type-classes

Type Classes Pattern

Intent

In computer science, a type class is a type system construct that supports ad hoc polymorphism. The type classes pattern supports retroactive extension: the ability to extend existing software modules with new functionality without needing to touch or re-compile the original source.

Applicability

Use type-classes pattern when

• you want to ad-hoc and retroactive polymorphism
• avoiding using a lot of adapters

Structure

Participants

• TypeClass
  • always takes one or more type parameters.
  • is usually designed to be stateless.
• ImplicitObject
  • is the implicit value/object converting Target to TypeClass[Target].
• Target
  • defines the real object that used in client's function as a parameter.
• Client
  • defines functions which take parameter T and implicit parameter TypeClass[T].

Example

Imagine a human can say hello to some targets, but he has no idea what are concrete types of the targets. He only knows that the targets are Speakable. This example demonstrates how to make this happen.

Participants in this example:

• Speakable is the TypeClass.
• SpeakableMonkey/SpeakableLion is the ImplicitObject.
• Monkey/Lion is the Target.
• Human is the Client*.

Scala Tips

• As a shortcut for implicit parameters with only one type parameter, Scala provides so-called context bounds. For example:

  def sayHelloTo[A: Speakable[A]](target: A): String = {
    s"Human say hello, get reply ${implicitly[Speakable[A]].say()}"
  }

  If you want to access that implicitly available value, you need to call the implicitly method.

Reference

http://danielwestheide.com/blog/2013/02/06/the-neophytes-guide-to-scala-part-12-type-classes.html