scala> List(1,2,3,4).filter((_ % 2) == 0)
<console>:8: error: missing parameter type for expanded function ((x$1) => x$1.$percent(2))
          List(1,2,3,4).filter((_ % 2) == 0)

scala> List(1,2,3,4).filter(_ % 2 == 0)
res1: List[Int] = List(2, 4)