Type Class Polymorphism In Scala

Mayank BairagiSr. Software Consultant Knoldus

Polymorphism

Same Operation Working on different type of values

Type of Polymorphism

1. Parametric

2. ad hoc

Polymorphism By

Overloading

Inheritance

Pattern Matching

Trait/ Interface

Type parameters and Generic types

Type Classes

Type Classes are Introduce first in haskell language

Scala Type Classes are Pattern instead of language feature

Example: scala.math.Numaric, scala.math.Ordering

Every Thing in Scalaz is Type class ( monoids, monads, applicative , functors )

OverLoading

case class Book(title:String,author:String)case class Movie(title:String,director:String)

object OverLoading { def serialize(book:Book)= "Book(" +book.title+","+book.author+")" def serialize(movie:Movie)= "Book(" +movie.title+","+movie.director+")"}

OverLoading

case class Book(title:String,author:String)case class Movie(title:String,director:String)

object OverLoading { def serialize(book:Book)= "Book(" +book.title+","+book.author+")" def serialize(movie:Movie)= "Book(" +movie.title+","+movie.director+")"}

Interface

trait Serializable { def serialize:String}

class Book(title:String,author:String) extends Serializable{ override def serialize= "Book(" +this.title+","+this.author+")"}

class Movie(title:String,director:String) extends Serializable{ override def serialize= "Movie(" +this.title+","+this.director+")"}

Problem With Interface

We Have Coupling Problem Here , How each class is serialize , this information has to be in the class. In order to add more trait and override the methods I need to have control on these classes. I should be allowed to view and modify the source code.

Pattern Matching

object Serialize { def serialize(x:Any) { x match { case b:Book => "Book(" +b.title+","+b.author+")" case m:Movie => "Movie(" +m.title+","+m.director+")" } }}

Problem With Pattern Matching

Both the Movie and Book classes are unaware how actually there serialized.

If I need more than one type of serialization than I need more serialize method with it's own case match block.

Now we have fixed the coupling problem, but unfortunatly we have introduce new coupling

Method serialize need to know about all the classes which need to be serialize.

Problem of control and source code is still exist

Type Class

case class Book(title:String,author:String)case class Movie(title:String,director:String)

trait Serializable[T] { def ser(t:T):String}

object Serializable{ def serialize[T](t:T, s:Serializable[T])=s.ser(t)}

object BookIsSerialzabel extends Serializable[Book]{ def ser(book:Book)= "Book(" +book.title+","+book.author+")"}

object MovieIsSerialzabel extends Serializable[Movie]{ def ser(movie:Movie)= "Movie(" +movie.title+","+movie.director+")"}

Type Class With Implicit

case class Book(author:String) extends Card[Book]case class Movie(director:String) extends Card[Movie]

trait Serializable[T] { def ser(t:T):String=t.asInstanceOf[Card[T]].title}

object Serializable{ def serialize[T](t:T)(implicit s:Serializable[T])=s.ser(t) implicit object BookIsSerialzabel extends Serializable[Book] {override def ser(book:Book)= "Book(" +book.title+","+book.author+")"}

implicit object MovieIsSerialzabel extends Serializable[Movie] { override def ser(movie:Movie)= "Movie(" +movie.title+","+movie.director+")"}}

Type Variances and Context Bound

Co variance +TContra variance -T

Thank you