食品 技术支持 东莞网站建设,北京好的网页设计,有没有免费的网站,展示型建站模板平台Scala 作为一门函数式编程语言#xff0c;对习惯了指令式编程语言的同学来说#xff0c;会不大习惯#xff0c;这里除了思维方式之外#xff0c;还有语法层面的#xff0c;比如 underscore#xff08;下划线#xff09;就会出现在多种场合#xff0c;令初学者相当疑惑对习惯了指令式编程语言的同学来说会不大习惯这里除了思维方式之外还有语法层面的比如 underscore下划线就会出现在多种场合令初学者相当疑惑今天就来总结下 Scala 中下划线的用法。 ? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 1、存在性类型Existential types def foo(l: List[Option[_]]) ... 2、高阶类型参数Higher kinded type parameters case class A[K[_],T](a: K[T]) 3、临时变量Ignored variables val _ 5 4、临时参数Ignored parameters List(1, 2, 3) foreach { _ println(Hi) } 5、通配模式Wildcard patterns Some(5) match { case Some(_) println(Yes) } val (a, _) (1, 2) for (_ - 1 to 10) 6、通配导入Wildcard imports import java.util._ 7、隐藏导入Hiding imports import java.util.{ArrayList _, _} 8、连接字母和标点符号Joining letters to punctuation def bang_!(x: Int) 5 9、占位符语法Placeholder syntax List(1, 2, 3) map (_ 2) _ _ 10、偏应用函数Partially applied functions List(1, 2, 3) foreach println _ 11、初始化默认值default value var i: Int _ 12、访问元组tuple getters t._2 13、参数序列parameters Sequence _*作为一个整体告诉编译器你希望将某个参数当作参数序列处理例如val s sum(1 to 5:_*)就是将1 to 5当作参数序列处理。 这里需要注意的是以下两种写法实现的是完全不一样的功能 ? 1 2 3 foo _ // Eta expansion of method into method value foo(_) // Partial function application Example showing why foo(_) and foo _ are different: ? 1 2 3 4 5 6 7 8 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 _ isnt), 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中的下划线到底有多少种应用场景 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简明教程 http://colobu.com/2015/01/14/Scala-Quick-Start-for-Java-Programmers/转载于:https://www.cnblogs.com/tonychai/p/4546264.html
