scala - Extractor with pattern matching -

this scala code

object twice {   def apply(x: int): int = x * 2   def unapply(z: int): option[int] = if (z%2 == 0) some(z/2) else none }  object twicetest extends app {   val x = twice(5)   println(x)   x match { case twice(n) => println(n) } } 

prints

10 5 

when twicetest run. question is: x integer , not function twice(n), why pattern matching works?

it's because of pattern matching expression.

let's take @ it:

x match {   case twice(n) => println(n) } 

you matching using twice. in background, call twice.unapply() parameter x. if result some(foo), substitute value of foo value of n. otherwise won't match.

see this longer explanation.