CODEWITHSUNDEEP
scala
Tom
Tom

Reputation: 6332

Lower bound seems not working with covariance

Given the following code:

class Animal
class Cat extends Animal
class BlackCat extends Cat
class MyOption[+A](val x: A) {
      def get(): A = x
      def getOrElse[B >: A](default: => B): B = {
        if (x != null) x else default
      }
}

object ThirdParty {
  def getAnimal : MyOption[Animal]= new MyOption[Cat](null)
}

object MyOptionRunner {
    def main(args: Array[String]) {
    val opt3: Animal = ThirdParty.getAnimal.getOrElse(new BlackCat)
    println(opt3)
    }
}

I am surprised that val opt3: Animal = ThirdParty.getAnimal.getOrElse(new BlackCat) can pass the compiling.

The type of ThirdParty.getAnimal is Myption[Cat],then the call is equivalent to Myption[Cat].getOrElse(new BlackCat),it doesn't conform to the definition of getOrElse, A is Cat and B is BlackCat here, which breaks B>:A

Upvotes: 1

Views: 44

Answers (1)

Silvio Mayolo
Silvio Mayolo

Reputation: 70267

val opt3: Animal = ThirdParty.getAnimal.getOrElse(new BlackCat)

B is not BlackCat; the type of B will be the most specific type that conforms to the specification required. Since the argument is new BlackCat, we know that B >: BlackCat. By the requirement of getOrElse, we know that B >: Cat. The most specific type that satisfies both of these requirements is, naturally, Cat.

Upvotes: 1

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