Introduction

This section explores four progressive approaches to handling effectful domain workflows in F#, each building upon the limitations of the previous one. Understanding this evolution will help you choose the right pattern for managing dependencies and side effects in your functional applications.

The Challenge

In object-oriented programming, the building blocks are objects and classes. When a class collaborates with other classes, these collaborators are called dependencies. Managing these dependencies effectively—especially when they involve side effects like database access, HTTP calls, or file I/O—is crucial for building testable, maintainable applications.

Five Approaches

This introduction presents five progressive solutions to the dependency problem:

Approach	Version	Key Concept	Main Benefit	Main Limitation
Dependency Injection	–	Pass dependencies as inputs	Widely adopted, simple	Async leak, runtime configuration
Dependency Interpretation	program V1	Abstract dependencies as data	Testability, separation of what/how	Monolithic instruction type
Free Monad	program V2	Separate instructions by domain	Better organization, domain isolation	All domains still coupled in Program type
Algebraic Effects	program V3	Effect handlers with type safety	Complete domain isolation, vertical slicing	No parallel execution, high boilerplate
Tagless Final	program V4	Async reader with instruction interface	Simplest, parallel execution, undo support	–

Prerequisites

To get the most out of this introduction, you should be familiar with F# basics:

• Functional programming concepts: pure functions, side effects

• Types: discriminated unions, records

• Pattern matching

• Computation expressions—at least async

If not, you can consult my free e-book F# Training.

How to Read This Introduction

Each page builds upon the previous one:

1. Start with Dependency Injection to understand the baseline approach and its limitations.

2. Progress through Dependency Interpretation to see how functional programming addresses these limitations.

3. Explore Free Monad to understand domain separation.

4. Review Algebraic Effects to understand the V3 attempt and its limitations.

5. Conclude with Tagless Final to see the current, simplest approach (V4) that powers the rest of this guide.

You can also jump directly to a specific approach if you're already familiar with the earlier patterns.