¶ ReasonML
¶ Programming Languages
ReasonML has always had a curious way of entering a developer’s world. Some find it through React, others through BuckleScript, some through OCaml, and a growing number through the search for a language that brings sanity, predictability, and a sense of calm back into frontend development. ReasonML doesn’t arrive with the fanfare of trendy languages. Instead, it sneaks up on you, quietly promising something modern and familiar, yet deeply grounded in decades of solid language design. It offers type safety without headache, functional programming without ceremony, and a syntax that feels comfortable even to someone who has spent most of their career in JavaScript.
This course of 100 articles is meant to give you a wide and deep understanding of ReasonML—its philosophy, its abilities, its strengths, its habits, and the mindset it encourages. ReasonML is one of those languages that becomes more enjoyable the longer you stay with it. At first glance, it feels clean and pleasant. Then, as you explore deeper, you start noticing how thoughtfully everything fits together. The types aren’t just strict; they’re expressive. The syntax isn’t just pretty; it’s practical. The ecosystem isn’t just functional; it’s purposeful. And the more you use ReasonML, the more you begin to understand why developers who appreciate correctness and clarity gravitate toward it.
One of the reasons ReasonML feels refreshing is because it’s built on the rock-solid foundations of OCaml. OCaml has been around for decades, quietly powering compilers, financial systems, formal verification tools, and performance-critical infrastructure. It’s a language known for its reliability, its expressive type system, and its impressive speed. But OCaml’s syntax can feel foreign to newcomers from the JavaScript ecosystem, and that’s where ReasonML comes in. ReasonML reshapes the surface of OCaml into something more familiar—brackets, curly braces, readable patterns, and a style that feels friendlier to frontend developers. Underneath that friendlier face, however, lies the full power of OCaml’s type system and compiler.
It’s this blend—familiar surface, powerful core—that has made ReasonML so appealing to developers looking for something better than the chaos of loosely typed JavaScript or the complexity creep of many modern UI frameworks. ReasonML gives you the ability to write predictable, maintainable, safe code without sacrificing the expressiveness that makes day-to-day development feel creative. And because it compiles so efficiently to JavaScript, it offers a way to build large applications without the fear that hidden runtime errors will make their way into production.
One of the threads running through the entire ReasonML experience is the feeling that the language wants to help you. It nudges you toward clarity. It nudges you away from ambiguity. It encourages you to think about the shape of your data and the flow of your logic. And yet, it never feels heavy-handed. It doesn’t make you fight the type system. It doesn’t punish you for mistakes. Instead, it guides you with compiler messages that are surprisingly friendly, giving you clear hints about what’s wrong and how to fix it. Many developers describe the ReasonML compiler as one of the best teachers they’ve ever had.
This is one of the reasons the course you’re about to start takes a patient, explorative approach. ReasonML is not simply a language to learn; it’s a way of thinking. It’s a mindset shift toward correctness, expression, and clarity. Over the next hundred articles, you’ll see how ReasonML brings out the best habits in a developer—habits that stay with you even when you switch languages. You’ll understand why pattern matching becomes addictive, why variant types feel like a superpower, why immutable data structures reduce mental overhead, and why writing code with strong correctness guarantees brings a kind of confidence you don’t want to lose once you’ve had it.
ReasonML is often introduced as a safer, more expressive alternative to JavaScript, but it deserves to be understood in its own right. It’s not just a tool to avoid bugs. It’s a language shaped with elegance and intention. It lets you model your ideas in a way that feels direct and uncluttered. When you write Reason code, you often feel like the language is sitting beside you, quietly encouraging you not just to make the code work, but to make it right. This is the kind of experience that’s hard to describe without trying it for yourself, and it’s one of the reasons developers who fall in love with ReasonML tend to become passionate advocates.
Another remarkable quality of ReasonML is how it scales. Many languages are pleasant at small sizes and messy at large ones. Others excel at large-scale systems but feel burdensome for small scripts or prototypes. ReasonML manages both worlds gracefully. You can write a tiny experiment with just a few lines of code, or you can architect a full-scale application with complex data models, multiple modules, and long-term maintainability. The consistency of the language and its strong type system make large projects surprisingly easy to navigate. And because the compiler catches so many potential issues, refactoring becomes far less stressful. This is one of those hidden joys Reason developers talk about—you can reorganize major parts of your codebase and trust the compiler to walk with you, pointing out any place where your changes need attention.
This course aims to help you discover that confidence. Whether you’re coming to ReasonML from JavaScript, TypeScript, Rust, Haskell, or somewhere else entirely, the journey you’re about to begin will give you a thorough, grounded understanding of the language’s design and practical usage. You’ll learn how Reason thinks about types, how it handles functions and modules, how it approaches error-handling, how it describes data, and how it encourages patterns that are both expressive and safe. You’ll see how ReasonML’s strengths shine most clearly when you embrace the idea that types are not limits—they’re tools for expression.
Throughout the series, you’ll also explore ReasonML’s relationship with frameworks, build systems, and broader development ecosystems. ReasonML is not only a language; it’s part of a larger landscape that includes ReScript, BuckleScript (its original JavaScript backend), native compilation through OCaml, and a growing set of tools that make Reason a compelling choice for frontend as well as backend projects. You’ll see how ReasonML can integrate into modern workflows, how it interacts with JavaScript libraries, and how you can adopt it incrementally within existing codebases. This flexibility is one of its best qualities: you don’t have to rewrite everything to enjoy the benefits of ReasonML. You can start small and grow naturally.
One thing that often surprises developers is how creative ReasonML feels. Functional languages often carry a reputation for being academic or rigid, but ReasonML is playful. Its syntax encourages expressiveness. Its variants let you model your domain with elegance. Its pattern matching lets you describe data transformations in a way that feels both natural and powerful. When you write Reason code, you often end up with something that reads more like a clear description of intent than a set of mechanically composed statements. This expressiveness is one of the reasons many developers feel so at ease with Reason once they get the hang of it.
Another important part of the ReasonML experience is its approach to safety. Safety, in this context, doesn’t mean slowing you down or preventing creativity. It means giving you a foundation where you can build confidently. With ReasonML, whole categories of bugs simply disappear. You stop worrying about undefined or null showing up unexpectedly. You stop worrying about properties that may or may not exist. You stop worrying about fragile type assumptions scattered throughout a codebase. Instead, you work in a system where the shape of your data and the flow of your logic are consistently and predictably enforced. This doesn’t eliminate all problems, but it removes many of the most frustrating ones.
As you progress through these hundred articles, you’ll also explore advanced aspects of the language—areas where ReasonML reveals its depth and subtlety. You’ll see how modules and functors enable scalable architecture. You’ll learn how to design type-driven APIs. You’ll experiment with ReasonML’s powerful variant system to model complex states with clarity. And you’ll gain an appreciation for Reason’s ability to combine functional purity with practical efficiency in a way that feels grounded rather than theoretical.
You’ll also develop a deeper understanding of how ReasonML compiles to JavaScript, what the generated output looks like, how it behaves, and why the compilation pipeline is one of the major reasons ReasonML applications perform well in production. You might even explore ReasonML’s native compilation capabilities if you’re curious about using Reason beyond the browser. The language is versatile enough to live comfortably in multiple domains, and its strong type system keeps it consistent across all of them.
Most importantly, you’ll come to understand the spirit of ReasonML. Every language has a personality, and ReasonML’s personality is thoughtful, clear-headed, and quietly confident. It doesn’t shout. It doesn’t try to impress with overly clever features. It offers a mature foundation paired with a modern feel, resulting in an experience that is both calming and empowering. Many developers describe ReasonML as a language that rekindles their enjoyment of programming, stripping away unnecessary friction and leaving behind the essential joy of expressing ideas through code.
This introduction is simply the first step. Ahead of you lies a rich journey—a hundred articles designed to help you not only learn ReasonML, but internalize its philosophy and embrace its strengths. By the end, ReasonML will no longer feel like a new language you’re studying; it will feel like a natural extension of how you think about programming. You’ll have the confidence to build real applications, the understanding to design thoughtful abstractions, and the fluency to write expressive code that feels right.
ReasonML has a way of reshaping how developers think about software. It shows that safety can feel liberating, that clarity can feel expressive, and that the right abstractions can make programming more enjoyable. This course will guide you through every layer of that experience, helping you build a strong, intuitive relationship with the language.
Welcome to the journey. ReasonML has a lot to offer, and you’re about to discover just how elegant and empowering a language can be when it’s designed with intention, informed by decades of research, and shaped with real developer happiness in mind. Enjoy every step—you’re in for a rewarding experience.
¶ ReasonML
¶ Beginner Level
1. Introduction to ReasonML: What is Reason and Why Use It?
2. Setting Up Your Development Environment for ReasonML
3. Your First ReasonML Program: "Hello, World!"
4. Understanding ReasonML’s Syntax and Structure
5. Variables and Constants in ReasonML
6. Basic Data Types in ReasonML: Numbers, Strings, and Booleans
7. Working with Tuples in ReasonML
8. Working with Arrays in ReasonML
9. Lists in ReasonML: Immutable Data Structures
10. Records in ReasonML: Creating and Using Structured Data
11. Basic Operators in ReasonML: Arithmetic and Logical
12. Defining Functions in ReasonML
13. Using Functions: Parameters, Return Values, and Recursion
14. Control Flow: if, else, and switch Statements
15. Loops in ReasonML: for, while, and recurse
16. Pattern Matching in ReasonML
17. Basic String Manipulations in ReasonML
18. Error Handling in ReasonML: Using Result and Option
19. Working with Immutable Data in ReasonML
20. Modules and Namespaces in ReasonML
¶ Intermediate Level
21. Type System in ReasonML: Type Inference and Type Annotations
22. Using Variants in ReasonML for More Flexible Data Structures
23. Record and Variant Types in ReasonML
24. Working with Functions in ReasonML: Higher-Order Functions
25. Creating and Using Recursive Types in ReasonML
26. Understanding Type Aliases in ReasonML
27. Using ref and Mutable Data in ReasonML
28. Currying and Partial Application in ReasonML
29. Working with the option and result Types for Safe Operations
30. Interoperability Between JavaScript and ReasonML
31. ReasonReact: An Introduction to React in ReasonML
32. Using JSX in ReasonML with ReasonReact
33. Creating Components in ReasonReact
34. Handling Events in ReasonReact
35. State Management in ReasonReact
36. Working with Hooks in ReasonReact
37. Basic Routing in ReasonReact
38. Working with Forms in ReasonReact
39. Managing Side Effects in ReasonReact
40. Using Pattern Matching with Components in ReasonReact
41. ReasonML and Async Programming: Working with Promises
42. Introduction to Functional Programming in ReasonML
43. Working with Closures and Lexical Scoping
44. Data Flow in Functional Programs with ReasonML
45. Creating and Using Custom Modules in ReasonML
46. Testing in ReasonML: Writing Unit Tests
47. Debugging ReasonML Code with Visual Studio Code
48. Understanding and Using Type Classes in ReasonML
49. Functional Programming Patterns in ReasonML
50. Immutable Data Structures: Lists, Arrays, and Records in ReasonML
¶ Advanced Level
51. Advanced Type System Features in ReasonML: Polymorphism
52. Type Variance in ReasonML: Covariant and Contravariant Types
53. Advanced Pattern Matching: Exhaustiveness and Guards
54. Creating and Using Abstract Data Types in ReasonML
55. Building a Type-safe Web API with ReasonML
56. Integrating ReasonML with GraphQL
57. Error Handling Strategies in Complex ReasonML Applications
58. Creating and Managing Libraries in ReasonML
59. Designing Large-Scale Applications in ReasonML
60. Integrating ReasonML with OCaml Libraries
61. Concurrency and Parallelism in ReasonML
62. Managing Asynchronous Operations with async and await in ReasonML
63. Using External JavaScript Libraries with ReasonML
64. ReasonML and WebAssembly: Running Reason on the Web
65. Optimizing ReasonML Code for Performance
66. Advanced Functional Programming Techniques in ReasonML
67. Creating Custom Operators in ReasonML
68. Building Custom Middleware in ReasonReact
69. Integrating ReasonML with Backend Frameworks
70. Understanding and Using Lenses in ReasonML
71. Using Monads and Applicatives in ReasonML
72. Domain-Specific Languages (DSLs) in ReasonML
73. Building a Static Site Generator with ReasonML
74. Creating a Compiler with ReasonML
75. Advanced Interfacing with JavaScript: FFI (Foreign Function Interface)
76. Working with Streams and Event Loops in ReasonML
77. Using Type-Level Programming in ReasonML
78. Memory Management in ReasonML: Optimizing for Low Memory Consumption
79. Building and Managing Real-Time Applications in ReasonML
80. ReasonML for Data Science: Integrating with Python and R
81. Deploying ReasonML Applications to the Web
82. Creating a Native Mobile App with ReasonML and React Native
83. Building an End-to-End Application with ReasonML and ReasonReact
84. Designing a REST API in ReasonML
85. Error Boundaries and Exception Handling in ReasonReact
86. Automating Build and Deployment Pipelines for ReasonML
87. Exploring Advanced Compiler Techniques with ReasonML
88. Exploring the ReasonML Ecosystem: Tools, Libraries, and Frameworks
89. Building a Complex UI with ReasonReact
90. Testing in ReasonML: End-to-End Testing Strategies
91. Creating a Server-Side Rendered App with ReasonReact
92. WebSockets and Real-Time Communication in ReasonML
93. Using ReScript with ReasonML for JavaScript Interoperability
94. Refactoring and Maintaining Large Codebases in ReasonML
95. Introduction to Property-Based Testing in ReasonML
96. Working with Typed Arrays in ReasonML
97. Advanced Usage of ReasonReact: Context, Reducers, and More
98. Exploring the Functional Reactive Programming (FRP) Paradigm in ReasonML
99. Building a Full-Stack Application with ReasonML and ReasonReact
100. The Future of ReasonML: Trends, Ecosystem, and New Features