scala – 按键设置过滤地图

你可以利用Set [A]是谓词的事实;即A =布尔值

map filterKeys set

这里是在工作：

scala> val map = Map("1" -> 1,"2" -> 2,"3" -> 3)
map: scala.collection.immutable.Map[java.lang.String,3 -> 3)

scala> val set = Set("1","2")
set: scala.collection.immutable.Set[java.lang.String] = Set(1,2)

scala> map filterKeys set
res0: scala.collection.immutable.Map[java.lang.String,2 -> 2)

或者如果您喜欢：

scala> map filterKeys Set("1","2")
res1: scala.collection.immutable.Map[java.lang.String,2 -> 2)

谓语

它实际上是有用的有一个谓词的包装。像这样：

scala> class PredicateW[A](self: A => Boolean) {
   | def and(other: A => Boolean): A => Boolean = a => self(a) && other(a)
   | def or(other: A => Boolean): A => Boolean = a => self(a) || other(a)
   | def unary_! : A => Boolean = a => !self(a)
   | }
defined class PredicateW

和一个隐式转换：

scala> implicit def Predicate_Is_PredicateW[A](p: A => Boolean) = new PredicateW(p)
Predicate_Is_PredicateW: [A](p: A => Boolean)PredicateW[A]

然后你可以使用它：

scala> map filterKeys (Set("1","2") and Set("2","3"))
res2: scala.collection.immutable.Map[java.lang.String,Int] = Map(2 -> 2)

scala> map filterKeys (Set("1","2") or Set("2","3"))
res3: scala.collection.immutable.Map[java.lang.String,3 -> 3)

scala> map filterKeys !Set("2","3")
res4: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1)

这可以扩展到xor，nand等等，如果你包括符号unicode可以令人惊讶地可读代码：

val mustReport = trades filter (uncoveredShort ∨ exceedsDollarMax)

val european = { 
  val Europe = (_ : Market).exchange.country.region == Region.EU
  trades filter (_.market ∈: Europe)
}