ReScript Compiler

9.0.2 verified Fri May 01 auth: no javascript

ReScript (bs-platform) is a strongly typed language that compiles to efficient, readable JavaScript. This package provides the compiler, standard library (Stdlib, Belt), and build system (ninja-based). Current latest stable version is 12.2.0 (as of this entry), with v13 in alpha. Release cadence is roughly quarterly, with breaking changes in major versions. Key differentiators vs TypeScript: sound type system, no type errors at runtime, fast compilation, and seamless interop with JavaScript via special syntax and attributes. The package is primarily for tooling and compiler development; most users should use the `rescript` npm package or ReScript via the VSCode extension.

Common errors

error Fatal error: Parmatch.all_record_args ↓

cause Pattern matching on empty dict/record patterns in older versions.

fix

Update to ReScript v12.2.0 or later. If you must stay on an older version, avoid matching empty dict/record patterns.

error Cannot find module 'bs-platform' ↓

cause Trying to require or import 'bs-platform' after it has been deprecated and possibly not installed.

fix

Install the 'rescript' package and use 'require('rescript')' or adjust your build pipeline to use the 'rescript' package instead.

error exponential compilation blowup with large unboxed variants and dict pattern matching ↓

cause A bug in ReScript v12.0.0 that caused exponential time/memory usage.

fix

Update to v12.0.1 or later where this is fixed. If you cannot update, refactor large unboxed variants.

error Intl.Collator.compare: expected int, got float ↓

cause Breaking change in v13.0.0-alpha: the return type changed from int to Ordering.t (float).

fix

Update your code to handle Ordering.t (float) or use the previous version if you depend on int.

Warnings

deprecated The 'bs-platform' package is deprecated. Use the 'rescript' package instead. ↓

fix Run 'npm install rescript' and update your imports and build config to use the 'rescript' package.

breaking Since v13.0.0-alpha, 'bsconfig.json' is no longer supported. Use 'rescript.json'. ↓

fix Rename 'bsconfig.json' to 'rescript.json' and adjust the schema if needed.

breaking Since v13.0.0-alpha, the legacy build system is removed. Only the modern build system is supported. ↓

fix Use 'npx rescript build' instead of 'npx rescript-legacy build'. Ensure your rescript.json is configured for the modern build system.

breaking In v13.0.0-alpha, 'Int.fromString' and 'Float.fromString' use stricter number parsing and no longer accept an explicit radix argument. ↓

fix Remove the radix argument from calls to Int.fromString and Float.fromString. Use parseInt for radix-based parsing.

breaking In v13.0.0-alpha, the deprecated '%external' extension has been removed. ↓

fix Replace '%external' with the appropriate ReScript binding syntax, e.g., using '@external' decorator.

breaking In v12.0.0, the build system changed fundamentally. The 'bsc' and 'bsb' commands are deprecated. ↓

fix Use 'npx rescript build' instead of 'bsc' or 'bsb'. Update your rescript.json to the new schema.

Install

npm install bs-platform

yarn add bs-platform

pnpm add bs-platform

Imports

Belt
wrong
```
const Belt = require('bs-platform').Belt
```
correct
```
import Belt from 'rescript'
```
Since ReScript 9.0, the recommended way is via the 'rescript' package. The 'bs-platform' package is legacy and no longer updated for new ReScript versions.
Js
wrong
```
import * as Js from 'bs-platform'
```
correct
```
open Js;
```
Js module is part of the standard library. Use `open Js;` in ReScript source files. For JavaScript consumers, the module is not exported as a package import.
Intl
wrong
```
import Intl from 'bs-platform'
```
correct
```
open Intl;
```
Intl module is part of the standard library. Use `open Intl;` in ReScript source files. Breaking changes in v13: Intl.Collator.compare returns Ordering.t (float) instead of int.
rescript
wrong
```
npm install bs-platform
```
correct
```
npm install rescript
```
bs-platform is the legacy package name. Since v9, the recommended package is 'rescript'. Install 'rescript' for new projects.

Quickstart

Create a minimal ReScript project, compile a simple script, and run the generated JavaScript.

// Create a new ReScript project
mkdir my-project
cd my-project
npm init -y
npm install rescript

// Create rescript.json with minimal config
cat > rescript.json << 'EOF'
{
  "name": "my-project",
  "version": "0.1.0",
  "sources": {
    "dir": "src",
    "subdirs": true
  },
  "package-specs": {
    "module": "commonjs",
    "in-source": true
  },
  "suffix": ".bs.js",
  "bs-dependencies": []
}
EOF

mkdir src
cat > src/Demo.res << 'EOF'
let greeting = "Hello, ReScript!"
let add = (a: int, b: int) => a + b

let result = add(2, 3)
Js.log(greeting)
Js.log(result)
EOF

// Compile
npx rescript build

// Run the output (src/Demo.bs.js)
node src/Demo.bs.js