CODEWITHSUNDEEP
scala
codefly
codefly

Reputation: 640

Can I perform matching on a type parameter in Scala to see if it implements a trait?

I would like to have a method which returns a class of a certain type, but I want the method to behave differently depending on whether or not the class extends a particular trait as follows: 


case class ClassA extends TraitA
case class ClassB extends TraitB
case class ClassC extends TraitA
...
def myfunc[T]():T = {
  T match {
    case TraitA => // return new T in a particular way 
    case TraitB => // ditto
  }
}

Is this possible, or am I going about it the wrong way?

Thanks

Upvotes: 22

Views: 17966

Answers (3)

Rex Kerr
Rex Kerr

Reputation: 167891

You can't compare types directly, because there isn't anything there to compare (at runtime, due to erasure). You could work on a representation of your class:

trait TraitA { }
trait TraitB { }
class ClassA extends TraitA { }
class ClassB extends TraitB { }

def myFunc[T](clazz: Class[T]) = {
  if (classOf[TraitA] isAssignableFrom clazz) println("A")
  else if (classOf[TraitB] isAssignableFrom clazz) println("B")
  else println("?")
}

scala> myFunc(classOf[ClassA])
A

scala> myFunc(classOf[String])
?

or you can pattern match on instances of the class:

def myFunc2[T](t: T) = t match {
  case _: TraitA => println("A")
  case _: TraitB => println("B")
  case _ => println("?")
}

scala> myFunc2(new ClassA)
A

scala> myFunc2(Some(5))
?

You can also use the first approach in a syntactically less obtrusive way via class manifests:

def myFunc3[T](implicit mf: ClassManifest[T]) = {
  val clazz = mf.erasure
  if (classOf[TraitA] isAssignableFrom clazz) println("A")
  else if (classOf[TraitB] isAssignableFrom clazz) println("B")
  else println("?")
}

scala> myFunc3[ClassA]
A

scala> myFunc3[String]
?

and you can choose different sorts of dispatch also if the if/else becomes wieldy:

object MyFunc {
  val dispatch = Map(
    classOf[TraitA] -> (() => println("A")),
    classOf[TraitB] -> (() => println("B"))
  )
  val default = () => println("?")
  def apply[T](implicit mf: ClassManifest[T]) = 
    dispatch.find(_._1 isAssignableFrom mf.erasure).map(_._2).getOrElse(default)()
}

scala> MyFunc[ClassA]
A

scala> MyFunc[String]
?

Note that any generic code that you use this from will need to have a class manifest available (either as an implicit parameter or in shorthand, [T: ClassManifest].

Upvotes: 19

Paolo Falabella
Paolo Falabella

Reputation: 25844

You need an instance to check the type. A generic by itself is just a placeholder for a type. You could do something like this:

trait bTrait //common base trait
trait TraitA extends bTrait
trait TraitB extends bTrait

class ClassA extends TraitA
class ClassB extends TraitB

def myFunc[T <: bTrait](t:T) : String = //passing explicitly an instance of type T
{
  t match {
    case _ : TraitA => "TraitA"
    case _ : TraitB => "TraitB" 
  }
}

println(myFunc(new ClassA)) //prints TraitA

Upvotes: 3

The Archetypal Paul
The Archetypal Paul

Reputation: 41749

"want the method to behave differently depending on whether or not the class extends a particular trait"

This is almost a canonical description of inheritance. Can't you have a method in each trait that encapsulates the differing behaviour you're after?

Upvotes: 2

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