ReScript Basics

This page helps you get a ReScript project up and running if you’re new to ReScript.

Tip

For comprehensive ReScript documentation, visit the official ReScript docs.

Setting Up a New Project

Option 1: Using the Project Wizard

1. File → New → Project

2. Select ReScript from the generator list

3. Fill in the project name and location

4. Click Create

The wizard generates a basic project structure with rescript.json and source files.

Option 2: Manual Setup

1. Create a new directory for your project

2. Initialize with npm:

mkdir my-rescript-project
cd my-rescript-project
npm init -y

3. Install ReScript:

npm install rescript @rescript/core

4. Install the Language Server:

npm install @rescript/language-server

5. Create rescript.json:

{
  "name": "my-rescript-project",
  "sources": [
    { "dir": "src", "subdirs": true }
  ],
  "package-specs": [
    { "module": "esmodule", "in-source": true }
  ],
  "bs-dependencies": ["@rescript/core"]
}

6. Create src/ directory and your first .res file:

mkdir src

// src/Main.res
let greeting = "Hello from ReScript!"
Console.log(greeting)

7. Build:

npx rescript build

Project Structure

A typical ReScript project looks like:

my-project/
├── rescript.json          # ReScript configuration
├── package.json           # npm dependencies
├── node_modules/
│   ├── rescript/          # ReScript compiler
│   └── @rescript/
│       ├── core/          # Standard library
│       └── language-server/  # LSP server
├── src/
│   ├── Main.res           # Source files
│   └── Utils.res
└── lib/                   # Compiled output (auto-generated)

Key Files

rescript.json

The main configuration file for your ReScript project. The plugin provides JSON Schema validation and auto-completion for this file.

Key fields:

• name — Package name

• sources — Source directories to compile

• bs-dependencies — ReScript package dependencies

• package-specs — Output format (commonjs or esmodule)

.res Files

ReScript source files. Each .res file defines a module with the same name as the file.

.resi Files

ReScript interface files. They define the public API of the corresponding .res module. Use Alt+O to switch between .res and .resi files.

Basic Syntax Quick Reference

// Variables
let name = "ReScript"
let age = 25
let isReady = true

// Functions
let add = (a, b) => a + b
let greet = (name) => `Hello, ${name}!`

// Types
type color = Red | Green | Blue
type user = {name: string, age: int}

// Pattern matching
let describe = (color) =>
  switch color {
  | Red => "red"
  | Green => "green"
  | Blue => "blue"
  }

// Modules
module Math = {
  let pi = 3.14159
  let square = (x) => x *. x
}

// Pipe operator
let result = [1, 2, 3]->Array.map(x => x * 2)->Array.filter(x => x > 2)