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).