scala - Why can't a variable be a stable identifier?

Question

The following

def mMatch(s: String) = {
    var target: String = "a"
    s match {
        case `target` => println("It was " + target)
        case _ => println("It was something else")
    }
}

does not compile:

error: stable identifier required, but target found. case target => println("It was " + target)

Why does Scala require a val not a var. I guess "Because" would be an acceptable answer but I have the feeling there is a deeper reason I am missing.

Answer 1

I suspect that it's to enable table-switching optimizations for those cases where it is possible (without piles of checking to see whether it's valid). For example, with the code

class Sw {
  def m(i: Int) = {
    val a = 3
    val b = 2
    val c = 1
    i match {
      case `a` => 0
      case `b` => -1
      case `c` => 4
      case _ => 2
    }
  }
}

you get the bytecode

public int m(int);
  Code:
   0:   iconst_3
   1:   istore_2
   2:   iconst_2
   3:   istore_3
   4:   iconst_1
   5:   istore  4
   7:   iload_1
   8:   istore  5
   10:  iload   5
   12:  tableswitch{ //1 to 3
        1: 48;
        2: 44;
        3: 52;
        default: 40 }
   40:  iconst_2
   41:  goto    53
   44:  iconst_m1
   45:  goto    53
   48:  iconst_4
   49:  goto    53
   52:  iconst_0
   53:  ireturn

which would be much more complicated to do if you used vars (you'd have to detect whether they had changed to know whether that table expression was still valid).