2 What is ReasonML?

This chapter gives a brief high-level explanation of Facebook’s new programming language, ReasonML.

2.1 What is ReasonML?

ReasonML is a new object-functional programming language created at Facebook. In essence, it is a new C-like syntax for the programming language OCaml. The new syntax is intended to make interoperation with JavaScript and adoption by JavaScript programmers easier. Additionally, it removes idiosyncrasies of OCaml’s syntax. ReasonML also supports JSX (the syntax for HTML templates inside JavaScript used by Facebook’s React framework). Due to ReasonML being based on OCaml, many people use the two names interchangeably. The following diagram shows how ReasonML fits into the OCaml ecosystem.

At the moment, ReasonML’s default compilation target is JavaScript (browsers and Node.js).

This is what ReasonML code looks like:

type color = Red | Green | Blue;

let stringOfColor = (c) =>
  switch (c) {
  | Red => "Red"
  | Green => "Green"
  | Blue => "Blue"
  };

Several things are notable:

• Many elements of the syntax are borrowed from JavaScript. For example:
  • The name switch (OCaml: match)
  • The syntax `(x) => ···` for functions
  • Semicolons
• Other elements are typical for a functional programming language. For example:
  • color is a variant type
  • switch performs pattern matching
• No type annotations were needed (e.g. the parameter c of stringOfColor does not have one).

2.2 The benefits of OCaml

ReasonML’s foundation, OCaml, brings the following benefits:

• It is an established language (created in 1996) that has proven itself in many projects. Facebook itself is using it in several projects (e.g. Flow).

• Its core is a functional programming language with a full-featured type system. But it also supports object-orientation and mutable state.

• It can be compiled to either bytecode, fast native code or JavaScript.

• Compilation to JavaScript is fast. Quoting the blog post “Messenger.com Now 50% Converted to Reason”:

  Full rebuild of the Reason part of the codebase is ~2s (a few hundreds of files), incremental build (the norm) is <100ms on average. The BuckleScript author estimates that the build system should scale to a few hundred thousands files in the current condition.

2.3 Improving OCaml

The ReasonML team also aims to improve the OCaml ecosystem:

• Better tooling (testing, documentation, editor support, etc.).
• Better interoperation with JavaScript. The ReasonML-to-JavaScript compiler is already very fast and produces relatively readable code.
• Better standard library (there is a fair amount of competition in this space in OCaml, without a clear winner).

2.4 Conclusion

ReasonML feels much like what you’d get if you cleaned up JavaScript and turned it into a statically typed functional programming language. I’m ambivalent about JSX in ReasonML – it has pros and cons. I’m glad that ReasonML doesn’t reinvent the wheel and is strictly based on the established OCaml.

OCaml’s pragmatism means that you don’t get some of the more fancy functional features (that, e.g., Haskell has), but it also leads to fast compilation, efficient code and decent error messages.