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Code, exercises, answers, and hints to go along with the book "Functional Programming in Scala"

Join the chat at https://gitter.im/fpinscala/fpinscala

This repository contains exercises, hints, and answers for the book Functional Programming in Scala. Along with the book itself, it's the closest you'll get to having your own private functional programming tutor without actually having one.

There are two main branches in this repository:

Be sure to select the branch which matches the edition of the book you are reading!

Here's how to use this repository:

Each chapter in the book develops a fully working library of functions and data types, built up through a series of exercises and example code given in the book text. The shell of this working library and exercise stubs live in src/main/scala/fpinscala/exercises/<chapter-description>, where <chapter-description> is a package name that corresponds to the chapter title (see below). When you begin working on a chapter, we recommend you open the exercise file(s) for that chapter, and when you encounter exercises, implement them in the exercises file and make sure they work.

If you get stuck on an exercise, let's say exercise 4 in the chapter, you can find hints in answerkey/<chapter-description>/04.hint.md (if no hints are available for a problem, the file will just have a single '-' as its contents) and the answer along with an explanation of the answer and any variations in answerkey/<chapter-description>/04.answer.md. The finished Scala modules, with all answers for each chapter live in src/main/scala/fpinscala/answers/<chapter-description>. Please feel free to submit pull requests for alternate answers, improved hints, and so on, so we can make this repo the very best resource for people working through the book.

Chapter descriptions:

  • Chapter 2: gettingstarted
  • Chapter 3: datastructures
  • Chapter 4: errorhandling
  • Chapter 5: laziness
  • Chapter 6: state
  • Chapter 7: parallelism
  • Chapter 8: testing
  • Chapter 9: parsing
  • Chapter 10: monoids
  • Chapter 11: monads
  • Chapter 12: applicative
  • Chapter 13: iomonad
  • Chapter 14: localeffects
  • Chapter 15: streamingio

Setup build environment

The project is setup to use Scala CLI. First install Scala CLI by following the installation instructions.

You'll also likely want an editor that's aware of Scala syntax. VSCode with the Metals extension works great.


To build the code for the first time, from the root directory of the project (i.e., the directory where this README.md is located):

$ scala-cli compile .

This compiles all exercises and answers. You can also do:

$ scala-cli console .

to get a Scala REPL (prompt scala>) with access to exercises and answers, and then for example:

scala> import fpinscala.exercises.datastructures.List

to import the List data type.

To run the sample programs:

$ scala-cli run .

This gives a list of possible main methods to execute. To run one of them:

$ scala-cli run . --main-class fpinscala.answers.gettingstarted.printAbs

To run a single unit-test, you can specify the fully qualified class name and test name:

$ scala-cli test . -- 'fpinscala.exercises.gettingstarted.GettingStartedSuite.MyProgram.factorial'

To run unit-tests for a package you can do:

$ scala-cli test . -- 'fpinscala.exercises.gettingstarted.*'

To run all unit-tests:

$ scala-cli test .

Note, running all tests will result in failures. As you solve exercises, the tests will start to pass.


Note: an SBT build is also provided.


All code in this repository is MIT-licensed. See the LICENSE file for details.

Have fun, and good luck! Also be sure to check out the community wiki for the chapter notes, links to more reading, and more.

Paul, Rรบnar, and Michael