Docs / GenType / Introduction


genType is a code generation tool that lets you export ReScript values and types to use in TypeScript (TS), and import TS values and types into ReScript.

Converter functions between the two runtime representations are generated when required based on the type of the values. In particular, conversion of rescript-react components both ways is supported, with automatic generation of the wrappers.

Here's an article describing how to use genType as part of a migration strategy where a tree of components is gradually converted to ReScript bottom-up (old article containing Reason / BuckleScript): Adopting Reason: strategies, dual sources of truth, and why genType is a big deal.

The implementation of genType performs a type-directed transformation of ReScript programs after compilation. The transformed programs operate on data types idiomatic to JS.

For example, a ReScript function operating on a ReScript variant type t = | A(int) | B(string) (which is represented as custom objects with tags at runtime) is exported to a JS function operating on the corresponding JS object of type { tag: "A"; value: number } | { tag: "B"; value: string }.

A Quick Example

Let's assume we are working on a TypeScript codebase and we want to integrate a single rescript-react component.

We want to be able to import the rescript-react component like any other React component in our existing TS code, but we also want to preserve all the ReScript types in the TS type system (and convert incompatible values if necessary).

That's exactly what genType was made for!

First we'll set up a rescript-react component:

/* src/MyComp.res */ @genType type color = | Red | Blue; @genType @react.component let make = (~name: string, ~color: color) => { let colorStr = switch (color) { | Red => "red" | Blue => "blue" }; <div className={"color-" ++ colorStr}> {React.string(name)} </div>; };

On a successful compile, genType will convert src/MyComp.res to a TS file called src/MyComp.gen.tsx which will look something like this:

// src/MyComp.gen.tsx /* TypeScript file generated from MyComp.res by genType. */ /* eslint-disable import/first */ import * as React from 'react'; const $$toRE818596289: { [key: string]: any } = {"Red": 0, "Blue": 1}; // tslint:disable-next-line:no-var-requires const MyCompBS = require('./'); // tslint:disable-next-line:interface-over-type-literal export type color = "Red" | "Blue"; // tslint:disable-next-line:interface-over-type-literal export type Props = { readonly color: color; readonly name: string }; export const make: React.ComponentType<{ readonly color: color; readonly name: string }> = function MyComp(Arg1: any) { const $props = {color:$$toRE818596289[Arg1.color],}; const result = React.createElement(MyCompBS.make, $props); return result };

genType automatically maps the color variant to TS via a string union type color = "Red" | "Blue", and also provides all the converters to convert between the ReScript & TS representation as well.

Therefore way we can seamlessly use ReScript specific data structures within TS without writing the converter code by hand!

Within our TypeScript application, we can now import and use the React component in the following manner:

// src/App.ts import { make as MyComp } from "./MyComp.gen.tsx"; const App = () => { return (<div> <h1> My Component </h1> <MyComp color="Blue" name="ReScript & TypeScript" /> </div>); };

That's it for our quick example.

For detailed information, head to the Getting Started or Usage section.


Since ReScript v10.1, genType is part of the compiler's GitHub repository.