Question

this works as expected

scala> 3 match { case x:Int => 2*x }
res1: Int = 6

why does this fail?

scala> 3 match { case $x:Int => 2*$x }
:1: error: '=>' expected but ':' found.
       3 match { case $x:Int => 2*$x }
                        ^

scala> 3 match { case `$x`:Int => 2*$x }
:1: error: '=>' expected but ':' found.
       3 match { case `$x`:Int => 2*$x }
                          ^

scala> 3 match { case `$x` : Int => 2*$x }
:1: error: '=>' expected but ':' found.
       3 match { case `$x` : Int => 2*$x }

‘$’ is supposed to be a valid identifier character, as demonstrated here:

scala> var y = 1
y: Int = 1

scala> var $y = 2
$y: Int = 2

Thanks

Answer 1

Actually, while all other answers are correct in some sense, the explanation here is simpler. A dollar sign is considered an uppercase letter according to the specification and, thus, is treated like a constant in a pattern match.