The "free monad" design pattern in Scala: how it works and what advantages it brings

The main topic of this presentation is to illustrate the use of the "free monad" as a design pattern. The free monad can replace imperative code by declarative data structures that are composable and can be "run" or "interpreted" later.

This session shows a straightforward implementation of the free monad and explains how it works in more detail. I also show how to use the `Free` type constructor as implemented in the `cats` library.

Contents:

How to transform an imperative API into a sequence of instructions
Specific example: a subset of API for Apache HDFS file system client (create / read / write / delete files)
Using case classes and a trait with a type constructor `HdfsOps[A]` to represent an imperative API
Showing why the result is not a monad and not even a functor
Motivating a monad (rather than, say, a `List`) as the data structure for a sequence of instructions
Defining the `pure` and the `flatMap` methods on a free monad
Transforming the free monad into an arbitrary target monad `M`, generically
Examples of "interpreters" for the `HdfsOps` free monad: one interpreter for `Writer` monad, another for the `Try` monad
Doing the same thing with the `cats` library
Final example: Encoding some Akka-Http routing directives as a free monad, and what advantages this could bring

Source code: https://github.com/winitzki/scala-examples/blob/master/chapter08/src/test/scala/example/FreeMonadPresentation.scala
https://github.com/winitzki/scala-examples/blob/master/chapter08/src/test/scala/example/AkkaHttpFreeMonad.scala