如何使用模式匹配案例类中声明的变量？

Question

I have the following trait:

sealed trait Tr

case class Test1(str: String) extends Tr

Now I want to use str in my code.

val tr: Tr = //...

tr match {
    case Test1(str) =>
        val str = //str declared in Test1(str) + "some another string"
}

Is it possible to directly refer to the str preserving the name str . I can use something like

tr match {
    case test1 @ Test1(str) =>
        str = test1.str + "some another string"
}

But this seems ugly to me.

Answer 1

str is immutable. Try another variable.

If you want to edit a case class try:

val test2 = test1.copy(str = test1.str + "some another string")

This will create a new instance, not mutate the case class .

Answer 2

You don't need the email operator:

sealed trait Tr
case class Test1(str: String) extends Tr
val tr = Test1 ("foo")
tr match {
   case Test1 (str) => println (str) 
}
foo

Of course, reassignment to str isn't possible.

Note, that in most cases, the parameter of a class has a meaning, and in an ideal world, one of few best meanings, so the stimulus, to repeat the same variable name is high. But in principal, you aren't bound to use str again. Maybe the new context provides a better name:

tr match {
   case Test1 (bar) => println (bar) 
}

The parameter name, whether 'str' or 'bar' is used to deconstruct the Ctor-call to Test1, and is binding, what fits, to the new name.

Note, that in your example:

tr match {
    case test1 @ Test1(str) =>
        str = test1.str + "some another string"
}

If you happen to need to refer to the whole thing, the name tr is in scope, so you might refer to it:

tr match {
    case Test1 (str) => dedodulute (tr)
}

You aren't obliged to use str on the right hand side at all.

Sometimes you even like to refer to the parameter of your case class on the left hand side of the case statement:

tr match {
      case Test1 (s) if (s.startsWith ("f"))  => println ("foo") 
      case Test1 (s) if (s.startsWith ("b"))  => println ("bar") 
      case _ => println ("unknown") 
}

You may even pick a single element from a list, but have to know it's position from left:

val l = List (4, 1, 3, 9) 
l match {
      case List (_, a, _*) => println ("2nd el is " + a) 
      case _ => println ("unknown") 
}

You can't use

case List (_*, a, _) => println ("2nd last el is " + a) 

But you may use literal values, to tighten your match:

l match {
    case List (3, a, _*) => println ("2nd el is " + a + "after 3") 
    case List (4, a, _*) => println ("2nd el is " + a + "after 4") 
    case _ => println ("unknown") 
 }

Answer 3

You can do something like this:

sealed trait Tr

case class Test1(str: String) extends Tr

val tem = Test1 ("I am immutable!!")
val str = tem match {
case Test1 (str) => str + "But you can assign me to something else :)."
}

println(str)

Answer 4

Your last case looks almost what you need:

tr match {
  case test1: Test1 =>
    val str = test1.str + "some another string"
}