Scala 中忽略变量语法的用例是什么? [英] What is the use cases of Ignored Variable syntax in Scala?

问题描述

根据这个答案 https://stackoverflow.com/a/8001065/1586965 我们可以在 Scala 中做到这一点:

According to this answer https://stackoverflow.com/a/8001065/1586965 we can do this in Scala:

val _ = 5

现在我理解了 lambda 表达式中忽略参数的意义，但我无法想象我想要声明一个根据定义我无法引用的变量的例子.我能想到的唯一例子是懒惰地命名隐式值，例如

Now I understand the point of ignored parameters in lambda expressions, but I cannot really imagine examples where I would ever want to declare a variable that by definition I cannot reference. The only example I can think of is being lazy about naming implicit values, e.g.

implicit val _: MyNumeric = ...
...
class A[T : MyNumeric] {
...

这是唯一的用例吗?我错过了什么吗?

Is this the only use case? Am I missing something?

如果这是唯一的用例，当 val 是 not 隐含的，因为它完全没有意义时，编译器/IDE 不应该给出警告/提示吗?

If it is the only use case, shouldn't the compiler/IDE give a warning/hint when the val is not implicit as it is utterly pointless?

澄清

变量/值是指单个变量/值，而不是作为提取声明一部分的变量/值.

By variable/value I mean a single one, not one that is part of an extraction declaration.

推荐答案

它使用一个值.

$ scala -Ywarn-value-discard
Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_11).
Type in expressions to have them evaluated.
Type :help for more information.

scala> def f() = { println("I ran.") ; 42 }
f: ()Int

scala> def g(): Unit = { f() }
<console>:8: warning: discarded non-Unit value
       def g(): Unit = { f() }
                          ^
g: ()Unit

scala> def g(): Unit = { val _ = f() }
g: ()Unit

scala> g
I ran.

scala> 

为了验证，它在-Ywarn-unused下也没有警告.

To verify, it also doesn't warn under -Ywarn-unused.

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