|1H|T|1I|1|1J]]|A|$]]|$1|Y|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|Z|$10|$5|11|12|13|A|$14|15]]]]

Also a simple function that filter all empty list would fail:

<pre><code>withoutEmpty = filter (== [])
</code></pre>

and that would be:

<pre><code>withoutEmpty = filter null
</code></pre>

notice that:

<pre><code>withoutEmpty ls = filter (==[]) ls
</code></pre>

will work just fine, but the important point is that in some cases like the other one could fail.

Also look at @cole answer, it complements all the answers here, the typeclass Foldable has the <code>null</code> function there to be implemented:

To see more info of <a href="http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Foldable.html" rel="nofollow noreferrer">Foldable here</a>

I am reading through the "Starting Out" chapter of <a href="http://learnyouahaskell.com/starting-out" rel="noreferrer">Learn You a Haskell for Great Good!</a>. It says:

<blockquote>
 <code>null</code> checks if a list is empty. If it is, it returns <code>True</code>, otherwise it returns <code>False</code>. Use this function instead of <code>xs == []</code> (if you have a list called <code>xs</code>)
</blockquote>

I tried in ghci:

<pre><code>xs = [] -- and then,

xs == []
null xs
</code></pre>

Both of them are <code>True</code>.

I wonder what's the difference. 

Should I use the <code>null</code> function instead of <code>== []</code> and why?

Why use null function instead of == [] to check for empty list in Haskell?

数据结构

我正在通读的“入门”章节。上面写着： null检查列表是否为空。如果是，则返回True，，否则返回False.使用此函数而不是xs == [] (如果您有一个名为xs)的列表我在ghci中尝试过：xs = []      -- and then,xs == []null xs它们都是True。我想知道有什么不同。我应该...

问为什么要使用null函数而不是== []来检查Haskell中的空列表？
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