How to pattern match a function in Scala?

Question

Is it possible to pattern match functions in scala ? Specially the return type of the function.

Here is an example. I'm trying to print Shoe if the function's return type is Shoe or Bag otherwise.

object Test extends App {

  trait ProductItem {
    val name: String
    val price: Int
  }

  case class Product(partialPerson: (String) => ProductItem)
  case class Shoe(name: String)(val price: Int) extends ProductItem
  case class Bag(name: String)(val price: Int) extends ProductItem

  val shoe = Shoe("nike")(_)
  val bag = Bag("addidas")(_)

  def printType(shoe: (Int) => ProductItem): Unit = {
    shoe match {
      case isShoe: ((Int) => Shoe) =>
        println("Is a shoe")
      case isBag: ((Int) => Bag) =>
        println("Is a bag")
    }
  }

  printType(shoe)
  printType(bag)
}

Output:
Is a shoe
Is a shoe

Expected output:
Is a shoe
Is a bag

chengpohi · Accepted Answer · 2015-11-14 13:28:07Z

5

This is caused by the type erasure in compile time:

<console>:17: warning: non-variable type argument Bag in type pattern Int => Bag is unchecked since it is eliminated by erasure

You can solve it by using TypeTags:

def printType[T](shoe: (Int) => T)(implicit tag: TypeTag[T]): Unit = {
    shoe match {
      case isShoe if tag.tpe =:= typeOf[Shoe]=>
        println("Is a shoe")
      case isBag if tag.tpe =:= typeOf[Bag] =>
        println("Is a bag")
    }
}

answered Nov 14, 2015 at 13:28

chengpohi

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1 Comment

Basil Over a year ago

Is there much perf cost to this? Or is it fine to do in a cross-cutting concern method (logging/metrics)

Maxim · Accepted Answer · 2015-11-15 14:22:55Z

2

When compiling this code you get a compiler warning tha explains the problem -

Warning:(20, 25) non-variable type argument c.n.p.Shoe in type pattern Int => c.n.p.Shoe is unchecked since it is eliminated by erasure case isShoe: (Int => Shoe) => ^

a possible solution around it -

  def printType[T <: ProductItem : ClassTag](item: Int => T): Unit = {
    implicitly[ClassTag[T]] match {
      case ClassTag(s) if s == classOf[Shoe] =>
        println("Is a shoe")
      case ClassTag(b) if b == classOf[Bag] =>
        println("Is a bag")
    }
  }

To understand this solution you'll have to be familiar with implicit parameters & ClassTag type.

edited Nov 15, 2015 at 14:22

answered Nov 14, 2015 at 13:25

Maxim

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