在Clojure中是否有除nil-punning以外的惯用替代方法? [英] Is there an idiomatic alternative to nil-punning in Clojure?
问题描述
我正在读取一些Clojure代码，该代码具有大量未初始化的值，例如nil，用于传递到记录中的数字值.
I'm reading some Clojure code at the moment that has a bunch of uninitialised values as nil for a numeric value in a record that gets passed around.
Now lots of the Clojure libraries treat this as idiomatic. Which means that it is an accepted convention.
但是它也会导致NullPointerException，因为并非所有Clojure核心函数都可以将nil作为输入处理. (他们也不应该).
But it also leads to NullPointerException, because not all the Clojure core functions can handle a nil as input. (Nor should they).
其他语言具有Maybe或Option的概念，以在值为null的情况下代替该值，以减轻NullPointerException风险.这在Clojure中是可能的-但不能很常见.
Other languages have the concept of Maybe or Option to proxy the value in the event that it is null, as a way of mitigating the NullPointerException risk. This is possible in Clojure - but not very common.
您可以使用fnil做一些技巧，但这不能解决所有问题.
You can do some tricks with fnil but it doesn't solve every problem.
另一种选择是简单地将未初始化的值设置为类似:empty-value的符号，以强制用户在所有处理代码中显式处理此方案.但这并不是nil的真正提升，因为您直到运行时才真正发现所有场景(用他人的代码).
Another alternative is simply to set the uninitialised value to a symbol like :empty-value to force the user to handle this scenario explicitly in all the handling code. But this isn't really a big step-up from nil - because you don't really discover all the scenarios (in other people's code) until run-time.
我的问题是:在Clojure中是否有除nil-punning以外的惯用替代方法?
推荐答案
不确定您是否已阅读过Lispcast nil-punning上的> post ，但我确实认为这是为什么它很惯用的一个很好的例子，并涵盖了我在其他SO问题中没有提到的各种重要考虑因素.
Not sure if you've read this lispcast post on nil-punning, but I do think it makes a pretty good case for why it's idiomatic and covers various important considerations that I didn't see mentioned in those other SO questions.
基本上，nil是Clojure中的头等物品.尽管具有固有的常规含义，它还是一个适当的值，并且可以在许多上下文中以上下文相关的方式处理.与宿主语言中的null相比，它更加灵活和强大.
Basically, nil is a first-class thing in clojure. Despite its inherent conventional meaning, it is a proper value, and can be treated as such in many contexts, and in a context-dependent way. This makes it more flexible and powerful than null in the host language.
例如，类似的东西甚至无法在java中编译:
For example, something like this won't even compile in java:
if(null) {
....
}
与clojure一样，(if nil ...)也可以正常工作.因此，在许多情况下，您可以安全地使用 nil.我还没有看到一个随处可见if(foo != null) { ...之类的代码的Java代码库.也许Java 8的Optional会改变这一点.
Where as in clojure, (if nil ...) will work just fine. So there are many situations where you can use nil safely. I'm yet to see a java codebase that isn't littered with code like if(foo != null) { ... everywhere. Perhaps java 8's Optional will change this.
我认为您可以容易遇到问题的地方是在Java interop场景中，您正在处理实际的null s.一个好的clojure包装器库在很多情况下也可以帮助您避免这种情况，这是在可能的情况下优于直接Java互操作的一个好理由.
I think where you can run into issues quite easily is in java interop scenarios where you are dealing with actual nulls. A good clojure wrapper library can also help shield you from this in many cases, and its one good reason to prefer one over direct java interop where possible.
鉴于此，您可能需要重新考虑与当前潮流作斗争.但是由于您正在询问替代方案，因此我认为这很棒:棱镜的模式.模式具有Maybe模式(以及许多其他有用的模式)，并且在许多情况下都可以很好地工作.该图书馆非常受欢迎，我已经成功地使用了它. FWIW，建议在最近的 clojure应用书中.
In light of this, you may want to re-consider fighting this current. But since you are asking about alternatives, here's one I think is great: prismatic's schema. Schema has a Maybe schema (and many other useful ones as well), and it works quite nicely in many scenarios. The library is quite popular and I have used it with success. FWIW, it is recommended in the recent clojure applied book.
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