Scala 占位符语法 [英] Scala placeholder syntax

问题描述

有些东西我不太明白，希望有人能解释一下..我有 Seq[String]

There is something that I can't quite understand hope someone can shed some light.. I have Seq[String]

val strDeps: Seq[String] = ...

我尝试使用 sortWith 方法对它进行反向排序，但出现以下错误.

and I tried to sort it on the reverse of the using the sortWith method and I get the following error.

scala> print(strDeps.sortWith(_.reverse.compareTo(_.reverse) < 0) mkString ("\n"))
<console>:15: error: wrong number of parameters; expected = 2
              print(strDeps.sortWith(_.reverse.compareTo(_.reverse) < 0) mkString ("\n"))
                                                                    ^

但是当我尝试对其进行排序而不进行反向操作时，它工作正常.

But when I try sort it without doing a reverse it works fine.

scala> print(strDeps.sortWith(_.compareTo(_) < 0) mkString ("\n"))
// this is fine

也可以在没有占位符语法的情况下正常工作

Also it works fine without the placeholder syntax

scala> print(strDeps.sortWith((a,b) => a.reverse.compareTo(b.reverse) < 0) mkString ("\n"))
// this works fine too

推荐答案

_ 仅扩展到尽可能小的范围.

_ expands only to the smallest possible scope.

内部 _.reverse 部分已经被解释为 x =>x.reverse 因此参数在 sortWith 中丢失.

The inner _.reverse part is already interpreted as x => x.reverse therefore the parameter is missing inside sortWith.

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