Workshop on Effectful Functional Programming with Eff/Monix/Monocle/Cats
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.

Getting Work Done With Effectful Functional Programming

A workshop on programming with Effectful Functional Programming. This can be described as a style of pure functional programming emphasizing the use of effects. It readily handles the complex or messy programming problems that are often encountered in industry.

It has a close connection to the Monad and Applicative type classes, as they are how we build large effectful programs out of small effectful programs.

What are Effects?

So what's a "effect" then? Like many abstract concepts, it is best grasped by seeing many examples, as we'll do in this workshop. But here's a brief overview:

  • Recall that functional programs can only act upon the world through the values they compute.
  • Effectful function programs compute effectful functional values, which have the form F[A].
  • The type A in F[A] represents the pure payload value the program computes.
  • The type that wraps around it, F[_], represents the effects that the program must resolve in order to, or additional to, yielding the A payload.

It turns out that a huge variety and complexity of different program behaviours can be represented as an effectful value F[A].

Expand diagram: Effectful Functional Programming - a visual intuition



The workshop consists of a series of practical exercises using the following open source libraries:

Use Case

Each exercise is an alternate implementation of the same use case:

Ever had a full disk? Where does the space go? Implement a program that can find the largest N files in a directory tree


  • Wifi/Internet required.

  • You will need Java 8+ and Simple Build Tool (sbt) installed.

  • While SBT will download Scala and the Eff libraries on-demand, this can be a slow process. Before the workshop, it is recommended to run sbt update in the base directory to pre-download the required libraries. This may take a few minutes up to 1 hour, depending what you have cached locally in ~/.ivy2/cache.

  • Import the base SBT project into your IDE: Intellij, Eclipse ScalaIDE or Ensime.

  • Or work with any editor and the SBT command line if you prefer.

    Be warned that IDE presentation compilers don't correctly handle some Eff code, and may flag valid code as invalid. Try your code with the full Scala compiler via SBT command line before concluding there is a problem.


The SBT base project contains nine exercise projects, each with a README with instructions to attempt. Each of them contains a different implementation of a file scanner program.

It is suggested to do the exercises in this order. The instruction pages are best viewed in a browser; reach them here:

  • Classic - File Scanning in a classic Scala style
  • Task effect - Using Monix task effect to defer execution
  • Reader effect - Using Reader effect for dependency injection and abstracting the environment
  • Error effect - Using Either effect for error handling
  • Writer effect - Using Writer effect for logging
  • State effect - Using State effect to keep track of Symlinks encountered
  • Concurrency - Scanning directories in parallel with applicative traversal
  • Optics - Using Optics to change the focus of a Reader effect
  • Custom Effects - Using a custom Filesystem effect

There are three types of tasks you'll encounter

  • 🔍 Study Code Study existing application and test code
  • 📝 Write Code Adding missing code or changing existing code at an indicated line or method.
  • ▶️ Run Code Run the file scanner (eg exercise1/run) or the unit tests (eg exercise1/test) from SBT prompt.

Each project can be compiled, run or tested separately; errors in one project won't affect the others.

*Initially, most exercises will not compile and/or run, until you complete the specified tasks. To try running the code, go to the corresponding solutions project. *


There is a solutions subfolder containing corresponding solution subprojects.

There is learning value in attempting a hard problem, getting stuck, then reviewing the solution. Use the solutions if you get blocked!

Using SBT

Start SBT in the base directory and then operate from the SBT prompt. Invoking each SBT command from the shell (eg sbt exercise1/compile) is slower due to JVM startup costs.

/Users/ben_hutchison/projects/GettingWorkDoneWithExtensibleEffects $ sbt
Getting org.scala-sbt sbt 0.13.13 ...
..further sbt loading omitted..

To list all exercise- and solution- subproject names:

> projects

Try running the file scanner (ie main method) of subproject solutionExerciseClassic on the current directory.

> solutionExerciseClassic/run .

To compile sources in subproject exercise1:

> exerciseClassic/compile

To run any unit tests (in src/test/scala/*) under subproject exerciseClassic

> exerciseClassic/test

SBT commands should be scoped to a subproject (eg exerciseClassic/test). Running eg test at the top level will load 10 copies of the classes into the SBT JVM, potentially leading to OutOfMemoryError: Metaspace

"Learn by Doing"

This project teaches Extensible Effects in practice; what it feels like to code with the Eff framework.

It doesn't make any attempt to cover the complex, subtle theory behind Eff, a refinement of 25 years experience of programming with monads, and isn't a complete picture of Eff by any means. At the time of writing however, there are more resources available covering the theory, than practice, of Eff, including:

Workshop History

April 2017

  • Initial version based on Eff 4.3.1, cats 0.9.0 and Monix 2.2.4. Includes 5 exercises introducing Reader, Either, Task and Writer effects.

  • Presented at Melbourne Scala meetup

May 2017

April 2018

  • Upgrade libraries to Eff 5.2, Monic 3.0, cats 1.1, sbt 1.1 and introduce Cats Effect 0.10.1 library to use IO effect rather than Task, and Monocle 1.5 optics library.

  • Rewrite existing exercises 1 - 5 to reflect updated libraries, slightly changed emphasis. Add three new exercises covering State, Optics and Custom effects

May 2018