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scalainheritancetypes
Georg
Georg

Reputation: 982

Scala - inferred as wrong type, leading to type mismatch?

In Scala 2.11.5, compiling this

object Tryout {
  trait A {
    def method(a: Int): Boolean
  }

  abstract class B extends A {
    def method(a: Int) = ???
  }

  new B {
    override def method(a: Int) = true  // type mismatch here
  }
}

yields a "type mismatch: found Boolean, required Nothing" at the "true". If I replace the ??? with true or false, it compiles. It also compiles if I specify the result type of "method" in the abstract class.

This is not a big issue. However I'm curious whether anybody can explain why ??? is not inferred correctly as Boolean?

Upvotes: 2

Views: 1944

Answers (2)

sarveshseri
sarveshseri

Reputation: 13985

Return type of def method(a: Int) = ??? is actually Nothing.

scala> def method(a: Int) = ???
method: (a: Int)Nothing

Now, method in class B is overriding the method from the parent trait. You should define your claas B like this,

abstract class B extends A {
  // def method is provided by trait A
}

or

abstract class B extends A {
  // def method with full signature
  override def method(a: Int): Boolean = ???
}

Upvotes: 4

Michael Zajac
Michael Zajac

Reputation: 55569

Scala allows you to make the return type of an inherited method more restrictive in a sub-class.

abstract class A {
    def foo: Any
}

abstract class B {
    def foo: Int
}

new B {
    def foo = 1
}

So when you declare def method(a: Int) = ??? in B, ??? is inferred as Nothing, because the scala compiler doesn't know if you want Boolean or Nothing. This is one of the reasons why it is always a good idea to explicitly declare return types.

Upvotes: 7

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