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1、存在性類型：Existential types def foo(l: List[Option[_]]) = ...2、高階類型參數(shù)：Higher kinded type parameterscase class A[K[_],T](a: K[T])3、臨時變量：Ignored variables val _ = 54、臨時參數(shù)：Ignored parameters List(1, 2, 3) foreach { _ => println("Hi") }5、通配模式：Wildcard patterns Some(5) match { case Some(_) => println("Yes") } match { case List(1,_,_) => " a list with three element and the first element is 1" case List(_*) => " a list with zero or more elements " case Map[_,_] => " matches a map with any key type and any value type " case _ => } val (a, _) = (1, 2) for (_ <- 1 to 10)6、通配導入：Wildcard imports import java.util._7、隱藏導入：Hiding imports // Imports all the members of the object Fun but renames Foo to Barimport com.test.Fun.{ Foo => Bar , _ }// Imports all the members except Foo. To exclude a member rename it to _import com.test.Fun.{ Foo => _ , _ }8、連接字母和標點符號：Joining letters to punctuation def bang_!(x: Int) = 59、占位符語法：Placeholder syntax List(1, 2, 3) map (_ + 2) _ + _ ( (_: Int) + (_: Int) )(2,3)val nums = List(1,2,3,4,5,6,7,8,9,10)nums map (_ + 2) nums sortWith(_>_) nums filter (_ % 2 == 0) nums reduceLeft(_+_) nums reduce (_ + _) nums reduceLeft(_ max _) nums.exists(_ > 5) nums.takeWhile(_ < 8)10、偏應用函數(shù)：Partially applied functions def fun = { // Some code } val funLike = fun _List(1, 2, 3) foreach println _1 to 5 map (10 * _)//List("foo", "bar", "baz").map(_.toUpperCase())List("foo", "bar", "baz").map(n => n.toUpperCase())11、初始化默認值：default value var i: Int = _12、作為參數(shù)名： //訪問mapvar m3 = Map((1,100), (2,200)) for(e<-m3) println(e._1 + ": " + e._2) m3 filter (e=>e._1>1) m3 filterKeys (_>1) m3.map(e=>(e._1*10, e._2)) m3 map (e=>e._2)//訪問元組：tuple getters (1,2)._213、參數(shù)序列：parameters Sequence _*作為一個整體，告訴編譯器你希望將某個參數(shù)當作參數(shù)序列處理。例如val s = sum(1 to 5:_*)就是將1 to 5當作參數(shù)序列處理。 //Range轉(zhuǎn)換為ListList(1 to 5:_*)//Range轉(zhuǎn)換為VectorVector(1 to 5: _*)//可變參數(shù)中def capitalizeAll(args: String*) = { args.map { arg => arg.capitalize } }val arr = Array("what's", "up", "doc?") capitalizeAll(arr: _*)

這里需要注意的是，以下兩種寫法實現(xiàn)的是完全不一樣的功能：

foo _ // Eta expansion of method into method value foo(_) // 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.

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轉(zhuǎn)載于:https://www.cnblogs.com/wzj4858/p/8204369.html

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