Scala/Structural Typing

From Wikibooks, open books for an open world
Jump to: navigation, search

Structural typing as defined by Wikipedia “A structural type system (or property-based type system) is a major class of type system, in which type compatibility and equivalence are determined by the type’s structure, and not by other characteristics such as its name or place of declaration “ Strutural types in Scala allows code modularity for some specific situations. For instance, if a behaviour is implemented across several classes and those behaviours need to invoked by the structure of the type.

This approach rules out the need for an abstract class or trait merely for the purpose of calling single overridden method. Structural typing not only adds syntatic sugar but also makes the code much modular.

Lets consider a behaviour ‘walk’ in classes Cat and Dog. The StrucType class’s whoIsWalking takes a type parameter which states that “Accept any object which has a method walk and that returns a string” type is aliased with a variable ‘c’ and with in the method the aliased variable can invoke ‘walk’.

class StrucType {
  def whoIsWalking(c:{def walk():String}) = println(c.walk)

Below are the classes which has the 'walk' method in common

class Cat {
  def walk():String = "Cat walking"
class Dog {
  def walk():String = "Dog walking"

The below is the class with the main method

 object Main {
    def main(args:Array[String]) {
    println("Hello Scala")
    val walkerStruct = new StrucType();
    walkerStruct.whoIsWalking(new Cat());
    walkerStruct.whoIsWalking(new Dog());