淺談 Scala 中底線的用途

標籤：

Scala 作為一門函數式程式設計語言，對習慣了指令式程式設計語言的同學來說，會不大習慣，這裡除了思維方式之外，還有文法層面的，比如 underscore（底線）就會出現在多種場合，令初學者相當疑惑，今天就來總結下 Scala 中底線的用法。

1、存在性類型：Existential typesdef foo(l: List[Option[_]]) = ...2、高階型別參數：Higher kinded type parameterscase class A[K[_],T](a: K[T])3、臨時變數：Ignored variablesval _ = 54、臨時參數：Ignored parametersList(1, 2, 3) foreach { _ => println("Hi") }5、通配模式：Wildcard patternsSome(5) match { case Some(_) => println("Yes") }val (a, _) = (1, 2)for (_ <- 1 to 10)6、通配匯入：Wildcard importsimport java.util._7、隱藏匯入：Hiding importsimport java.util.{ArrayList => _, _}8、串連字母和標點符號：Joining letters to punctuationdef bang_!(x: Int) = 59、預留位置文法：Placeholder syntaxList(1, 2, 3) map (_ + 2)_ + _   10、偏應用函數：Partially applied functionsList(1, 2, 3) foreach println _11、初始化預設值：default valuevar i: Int = _12、訪問元組：tuple getterst._2 13、參數序列：parameters Sequence _*作為一個整體，告訴編譯器你希望將某個參數當作參數序列處理！例如val s = sum(1 to 5:_*)就是將1 to 5當作參數序列處理。

這裡需要注意的是，以下兩種寫法實現的是完全不一樣的功能：

foo _               // Eta expansion of method into method valuefoo(_)              // Partial function application

Example showing why foo(_) and foo _ are different:

trait PlaceholderExample {  def process[A](f: A => Unit)  val set: Set[_ => Unit]  set.foreach(process _) // Error   set.foreach(process(_)) // No Error}

In the first case, process _ represents a method; Scala takes the polymorphic method and attempts to make it monomorphic by filling in the type parameter, but realizes that there is no type that can be filled in for A that will give the type (_ => Unit) => ? (Existential _ is not a type).

In the second case, process(_) is a lambda; when writing a lambda with no explicit argument type, Scala infers the type from the argument that foreach expects, and _ => Unit is a type (whereas just plain _ isn‘t), so it can be substituted and inferred.

This may well be the trickiest gotcha in Scala I have ever encountered.

Refer：

[1] What are all the uses of an underscore in Scala?

http://stackoverflow.com/questions/8000903/what-are-all-the-uses-of-an-underscore-in-scala

[2] Scala punctuation (AKA symbols and operators)

http://stackoverflow.com/questions/7888944/scala-punctuation-aka-symbols-and-operators/7890032#7890032

[3] Scala中的底線到底有多少種應用情境？

http://www.zhihu.com/question/21622725

[4] Strange type mismatch when using member access instead of extractor

http://stackoverflow.com/questions/9610736/strange-type-mismatch-when-using-member-access-instead-of-extractor/9610961

[5] Scala簡明教程

http://colobu.com/2015/01/14/Scala-Quick-Start-for-Java-Programmers/

淺談 Scala 中底線的用途