scala 方法调用后的下划线是什么意思? [英] What does an underscore after a scala method call mean?

问题描述

scala 文档 有一个代码示例，其中包括以下行:

The scala documentation has a code example that includes the following line:

val numberFunc = numbers.foldLeft(List[Int]())_

方法调用后的下划线是什么意思?

What does the underscore after the method call mean?

推荐答案

这是一个部分应用的函数.您只向 foldLeft 提供第一个参数(初始值)，但不提供第二个；你把它推迟到以后.在您链接的文档中，他们在下一行定义 squares:

It's a partially applied function. You only provide the first parameter to foldLeft (the initial value), but you don't provide the second one; you postpone it for later. In the docs you linked they do it in the next line, where they define squares:

val numberFunc = numbers.foldLeft(List[Int]())_
val squares = numberFunc((xs, x) => xs:+ x*x)

看到 (xs, x) =>xs:+ x*x，这是您在定义 numberFunc 时遗漏的第二个参数.如果您立即提供了它，那么 numberFunc 将不是函数 - 它是计算值.

See that (xs, x) => xs:+ x*x, that's the missing second parameter which you omitted while defining numberFunc. If you had provided it right away, then numberFunc would not be a function - it would be the computed value.

所以基本上整个事情也可以用咖喱的形式写成一行:

So basically the whole thing can also be written as a one-liner in the curried form:

val squares = numbers.foldLeft(List[Int]())((xs, x) => xs:+ x*x)

但是，如果你希望能够一次又一次地重用foldLeft，拥有相同的集合和初始值，但每次都提供不同的函数，那么定义一个单独的numbersFunc(就像他们在文档中所做的那样)并用不同的功能重用它，例如:

However, if you want to be able to reuse foldLeft over and over again, having the same collection and initial value, but providing a different function every time, then it's very convinient to define a separate numbersFunc (as they did in the docs) and reuse it with different functions, e.g.:

val squares = numberFunc((xs, x) => xs:+ x*x)
val cubes = numberFunc((xs, x) => xs:+ x*x*x)
...

请注意，编译器错误消息非常简单，以防您忘记下划线:

Note that the compiler error message is pretty straightforward in case you forget the underscore:

错误:trait 中方法 foldLeft 缺少参数列表LinearSeqOptimized 未应用的方法仅转换为函数当需要函数类型时.您可以进行此转换通过编写 foldLeft _ 或 foldLeft(_)(_) 而不是显式foldLeft.val numberFunc = numbers.foldLeft(ListInt)

Error: missing argument list for method foldLeft in trait LinearSeqOptimized Unapplied methods are only converted to functions when a function type is expected. You can make this conversion explicit by writing foldLeft _ or foldLeft(_)(_) instead of foldLeft. val numberFunc = numbers.foldLeft(ListInt)

哈哈，我刚刚意识到他们对文档中的 cubes 做了完全相同的事情.

Haha I just realized that they did the exact same thing with cubes in the documentation.

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