forst

README

Forst  

A programming language that helps you move TypeScript backends to Golang.

To accomplish this, Forst supports you in four key ways:

Benefit	How it works
Go ecosystem	Compiles directly to Go. You get full access to the Go package ecosystem, standard library and build tools.
TypeScript integration	Generates native TS definitions and a client directly from your backend source. No need for intermediate languages like GraphQL or Protobuf just for type safety.
Node.js interop	Call existing Node.js code directly from Forst to keep your existing tools and libraries during migration.
Incremental migration	Allows you to move small parts of your code at a time while your existing codebase keeps running, so you can keep shipping during migration.

Why?

Building backend APIs often forces a trade off between runtime performance and developer speed. Maintaining separate schema contracts leads to lots of glue code. Full backend rewrites into more performant languages are often impossible to do safely.

Forst exists to remove that friction. You can migrate incrementally or write completely new backends with native performance and an ergonomic DX by design.

Examples

Forst code builds on Go foundations. Here is how a handler evolves from a basic executable to full interop and type generation.

Hello World

Forst files use the .ft extension and support standard Go source structure.

package main

import "fmt"

func main() {
	fmt.Println("Hello World!")
}

Validation and nominal errors

Constraints on types validate boundary data automatically. You declare structured domain errors with error and check invariants with ensure.

type PlaceOrderInput = {
	stockKeepingUnit: String.Min(1).Max(64),
	quantity:         Int.Min(1).Max(99),
}

error InsufficientStock {
	requested: Int,
	available: Int,
}

func PlaceOrder(in: PlaceOrderInput, available: Int) Result(String, Error) {
	ensure in.quantity is Max(available) or InsufficientStock({
		requested: in.quantity,
		available: available,
	})
	return "ORDER-123"
}

Go interop

Call standard Go packages or existing Go functions directly in Forst source.

import "fmt"

func PlaceOrder(in: PlaceOrderInput, available: Int) Result(String, Error) {
	ensure in.quantity is Max(available) or InsufficientStock({
		requested: in.quantity,
		available: available,
	})
	orderID := fmt.Sprintf("ORDER-%d", 123)
	return orderID
}

Node.js interop

Import existing JavaScript or TypeScript modules into Forst with import node during migration.

import node "./legacy/payment"

func PlaceOrder(in: PlaceOrderInput, available: Int) Result(String, Error) {
	ensure in.quantity is Max(available) or InsufficientStock({
		requested: in.quantity,
		available: available,
	})
	receipt := payment.Charge({ amount: in.quantity * 10 })
	return receipt.id
}

TypeScript client generation

Run forst generate to create typed client packages for your frontend. Callers receive end to end type safety without writing schema glue code.

import { $orders } from "@forst/gen/orders";

const order = await $orders.PlaceOrder({
  stockKeepingUnit: "SKU-99",
  quantity: 2,
});

Effect TS interop

Enable Effect mode in ftconfig.json to emit native Effect services and layers.

import { Effect } from "effect";
import { $orders } from "@forst/gen/orders";

const program = Effect.gen(function* () {
  const order = yield* $orders.PlaceOrder({
    stockKeepingUnit: "SKU-99",
    quantity: 2,
  });
  return order;
}).pipe(Effect.provide($orders.Default));

await Effect.runPromise(program);

Features

Highlights from the docs feature comparison.

Language

Capability	Forst
Structural typing	Built-in records, signatures, and is narrowing
Validation on types	Built-in field constraints; boundary runtime checks
Error handling	ensure, nominal error types, and Result; no exceptions
Mocking and DI	use / with providers; no external DI framework
Type narrowing	is / ensure and type guards
Goroutines	Native go and defer via Go output

Interop and adoption

Capability	Forst
Go module ecosystem	Import Go packages natively
JS / npm ecosystem	Call legacy JS/TS via import node
Shared server ↔ client types	forst generate from the same .ft source
Call backend from Node	Generated client and HTTP invoke
Incremental migration	Mix .ft, .go, and legacy code in one codebase

See ROADMAP.md for experimental features and planned work.

Design Philosophy

See also PHILOSOPHY.md for what guides and motivates us.

Install and tooling

npm packages

Package	Purpose
@forst/cli	Forst compiler in JS/TS projects
@forst/sidecar	Dev server and HTTP client during migration

Install the compiler in a Node project:

npm i -D @forst/cli
npx forst version

@forst/cli pulls the matching native binary from GitHub Releases.

VS Code extension

Optional extension in packages/vscode-forst.

• Registers .ft files in the editor
• Diagnostics via the compiler HTTP LSP (forst lsp)

See packages/vscode-forst/README.md for installation and troubleshooting.

Linux (.deb)

On Debian or Ubuntu, install the compiler from GitHub Releases (pick amd64 or arm64):

wget https://github.com/forst-lang/forst/releases/download/vX.Y.Z/forst_X.Y.Z-1_amd64.deb
sudo apt install ./forst_X.Y.Z-1_amd64.deb
forst version

See docs/installation.mdx for other install paths (npm, native binary, Docker).

TypeScript client output

You can generate TypeScript types and a small client from your Forst code so front ends or Node callers get the same shapes your server uses, without copying types by hand.

Run forst generate with a .ft file or a folder of .ft files; it writes a generated/ tree (declarations plus helpers) and a client/ stub you can wire to your app. The dev server can also expose types over HTTP while you iterate.

Inspirations

Our primary inspiration is TypeScript's structural type system and its enormous success in making JavaScript development more ergonomic, robust and gradually typeable. We aim to bring similar benefits to Go development, insofar as they are not already present.

We also draw inspiration from:

• Zod — constraints and shape guards as composable runtime checks on nested data.
• tRPC — one source of truth for API shapes, with TypeScript types and a small client generated from Forst (forst generate, examples/client-integration/).
• Go and Rust — errors as values and explicit control flow (ensure … or …) instead of exceptions.