The document discusses the Scala Cats library, emphasizing its role in functional programming and the use of type classes to extend library functionality without altering source code. It explains key concepts such as type classes, their components, implicits, and implicit scope in Scala. Additionally, it highlights etiquette for sessions, including punctuality and constructive feedback.

1. Presented By: Mansi Babbar
Getting Started with
Scala Cats

2. Lack of etiquette and manners is a huge turn off.
KnolX Etiquettes
Punctuality
Respect Knolx session timings, you
are requested not to join sessions
after a 5 minutes threshold post
the session start time.
Feedback
Make sure to submit a constructive
feedback for all sessions as it is
very helpful for the presenter.
Mute
Please keep your window on mute
Avoid Disturbance
Avoid leaving your window
unmuted after asking a question

3. Agenda
What is Cats?
03 What are Implicits?
04 Implementation - The Cat Show
What are Type Classes?02
01

4. Meet Cats

5. ● An abstraction for functional programming in
the Scala programming language.
● Cats contain a large variety of tools for
functional programming majorly in form of
type classes which we can apply to the
existing Scala types.
What is Cats?

6. Type Classes

7. What are Type Classes?
● Type classes are a pattern in
programming originating in Haskell.
● Allow us to extend existing libraries
with new functionality, without using
traditional inheritance, and also without
altering the original library source code.

8. Type Class Components

9. Type Class :
● An API or interface that represents some functionality.
● Represented by a trait with at least one type parameter.
Type Class Components

10. Type Class Instances :
● Provide implementations of the type class for specific types.
● In Scala, we define instances by creating concrete implementations of the
type class and tagging them with the implicit keyword:
Type Class Components

11. Type Class Use :
● Any functionality which requires a type class instance to work.
● In Scala, this means any method that accepts instances of the type class as
implicit parameters.
● Cats provide utilities which make type classes a little bit easier to use, and
you may sometimes see these patterns in other libraries also.
● Type class can be used by creating Interface Objects.
Type Class Components

12. Interface Objects :
The simplest way of creating an interface that uses a type class is to place
methods in a singleton object:
Type Class Components

13. Interface Objects :
To use this object, we have to import any type class instances we care about
and then call the relevant method:
Type Class Components

14. Implicits

15. What are Implicits?
● Working with implicit values and the implicit
parameters.
● Placing the instances in a companion object to
the type class has a very special significance in
Scala because it actually plays into something
called implicit scope.

16. What is Implicit Scope?
● Compiler will search for the candidate type class instances by type.
● Places where the compiler will search for the candidate instances is known as the
implicit scope.
● It applies at the call site which is the point where we call a method with an
implicit parameter.
● Definitions will only be included in implicit scope if they are tagged with the
implicit keyword.
● Compiler will fail with an ambiguous implicit values error if it sees multiple
candidate definitions.

17. What is Implicit Scope?
For example, in the following expression it will look for an instance of type
JsonWriter[String]:

18. What is Implicit Scope?
Type class instances packaging can be done as:
● by placing them in an object such as JsonWriterInstances
● by placing them in a trait
● by placing them in the companion object of the type class

19. Demo: The Cat Show

20. References
● https://typelevel.org/cats/
● http://underscore.io/books/scala-with-cats/
● https://www.scalawithcats.com/dist/scala-with-
cats.html
● https://github.com/typelevel/cats
● https://medium.com/@terezk_a/haskell-in-elm-
terms-type-classes-415f1612b335

21. Thank You :)