Runtime in Effect

Overview

The Runtime is Effect's execution engine:

• Default Runtime - Built-in, zero configuration
• Custom Runtime - Configure services, context, execution
• ManagedRuntime - Lifecycle-managed custom runtimes

Default Runtime

Effects run via the default runtime:

import { Effect } from "effect";

const program = Effect.succeed(42);

const result = await Effect.runPromise(program);

const syncResult = Effect.runSync(program);

const exit = await Effect.runPromiseExit(program);

Run Methods

Method	Returns	Throws on Error
Effect.runPromise(e)	Promise<A>	Yes
Effect.runPromiseExit(e)	Promise<Exit<A, E>>	No
Effect.runSync(e)	A	Yes
Effect.runSyncExit(e)	Exit<A, E>	No

Locally Scoped Configuration

Modify runtime behavior for specific effects:

import { Logger, LogLevel } from "effect";

const program = Effect.gen(function* () {
  yield* Effect.log("This appears");
  yield* Effect.logDebug("This may not appear");
});

const withDebug = program.pipe(Logger.withMinimumLogLevel(LogLevel.Debug));

Effect.Tag for Services

Create typed service tags for dependency injection:

import { Effect, Context } from "effect";

class Database extends Context.Tag("Database")<
  Database,
  {
    readonly query: (sql: string) => Effect.Effect<unknown[]>;
    readonly execute: (sql: string) => Effect.Effect<void>;
  }
>() {}

const program = Effect.gen(function* () {
  const db = yield* Database;
  const users = yield* db.query("SELECT * FROM users");
  return users;
});

ManagedRuntime

For applications needing custom runtime configuration:

import { ManagedRuntime, Layer } from "effect";

const AppLive = Layer.mergeAll(DatabaseLive, LoggerLive, ConfigLive);

const runtime = ManagedRuntime.make(AppLive);

const main = async () => {
  const result = await runtime.runPromise(program);
  console.log(result);

  await runtime.dispose();
};

ManagedRuntime Benefits

• Pre-builds layer once
• Reuses services across effect runs
• Proper cleanup with dispose()
• Integration with frameworks

Framework Integration

Express Integration

import express from "express";
import { ManagedRuntime, Layer } from "effect";

const AppLive = Layer.mergeAll(DatabaseLive, AuthLive);
const runtime = ManagedRuntime.make(AppLive);

const app = express();

app.get("/users", async (req, res) => {
  const result = await runtime.runPromise(
    getUsers().pipe(Effect.catchAll((error) => Effect.succeed({ error: error.message }))),
  );
  res.json(result);
});

// Cleanup on shutdown
process.on("SIGTERM", () => {
  runtime.dispose().then(() => process.exit(0));
});

React Integration

import { ManagedRuntime } from "effect"
import { createContext, useContext } from "react"

// Create runtime context
const RuntimeContext = createContext<ManagedRuntime<AppServices> | null>(null)

// Provider component
export function AppProvider({ children }: { children: React.ReactNode }) {
  const [runtime] = useState(() => ManagedRuntime.make(AppLive))

  useEffect(() => {
    return () => { runtime.dispose() }
  }, [])

  return (
    <RuntimeContext.Provider value={runtime}>
      {children}
    </RuntimeContext.Provider>
  )
}

// Hook to use runtime
export function useRuntime() {
  const runtime = useContext(RuntimeContext)
  if (!runtime) throw new Error("Runtime not provided")
  return runtime
}

// Usage in components
function UserList() {
  const runtime = useRuntime()
  const [users, setUsers] = useState<User[]>([])

  useEffect(() => {
    runtime.runPromise(fetchUsers()).then(setUsers)
  }, [])

  return <ul>{users.map(u => <li key={u.id}>{u.name}</li>)}</ul>
}

Runtime Configuration

Custom Execution Context

import { Runtime, FiberRef } from "effect";

const customRuntime = Runtime.defaultRuntime.pipe(Runtime.withFiberRef(FiberRef.currentLogLevel, LogLevel.Debug));

Runtime.runPromise(customRuntime)(program);

Providing Services to Runtime

const runtimeWithServices = Runtime.defaultRuntime.pipe(
  Runtime.provideService(Database, databaseImpl),
  Runtime.provideService(Logger, loggerImpl),
);

Default Services

Effect provides these services automatically:

import { Clock, Random, Tracer, Console } from "effect";

const program = Effect.gen(function* () {
  const now = yield* Clock.currentTimeMillis;

  const rand = yield* Random.next;

  yield* Console.log("Hello");
});

Overriding Default Services

import { TestClock } from "effect";

const testProgram = program.pipe(Effect.provide(TestClock.layer));

const testWithTime = Effect.gen(function* () {
  const fiber = yield* Effect.fork(Effect.sleep("1 hour"));
  yield* TestClock.adjust("1 hour");
  yield* Fiber.join(fiber);
});

Interruption Handling

const program = Effect.gen(function* () {
  const fiber = yield* Effect.fork(longRunningTask);

  yield* Fiber.interrupt(fiber);
});

const critical = Effect.uninterruptible(
  Effect.gen(function* () {
    yield* startTransaction();
    yield* doWork();
    yield* commitTransaction();
  }),
);

Best Practices

1. Use ManagedRuntime for apps - Proper lifecycle management
2. Provide services via layers - Not runtime modification
3. Use Effect.Tag for services - Type-safe dependency injection
4. Handle cleanup properly - Always dispose() ManagedRuntime
5. Test with TestClock - Deterministic time in tests

Additional Resources

For comprehensive runtime documentation, consult ${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt.

Search for these sections:

• "Introduction to Runtime" for core concepts
• "ManagedRuntime" for managed runtime
• "Effect.Tag" for service tags
• "Integrations" for framework integration