Overview

This course provides an introduction to Functional Programming concepts in Kotlin using the Arrow library.

Prerequisites

One year of experience using Kotlin or similar Functional Programming-oriented languages, coroutines, structured concurrency, coroutine scopes, dispatchers, cancellation, suspend, Flow.

What you'll learn

In this intensive two-day course, we'll cover Functional Programming essentials in Kotlin. By the end of this course, you will have a solid grasp on the Fundamentals of the language.

Lesson 1 - Functional Programming

Functional programming fundamentals.​

• What is it, what is it good for.
• Pure functions.
• Side effects.
• Referential transparency & substitution model.
• Local reasoning.
• Is Kotlin a functional language?
• Referential transparency & substitution model.
• Higher order functions.
• Deferring execution.
• Exercises.

Lesson 2 - Domain Modeling

Modeling our domain from a functional perspective.

• Kotlin type system. Leveraging compiler aid via types.
• Modeling concerns and making them explicit with types.
• Unit and Nothing to model meaningful scenarios.
• Inline classes for type safety and preciseness.
• Algebraic Data Types: Product and Sum types.
• Complexity of ADTs.
• Modeling choice and control flow with sum types.
• Arrow data types as ADTs.
• Eliminating bugs and tests for invariants via type safety.
• Exercises.

Lesson 3 - Immutable Data and Optics

Optimizing reusability and composition for immutable data structures using Optics.

• Immutability.
• Issues with Kotlin data class copy.
• Optics for working efficiently with nested immutable data.
• Optics composition and reusability.
• The Optics DSL.
• Stacking optics to create more powerful ones 🔭
• Lens for reading / modifying nested data.
• Prism for conditional reading / modification.
• Optional as the composition of Lens and Prism.
• Every for working over lists of elements.
• FilterIndex for working over lists of elements based on a predicate.
• Index for reading or modifying elements obtained by their index.
• Iso for isomorphic conversions.
• Exercises.

Lesson 4 - Domain Modeling II - Error Handling

Learning how to model and handle errors in functional programs.

• Railway oriented programming 🚂 - Happy path vs alternative path.
• Making errors vs success explicit with types.
• The Either datatype.
• Exhaustive evaluation.
• Either composition and computational blocks (bindings).
• Either bias towards the happy case.
• Capturing third party library errors.
• Mapping third party errors to strongly typed domain errors.
• Error accumulation.
• Validated and ValidatedNel.
• Either and Validated, when to use each? Fail fast vs error accumulation.
• Either and Validated interoperability.
• Flattening lists of Either or Validated.
• Either or Validated for domain model validation.
• Modeling potential absence of a value.
• Exercises.

Lesson 5 - Functional Effects

Keeping side effects under control.

• Recap on side effects.
• Suspend for tracking effects at compile time.
• The benefits of modeling effects with suspend.
• Making effect declaration pure.
• Concern separation - pure logics vs runtime.
• Internals of suspend - Continuation passing style.
• The impact of suspend in architecture.
• Leveraging Either and Validated in suspended contexts.
• Arrow integration with KotlinX Coroutines.
• Structured concurrency & cancellation via KotlinX Coroutines.
• Providing a runtime for our suspended program.
• Functional effect operators via Arrow Fx.
• Parallel computations, races, repeating and retrying suspended effects.
• Handling errors in suspended contexts.
• Exercises.

Lesson 6 - Safe Resource Handling

Functional operators to work safely with resources across boundaries.

• What is a resource?.
• The issues of working with resources in concurrent contexts.
• Why try / catch is not enough.
• Acquiring, using, and releasing a resource.
• Abstraction as a means to leverage composability.
• Ensuring resource disposal to avoid leaks.
• bracket and bracketCase operators.
• guarantee and guaranteeCase operators.
• Limitations of bracket and bracketCase.
• Resource data type.
• Resource and bracket to work with third party libraries.
• Exercises.

Lesson 7 - Functional Streams

Streams able to embed and evaluate functional effects.

• The need for streams.
• The need for suspend support in streams.
• Streams in Kotlin via Flow data type.
• Modeling functional effects within a Flow.
• Flows are cold.
• Consuming functional streams safely - suspend terminal operators.
• Flow integration with Arrow Fx operators.
• Cancellation of streams with functional effects via structured concurrency.
• Sneak peek into Flow builders and operators.
• Composing Flows together.
• Error path vs happy path in the context of a Flow.
• Retrying Flows.
• Exercises.

Lesson 8 - Functional Architecture

How to organize and structure functional programs.

• Concern separation from an architecture perspective.
• Keeping architecture pure edge to edge.
• The concept of algebras.
• Isolating side effects, pushing them to the edges.
• Dependency injection in functional codebases.
• Replacing side effects with test doubles.
• The importance of pure functions and determinism for testing.
• The importance of a precise self validated domain for testing.
• Removing tests for invariants with proper types.
• Testing functional codebases.
• Sneak peek into Property Based Testing (PBT) with Kotest.
• Generating input data for our tests.
• Failure shrinking to track down failing cases.
• Detecting edge cases thanks to PBT.
• Exercises.