I write the following code:
class Animal
class Bird extends Animal
val animalList = List(new Animal, new Animal)
new Bird :: animalList
The implementation of the :: method looks like this:
def ::[B >: A] (x: B): List[B] =
new scala.collection.immutable.::(x, this)
I expected the return type to be List[Bird]. Why is it List[Animal]?
The dotless infix notation of
new Bird :: animalList
desugars to
animalList.::(new Bird)
because method names ending with : are invoked on the object on the right-hand side of the expression.
If we explicitly specify Bird as the type argument...
animalList.::[Bird](new Bird)
... then the compiler tells us why that doesn't work:
error: type arguments
[Bird]do not conform to method::'s type parameter bounds[B >: Animal]
There are two constraints on the type of B:
B >: A in the method signature means B has to be a supertype of the list's type (A, which here is Animal).new Bird) has to be an instance of B (because method arguments are covariant).The compiler chooses the most specific type that satisfies both of these constraints, which is Animal.
animalList.::[Animal](new Bird)