@forst/runtime

Node runtime for Forst → TypeScript interop. Compiled Go binaries call your legacy .ts and .js modules over a closed RPC channel. The Forst compiler uses this package at build time to index TypeScript exports.

Status: experimental. Pin this package and verify with the examples before production use.

Full guide → Call JavaScript from Forst

Install

npm install @forst/runtime

npx jsr add @forst/runtime

Requires Node.js 18+. When your Forst program uses a JS import (import "./path" node), you also need tsx on the path for TypeScript loading.

| Registry | Package | | --- | --- | | npm | @forst/runtime | | JSR | @forst/runtime |

What you get

The runtime is built on Effect: structured logs via Effect.log* and Effect.fn spans, with ForstRuntimeLayer (stderr pretty logging, tracing, and FORST_BRIDGE_LOG_LEVEL) provided at process boundaries via NodeRuntime.runMain or Effect.runPromise.

RPC and runtime hot paths are wrapped in Effect.fn spans (Rpc.dispatch, Runtime.handleSyncCall, …). Span attributes (rpc_method, module_id, …) appear on log lines when FORST_BRIDGE_LOG_LEVEL is debug or trace.

Custom Effect runtime

Default entrypoints (bootstrap.js, @forst/runtime/host) use ForstRuntimeLayer. To bring your own logging, tracing, or services, build a setup and pass it at the process boundary:

import { NodeRuntime } from "@effect/platform-node";
import { Effect, Layer } from "effect";
import {
  bootstrapMain,
  bootstrapFatal,
  createNodeRuntimeSetup,
  makeForstRuntimeLayer,
  startForstNodeHost,
} from "@forst/runtime";

// Standalone child: Forst owns stderr logging and tracing.
const myLayer = makeForstRuntimeLayer();
const { layer, runtime } = createNodeRuntimeSetup(myLayer);

// Embedded in an Effect app: keep the parent logger and tracer.
const embeddedLayer = Layer.merge(
  appLayer,
  makeForstRuntimeLayer({ replaceLogger: false })
);
const embedded = createNodeRuntimeSetup(embeddedLayer);

// Bootstrap child (local socket RPC). disablePrettyLogger avoids duplicate stdout logging.
NodeRuntime.runMain(
  bootstrapMain({ runtime }).pipe(
    Effect.catchAllDefect((cause) => bootstrapFatal(cause)),
    Effect.provide(layer)
  ),
  { disablePrettyLogger: true }
);

// Host mode: pass runtimeLayer so RPC forks use the same setup
await Effect.runPromise(
  startForstNodeHost({ runtimeLayer: embedded.layer }).pipe(
    Effect.provide(embedded.layer)
  )
);

Use the same layer for Effect.provide and the matching runtime for async RPC dispatch sites (bootstrapMain, host connection forks).

OpenTelemetry (optional)

Install peer packages when you want RPC spans exported to OTLP or the console:

npm install @effect/opentelemetry @opentelemetry/api @opentelemetry/sdk-trace-base @opentelemetry/sdk-trace-node @opentelemetry/exporter-trace-otlp-http

Bootstrap auto-export: the bootstrap child calls resolveForstRuntimeLayer() at startup. When OTEL_EXPORTER_OTLP_ENDPOINT or FORST_BRIDGE_OTEL=1 is set and the peers above are installed, Effect spans (Rpc.dispatch, Runtime.handleSyncCall, …) export through OpenTelemetry automatically.

OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318/v1/traces \
OTEL_SERVICE_NAME=my-app \
FORST_BRIDGE_LOG_LEVEL=debug \
npx forst run -root ./my-service ./main.ft

If OTEL env is set but peers are missing, bootstrap logs a warning and continues with stderr-only spans.

Host / embedded apps: merge OpenTelemetry at your app boundary so you do not duplicate exporters:

import { Layer } from "effect";
import { createNodeRuntimeSetup } from "@forst/runtime";
import {
  mergeForstNodeRuntimeWithOpenTelemetry,
  openTelemetryLayerFromEnv,
} from "@forst/runtime/opentelemetry";

const layer = mergeForstNodeRuntimeWithOpenTelemetry(
  openTelemetryLayerFromEnv(),
  { replaceLogger: false }
);
const { layer: runtimeLayer, runtime } = createNodeRuntimeSetup(
  Layer.merge(appLayer, layer)
);

Import helpers from @forst/runtime/opentelemetry. The main @forst/runtime entry also exports resolveForstRuntimeLayer for bootstrap-style resolution without static OTEL imports.

| Piece | Role | | --- | --- | | bootstrap.js | RPC server process Go spawns in bootstrap mode | | @forst/runtime/host | In process RPC when your app runs the Node child | | forst-runtime-index | CLI the compiler invokes to read TypeScript exports | | Schema types | forst-node-manifest-v1 and forst-index-v1 validation |

Project setup

Enable node interop in ftconfig.json:

{
  "files": {
    "include": ["**/*.ft", "**/*.ts"]
  },
  "bridge": {
    "enabled": true,
    "runtimeEnabled": true
  }
}

Use opt in imports in Forst source:

import "./legacy/payment" js

func main() {
    result := payment.create(100.0, "USD")
}

Build and run with the Forst compiler (@forst/cli):

npx forst build -root . ./main.ft
npx forst run -root . ./main.ft

Runtime modes

Bootstrap (default): Go starts a dedicated Node child that runs dist/bootstrap.js. Isolated process. RPC over a local socket (default .forst/node-bootstrap.sock); child stdout and stderr are forwarded as logs.

Host: Go starts your app (bridge.binary + bridge.args). RPC listens on a local socket inside that process so module cache and globals stay shared. Import from @forst/runtime/host and call signalForstAppReady() when your app is ready.

See runtime modes in the docs.

CLI

forst-runtime-index --root . --format forst-index-v1 --files legacy/payment.ts

The compiler calls this during type checking. You rarely run it yourself.

Environment variables

Bootstrap and logging

| Variable | Purpose | | --- | --- | | FORST_BRIDGE_LOG_LEVEL | Log verbosity: trace, debug, info, warn, or error (default info). Per-RPC flow uses Effect spans; set debug or trace to see span annotations (effect.spanName, rpc_method, …) on log lines. | | FORST_BRIDGE_OTEL | When "1", enables OpenTelemetry export in bootstrap (console spans when no OTLP endpoint is set). Requires optional OTEL peer packages. | | OTEL_EXPORTER_OTLP_ENDPOINT | Standard OTLP traces URL (e.g. http://localhost:4318/v1/traces). When set, bootstrap exports RPC spans to this endpoint. | | OTEL_SERVICE_NAME | OpenTelemetry service name (default forst-runtime). | | OTEL_EXPORTER_OTLP_HEADERS | Optional OTLP headers (key=value,key2=value2). | | FORST_BRIDGE_LOG_FORMAT | Log format: pretty (default) or json for structured stderr lines. | | FORST_BRIDGE_BOOTSTRAP | Absolute path to bootstrap.js (bootstrap mode spawn planning) | | FORST_BRIDGE_MODULES_DIR | Absolute path to compiled bridge modules (.js manifest IDs resolve here in compiled format) | | FORST_BRIDGE_SOCKET | Absolute Unix socket path (TCP URL on Windows) for Go↔Node RPC. Bootstrap default: {boundaryRoot}/.forst/node-bootstrap.sock. Host default: {boundaryRoot}/.forst/node.sock. | | FORST_BRIDGE_HOST_READY | Absolute path to JSON readiness file ({socket}.ready); Go waits for phase: "app" before dialing. |

Host mode (set by Go on the direct app-shim child)

| Variable | Purpose | | --- | --- | | FORST_BRIDGE_HOST | When "1", enables in-process host RPC (startForstNodeHost). Unset in bootstrap mode. | | FORST_BRIDGE_HOST_LEADER | When "1", marks the Go-spawned leader process. Required together with register.mjs in process.execArgv; workers skip binding. | | FORST_BRIDGE_APP_READY_MODULE | Optional path to a module loaded before app readiness when bridge.hostAppReadyModule is configured. |

See Environment variables for socket RPC for spawn layout, readiness phases, and troubleshooting.

Development

From the monorepo:

cd packages/runtime
bun run build
bun test

Publishing

Release Please tags runtime-v* bump package.json and jsr.json. CI publishes to npm and JSR via .github/workflows/publish-packages.yml.

Manual publish from packages/runtime:

bun run build
npm publish --access public --workspaces=false
npx jsr publish

Dry run:

bun run pack:dry
npx jsr publish --dry-run

npm trusted publishing (one-time)

Before CI can publish, add a trusted publisher for @forst/runtime on npmjs.com (same settings as @forst/cli — each package needs its own):

• Repository: forst-lang/forst
• Workflow: publish-packages.yml
• Environment: (none, unless you use one for other packages)

Without this, CI fails with OIDC token exchange error - package not found / ENEEDAUTH. Re-publishing an already-published version is a no-op in CI.

License

MIT. See LICENSE.