11.4.3 Replacing Inheritance with Composition

Transitioning from Inheritance to Composition in Clojure

In traditional object-oriented programming, inheritance is a common mechanism for achieving code reuse and polymorphism. However, in the functional programming paradigm, especially in Clojure, composition often serves as a more powerful and flexible tool.

Leveraging Composition and Higher-Order Functions

Code Reuse Without Inheritance:

In Java, inheritance is typically used to share code across classes:

// Java example using inheritance
class Animal {
    void makeSound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Bark");
    }
}

In Clojure, you can achieve similar code reuse through composition:

;; Clojure example using composition
(defn make-sound [sound]
  (fn [] (println sound)))

(def dog (make-sound "Bark"))

(dog) ;; Prints "Bark"

Using functions that return functions (higher-order functions) allows you to create reusable and composable units of behavior.

Protocols and Multimethods for Polymorphism

Achieving Polymorphic Behavior:

Instead of relying on class hierarchies, Clojure utilizes protocols and multimethods to provide polymorphic operations across disparate data types.

Protocols:

Protocols define a set of functions without binding them to any specific data structure, allowing different data types to implement the same operations:

;; Clojure protocol example
(defprotocol AnimalSound
  (make-sound [this]))

(defrecord Dog []
  AnimalSound
  (make-sound [this] (println "Bark")))

(def dog (->Dog))

(make-sound dog) ;; Prints "Bark"

Multimethods:

Multimethods provide an even more flexible way to define said operations by dispatching based on arbitrary criteria:

;; Clojure multimethod example
(defmulti make-sound :type)

(defmethod make-sound :dog [_]
  (println "Bark"))

(make-sound {:type :dog}) ;; Prints "Bark"

Benefits of Using Composition over Inheritance

• Avoids Hierarchical Rigidities: Composition allows more flexibility than the rigid hierarchies of inheritance.
• Promotes Code Reusability: Functions (especially higher-order functions) make it easy to reuse and compose behavior.
• Enhances Testability: Smaller, composable functions and protocols are easier to test independently than tightly-coupled class hierarchies.

Conclusion

By replacing inheritance with composition in Clojure, you’re adopting a more modular and flexible approach to software design. Protocols and multimethods further extend these capabilities, providing powerful tools for polymorphic behavior.

Embark on your functional programming journey by embracing the power of composition and Clojure’s advanced polymorphic features in your Java to Clojure migration efforts.