Most applications are just reading data, transforming it and writing it somewhere else. And there are great libraries in the Scala eco-system to support these use cases: Akka-Stream, fs2, Monix,... But if you look under the hood and try to understand how those libraries work you might be a bit scared by their complexity!

In this talk you will learn how to build a very minimal "streaming library" where all the difficult concerns are left to other libraries: eff for asynchronous computations and resources management, origami for extracting useful data out of the stream. Then you will decide how to spend your complexity budget and when you should pay for more powerful abstractions.

This talk presents how we use the Eff monad at Zalando to structure a REST microservice. Even when programming with functions, modules and components still matter, and I will present a new approach for dependency injection using the Reader monad (classic) and tree rewriting (what?). Then we will see how to specify more precisely several components (authentication, request to other services and database access) with the Eff monad and support additional concerns like logging or asynchronous execution.

Once you get the basic idea, the possibilities are endless! Rate limiting is just another component and another set of effects involving time and state, testing becomes really clean and easy.

Monad transformers are a neat way to compose effectful computations: pass some configuration, read/write from disk, track errors... However they can be a bit cumbersome to deal with, especially if you want to use different stacks sharing some common effects: for example a stack to interact with AWS and another one for Hadoop computations.

This talk will shortly introduce monad transformers in Scala and their drawbacks then present the Eff monad.

The Eff monad has some really interesting properties:

• effects can be developed completely independently from each other
• effects can be interpreted in different ways and the interpreters can be composed together
• it is based on an efficient "Free monad" (avoiding the issue of quadratic flatMaps)
• with the Eff monad you will never have to re-implement the Monad operations (point / flatMap) again!