This document provides an overview of the Typelevel stack, including Cats, Cats-Effect, Http4s, Circe, and Doobie. It describes these libraries as providing abstractions for pure functional programming (Cats), modeling programs as values (Cats-Effect), building HTTP applications (Http4s), working with JSON (Circe), and interacting with databases (Doobie). It includes examples of using the Http4s DSL to define an HTTP service and using Doobie to run database queries.

1. Up and Running with the
Typelevel Stack
Scala Italy 2018

2. Cats & Cats-Effect
● Lightweight functional programming library
● Provides abstractions that help with pure FP
● Aims to make code highly composable and reusable
● One of the most important concept is “Programs as values”

3. Pure Functional Programming
● Code completely free of side-effects
● Effects are modelled as values to be executed by the runtime
● The IO data type is an example of such an effect type
● We can model our entire application as a single value of IO

4. Http4s
● Lightweight typeful, pure functional http library
● Makes it very easy to build classic server applications
● Purely functional, but also very performant
● Offers a DSL for easily matching HTTP routes

5. type Service = Request => IO[Response]

6. Http4s DSL - example
HttpService[IO] {
case GET -> Root / name =>
Ok(s“Hello $name”)
}

7. Http4s DSL - example
HttpService[IO] {
case GET -> Root / name =>
Ok(s“Hello $name”)
}
Request

8. Http4s DSL - example
HttpService[IO] {
case GET -> Root / name =>
Ok(s“Hello $name”)
}
IO[Response]

9. Http4s DSL - example
HttpService[IO] {
case GET -> Root / name =>
Ok(s“Hello $name”)
}
Method

10. Http4s DSL - example
HttpService[IO] {
case GET -> Root / name =>
Ok(s“Hello $name”)
}
Path

11. Http4s DSL - example
HttpService[IO] {
case GET -> Root / name =>
Ok(s“Hello $name”)
}
String

12. Circe
● Lightweight JSON library
● Built upon Cats and Shapeless
● Allows us to derive encoders and decoders for our custom
data types
● One of the most widely used JSON libraries in Scala

13. Doobie
● Functional abstraction over JDBC
● Allows us to represent database programs as values of
the ConnectionIO data type
● We can then turn values of ConnectionIO into
values of IO as a transaction.
● Automatic mapping of query results to case classes

14. Doobie - example
val allUsers: ConnectionIO[List[User]] =
sql"SELECT * FROM user".query[User].to[List]
val luka: ConnectionIO[Option[User]] =
sql"SELECT * FROM user WHERE $name = ‘Luka’"
.query[User].option
val program: IO[List[User]] = allUsers.transact(xa)

15. Let’s start!
Clone the workshop repo:
https://github.com/LukaJCB/scala-italy-workshop
Documentation and assignments are in the README.md