- 用scala 表示JSON数据
package week1 abstract class JSON case class JObj(bindings: Map[String, JSON]) extends JSON case class JSeq(elems: List[JSON]) extends JSON case class JNum(num: Double) extends JSON case class JStr(str: String) extends JSON case class JBool(b: Boolean) extends JSON case object JNull extends JSON
将JSON对象显示为JSON串
package week1
object Main extends App {
val data = JObj(Map(
"firstName" -> JStr("John"),
"lastName" -> JStr("Smith"),
"address" -> JObj(Map(
"streetAddress" -> JStr("21 2nd Street"),
"state" -> JStr("NY"),
"postalCode" -> JNum(10021))),
"phoneNumbers" -> JSeq(List(
JObj(Map(
"type" -> JStr("home"), "number" -> JStr("212 555-1234"))),
JObj(Map(
"type" -> JStr("fax"), "number" -> JStr("646 555-4567")))))))
def show(json: JSON): String = json match {
case JNum(num) => num.toString
case JBool(b) => b.toString
case JNull => "null"
case JStr(str) => ‘\"‘ + str + ‘\"‘
case JSeq(elems) => "[" + (elems map show mkString ",") + "]"
case JObj(bindings) =>
val assoc = bindings map {
case (key, value) => "\""+key+"\""+":"+"\""+show(value)+"\""
}
"{"+ ( assoc mkString ",") + "}"
}
println(show(data))
}
2. case 序列作为函数：
val f:String => String = {case "ping" => "pong"}//> f : String => String = <function1>
f("ping") //> res0: String = pong
// f的定义等价于
val ff = new Function1[String, String] {
def apply(s: String) = s match {
case "ping" => "pong"
}
} //> ff : String => String = <function1>
但是f有可能抛mactch error
package week1
object Test {
val f:String => String = {case "ping" => "pong"}//> f : String => String = <function1>
f("ping") //> res0: String = pong
f("pong") //> scala.MatchError: pong (of class java.lang.String)
//| at week1.Test$$anonfun$main$1$$anonfun$1.apply(week1.Test.scala:4)
//| at week1.Test$$anonfun$main$1$$anonfun$1.apply(week1.Test.scala:4)
//| at week1.Test$$anonfun$main$1.apply$mcV$sp(week1.Test.scala:6)
//| at org.scalaide.worksheet.runtime.library.WorksheetSupport$$anonfun$$exe
//| cute$1.apply$mcV$sp(WorksheetSupport.scala:76)
//| at org.scalaide.worksheet.runtime.library.WorksheetSupport$.redirected(W
//| orksheetSupport.scala:65)
//| at org.scalaide.worksheet.runtime.library.WorksheetSupport$.$execute(Wor
//| ksheetSupport.scala:75)
//| at week1.Test$.main(week1.Test.scala:3)
//| at week1.Test.main(week1.Test.scala)
}
这时可以用偏函数,在调用前检查一下在参数处是否有定义
package week1
object Test {
val f:PartialFunction[String, String] = {case "ping" => "pong"}
//> f : PartialFunction[String,String] = <function1>
f.isDefinedAt("ping") //> res0: Boolean = true
f.isDefinedAt("pong") //> res1: Boolean = false
}
3.但是检查isDefinedAt并不能保证不抛MatchError
val g: PartialFunction[List[Int], String] = {
case Nil => "one"
case x :: rest =>
rest match {
case Nil => "two"
}
}
g.isDefinedAt(List(1, 2, 3)) //true
时间： 2024-10-09 21:37:20