No dynamic binding when abstract type involved in Scala?

问题

When I was trying the Animal/Food example for abstract types in Martin Odersky's Programming in Scala,

class Food
abstract class Animal {
  type SuitableFood <: Food
  def eat(food:SuitableFood)
}
class Grass extends Food
class Cow extends Animal {
  type SuitableFood=Grass
  override def eat(food:SuitableFood) {}
}
val bessy:Animal = new Cow
bessy.eat(new Grass)

I got the following error:

scala> <console>:13: error: type mismatch;
 found   : Grass
 required: bessy.SuitableFood
                  bessy.eat(new Grass)
                            ^

The original example by Martin was bessy.eat(new Fish), which would definitely fail, but I didn't expect it'd fail for Grass as well. The above error can be avoided by letting bessy be Cow instead of Animal: val bessy:Cow = new Cow.

Does this mean dynamic binding doesn't work here?

Edited: Simple dynamic binding for regular inheritance in Scala:

abstract class Parent {
  def sig:String = "Parent"
}
class Child extends Parent {
  override def sig:String = "Child"
}

And I had this, where x:Parent gave Child as well:

scala> new Child().sig
res1: String = Child

val x:Parent = new Child()
x: Parent = Child@3a460b07

x.sig
res2: String = Child

回答1:

Scala is statically typed. An arbitrary animal cannot eat grass, and you have just tried to feed grass to an arbitrary animal. It happens to be a cow, but you have stated (with : Animal) that the compiler may only assume that it is an animal.

If you allow the compiler to know that bessy is a Cow (val bessy = new Cow), then she'll eat grass just fine.

来源：https://stackoverflow.com/questions/20754143/no-dynamic-binding-when-abstract-type-involved-in-scala

标签

scala

dynamic-binding

abstract-type