Why does Scala PartialFunction isDefinedAt method always return true?

Question

my code is as below.

    case class C[T]() {
      val pf:PartialFunction[Any,Any] = {
        case i:T => i
      }
    }

    println(C[Int]().pf.isDefinedAt(-1.0))

this prints true. Why this happens?

Answer 1

Due to type erasure your code is basically equal to:

case class C[T]() {
  val pf:PartialFunction[Any,Any] = {
    case i:Any => i // this matches everything
  }
}

you can use TypeTags to fix it:

import scala.reflect.runtime.universe._
case class C[T: TypeTag]() {
  def pf[U: TypeTag]: PartialFunction[U, Any] = {
    case i if typeOf[U] <:< typeOf[T] => i
  }
}

in use:

@ C[Int]().pf.isDefinedAt("")
res41: Boolean = false

@ C[Int]().pf.isDefinedAt(34)
res42: Boolean = true

these are virtually equal to

@ C[Int]().pf[String].isDefinedAt("")
res41: Boolean = false

@ C[Int]().pf[Int].isDefinedAt(34)
res42: Boolean = true

where the type U is inferred - it has a limitation that it can only be as precise as compiler's knowledge about the type when TypeTag is required.

You can also try to use ClassTag[T] to use runtime reflection... but it would fail for primitives

case class C[T]()(implicit classTag: scala.reflect.ClassTag[T]) {
  def pf[U: TypeTag]: PartialFunction[U, Any] = {
    case i if classTag.runtimeClass.isInstance(i) => i
  }
}

which results in

@ C[Int]().pf.isDefinedAt(34)
res2: Boolean = false

@ C[Int]().pf.isDefinedAt("")
res3: Boolean = false

Thing is classTag would resolve to Scala's int while runtime would show java.lang.Int:

@ case class C[T]()(implicit classTag: scala.reflect.ClassTag[T]) {
    def pf: PartialFunction[Any, Any] = {
      case i => println(s"T = ${classTag.runtimeClass.getName}, here: ${i.getClass.getName}")
    }
  }
defined class C

@ C[Int]().pf.isDefinedAt(34)
res7: Boolean = true

@ C[Int]().pf(34)
T = int, here: java.lang.Integer
res8: Any = ()

In general there is no perfect solution here, you can read more about similar questions here and here.