Pattern match for variable in scope (Scala)

问题

In the following code

val x = 5
val y = 4 match {
  case x => true
  case _ => false
}

the value y is true. Scala interprets x to be a free variable in the pattern match instead of binding it to the variable with the same name in the scope.

How to solve this problem?

回答1:

Invoking the least astonishment principle, I will simply do:

val x = 5
val y = 4 match {
  case z if z == x => true
  case _ => false
}

回答2:

Backticking the variable indicates to bind a scoped variable:

val x = 5
val y = 4 match { case `x` => true; case _ => false }

returns false.

Alternatively, if a variable starts with an uppercase letter, it binds to a scoped variable without backticking.

来源：https://stackoverflow.com/questions/6754579/pattern-match-for-variable-in-scope-scala