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Type-driven development is a programming style where you write types first and use them to guide the definition of functions in your program. Scala is particularly well suited to it because of its powerful type system. Cats lets you take Scala and type-driven development even further by expressing side effecting operations as types.

During this talk, you will learn how to create a web service using Akka HTTP and Cats' Free, IO, Writer and Reader monads. The aim is to clearly delimit pure and impure code such that the former can be unit tested and reasoned about whilst the latter can be minimized.

The talk will have three parts. The first will show how to 'free' an Algebraic Data Type (ADT) that describes side effecting operations using the Free monad and an interpreter. The second part will introduce the IO monad and demonstrate how it lets you be both lazy and explicit when dealing with asynchronous operations. And in the final part, we will explore how to use the Reader and Writer monads to read configuration and write logs without sacrificing purity.

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