Structural type

Structural Type allow to set a behaviour very similar to what dynamic languages allow to do when they support duck typing.

The methods in a structural type are called via reflection, which is a lot slower on the JVM than a regular method call. You can define it either as a function parameter or as a type one.

Structural type definitions

val printSound = (animal: {def sound(): String}) => println(animal.sound())
// printSound: AnyRef{def sound(): String} => Unit = $$Lambda$5991/712224434@1dd12065

def printSound1[A <: { def sound(): String }](animal: A): Unit = println(animal.sound())
// printSound1: [A <: AnyRef{def sound(): String}](animal: A)Unit

class Dog {
  def sound(): String = "Woof!"
}

val dog = new Dog()
// dog: Dog = Dog@47d057ea

printSound(dog)
// Woof!

printSound1(dog)
// Woof!

An interface of object you’re passing as the parameter must have exactly the same signature as the structural type definition:

Interface object

def printSound(animal: {def sound(): String}): Unit = println(animal.sound())
// printSound: (animal: AnyRef{def sound(): String})Unit

class WhiteCat {
  def sound = "Meow!"
}

class RedCat {
  val sound = "Meow-meow!"
}

class BlackCat {
  def sound(): String = "Meow-meow-meow!"
}

printSound(new WhiteCat())
// <console>:14: error: type mismatch;
//  found   : WhiteCat
//  required: AnyRef{def sound(): String}
           printSound(new WhiteCat())
                      ^

printSound(new RedCat)
// <console>:14: error: type mismatch;
//  found   : RedCat
//  required: AnyRef{def sound(): String}
           printSound(new RedCat)
                      ^

printSound(new BlackCat)
// Meow-meow-meow!