Scala code - error: missing parameter type error

Question

What is wrong with this code?

object Numbers extends App {

  def decode(number: Int) : String = number match {
    case _ if _ % 15==0 => "fizzbuzz"
    case _ if _ % 3==0 => "fizz"
    case _ if _ % 5==0 => "buzz"
    case _ => _.toString
  }

  val test = List(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)
  test.map(decode).foreach(println)
}

I get the following error:

error: missing parameter type for expanded function
The argument types of an anonymous function must be fully known. (SLS 8.5)
Expected type was: String
case _ if _%15==0 => "fizzbuzz"

Why the compiler does not know the parameter type? Thanks

Kaito · Accepted Answer · 2012-08-10 12:11:29Z

6

(_ % 15 == 0) is expanded to a function (x: ?) => x % 15 == 0. Same for the other if checks.
The compiler can't infer it because it has no information about the parameter, it's a fresh name with no relation to the previous underscore in case _. So you can't use the underscore there to refer to the matched name, you have to actually assign it a name, as in case x if x % 15 == 0

answered Aug 10, 2012 at 12:11

Kaito

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