Essential F#
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Essential F#

About the Book

This is a practical book aimed at efficiently getting you up to speed with the essentials of functional-first programming in F#. 

Everywhere you look, programming languages are gaining functional features. The problem is that it's not the individual features that make functional programmers happy, it's the way that your approach to writing software and the features work together to help you write simple code to solve interesting problems. This book will help you to discover why F# is such a popular language with those who have spent time learning its secrets. 

This book started life as a series of blog posts I wrote for the Trustbit website ( The sum of the content is the core of what a developer needs to know to be able to work on one of their F# Line of Business codebases. I have improved and extended the content, upgraded the code to F# 5, and ensured that the code would run in VS Code with the ionide extension.

This book is FREE. If someone feels that they want to pay money for it, the author's royalties (80%) will continue to go to the F# Software Foundation.

You can report issues with this book to

About the Author

Ian Russell
Ian Russell

My name is Ian and I’ve been writing software for over 25 years, most of it in the .NET (Core) space. I have been using C# since 2003 and F# since 2012. I have been a regular presenter at .NET User Groups and community conferences around the UK, mainly talking about F#, Functional Programming, Domain-Driven Design, and many techniques and practices for writing better software.

I’m a qualified Teacher of Mathematics who decided in my final year of training that I preferred computers to children. :)

Many of my previous F# blog posts can be found on the Trustbit blog (

I’m currently learning Haskell, a pure functional programming language.

Reader Testimonials


I've been using F# for some time, mainly to automate tasks, so I have a bit of experience with the language. Your teaching approach is helping me connect the dots and get a better understanding of the concepts.



One of the essential cornerstones of #fsharp, freshly redone. Congrats @ijrussell, and enjoy it, future-f#-devs!


It's ok, I bought Ian's book and have still not read it either. Just knowing I have it makes my F# magically better.


Ian is a tremendous writer and does a great job in approaching #fsharp both in finding commonalities with other technologies as well as clearly showing where the language effectively stands apart. Highly recommended! I appreciate the conversational style -- #fsharp as an ecosystem is blessed with a broad set of explainers - each with their own perspective. The language and is deep & wide enough that it deserves expansive treatment. I feel like your book has helped me with fresh approaches.


Genetical Engineer

This is an excellent intro into a beautiful programming language.


This got me started on my F# journey, and now there's no going back!


Really an enjoyable book to read :)


Book is quite good :)

Table of Contents

  • Preface
    • Contents
    • F# Software Foundation
    • About the author
  • Acknowledgments
  • Getting Started
    • Setting up your environment
    • One Last Thing
  • 1 - An Introductory Domain Modelling Exercise
    • The Problem
    • Getting Started
    • Making the Implicit Explicit
    • Going Further
    • Alternative Approaches (1 of 2)
    • Alternative Approaches (2 of 2)
    • Summary
    • Postscript
  • 2 - Functions
    • Getting Started
    • Theory
    • In Practice
    • Unit
    • Anonymous Functions
    • Multiple Parameters
    • Partial Application (Part 1)
    • The Forward Pipe Operator
    • Partial Application (Part 2)
    • Summary
  • 3 - Null and Exception Handling
    • Null Handling
    • Interop With .NET
    • Handling Exceptions
    • Function Composition With Result
    • Summary
  • 4 - Organising Code and Testing
    • Getting Started
    • Solutions and Projects
    • Adding a Source File
    • Namespaces and Modules
    • Writing Tests
    • Writing Real Tests
    • Using FsUnit for Assertions
    • Summary
  • 5 - Introduction to Collections
    • Before We Start
    • The Basics
    • Core Functionality
    • Folding
    • Grouping Data and Uniqueness
    • Solving a Problem in Many Ways
    • Working Through a Practical Example
    • Summary
  • 6 - Reading Data From a File
    • Setting Up
    • Loading Data
    • Parsing Data
    • Testing the Code
    • Final Code
    • Summary
  • 7 - Active Patterns
    • Setting Up
    • Partial Active Patterns
    • Parameterized Partial Active Patterns
    • Multi-Case Active Patterns
    • Single-Case Active Patterns
    • Using Active Patterns in a Practical Example
    • Summary
  • 8 - Functional Validation
    • Setting Up
    • Solving the Problem
    • Where are we now?
    • Functional Validation the F# Way
    • Summary
  • 9 - Single-Case Discriminated Union
    • Setting Up
    • Solving the Problem
    • Using Modules
    • A Few Minor Improvements
    • Using a Record Type
    • Summary
  • 10 - Object Programming
    • Setting Up
    • Class Types
    • Interfaces
    • Object Expressions
    • Encapsulation
    • Equality
    • Summary
  • 11 - Recursion
    • Setting Up
    • Solving The Problem
    • Tail Call Optimisation
    • Expanding the Accumulator
    • Using Recursion to Solve FizzBuzz
    • Quicksort using recursion
    • Recursion with Hierarchical Data
    • Other Uses Of Recursion
    • Summary
  • 12 - Computation Expressions
    • Setting Up
    • Introduction
    • The Result Computation Expression
    • Introduction to Async
    • Compound Computation Expressions
    • Debugging Code
    • Further Reading
    • Summary
  • 13 - Introduction to Web Programming with Giraffe
    • Getting Started
    • Using Giraffe
    • Creating an API Route
    • Creating a Custom HttpHandler
    • Creating a View
    • Adding Subroutes
    • Reviewing the Code
    • Summary
  • 14 - Creating an API with Giraffe
    • Getting Started
    • Our Task
    • Sample Data
    • Routes
    • Handlers
    • Using the API
    • Summary
  • 15 - Creating Web Pages with Giraffe
    • Getting Started
    • Configuration
    • Adding a Master Page
    • Creating the Todo List View
    • Loading Data on Startup
    • Summary
  • Summary
    • F# Software Foundation
    • Resources
    • And Finally
  • Appendix 1
    • Creating Solutions and Projects in VS Code
  • Appendix 2
    • CSS and JavaScript for Chapter 15

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