What will be printed into the console?
val Constant = 'Q'
def tokenMe(ch: Char) = (ch: @switch) match {
  case ' ' | '\t' | '\n'  => 1
  case 'A' | 'Z' | '$'    => 2
  case '5' | Constant     => 3 
  case _                  => 4
}
println(tokenMe('5'))
Explanation
A non-literal may prevent switch generation: this would not compile
Theory
  • An annotation to be applied to a match expression. If present, the compiler will verify that the match has been compiled to a tableswitch or lookupswitch and issue an error if it instead compiles into a series of conditional expressions.

    Example usage:

    
    val Constant = 'Q'
    def tokenMe(ch: Char) = (ch: @switch) match {
      case ' ' | '\t' | '\n'  => 1
      case 'A' | 'Z' | '$'    => 2
      case '5' | Constant     => 3  // a non-literal may prevent switch generation: this would not compile
      case _                  => 4
    }
    
    Note: for pattern matches with one or two cases, the compiler generates jump instructions. Annotating such a match with @switch does not issue any warning.
    Scala Documetation

@vetalbabenko36 True! thanks :) it creates Warning:(4, 32) could not emit switch for @switch annotated match def tokenMe(ch: Char) = (ch: @switch) match { ^

2017 May 18, 1:04:03 AM

This example will be successfully compiled. It doesn't optimize to tableswtich , but it is compiled.

2017 May 3, 8:37:51 AM

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