effect

import { Effect, Console, pipe, Duration, Schedule } from 'effect'; interface User { id: number; name: string; email: string; } // Define a custom error type for better type safety class UserNotFoundError extends Effect.Error('UserNotFoundError')<{ userId: number }> {} class DatabaseConnectionError extends Effect.Error('DatabaseConnectionError')<{ message: string }> {} // Simulate fetching a user from a database const fetchUserFromDB = (userId: number): Effect.Effect<User, UserNotFoundError | DatabaseConnectionError, never> => Effect.sync(() => { if (userId === 1) { return { id: 1, name: 'Alice', email: 'alice@example.com' }; } else if (userId === 99) { throw new Error('Failed to connect to DB'); // Simulate a defect } return Effect.fail(new UserNotFoundError({ userId })); }).pipe( Effect.catchAllDefect((error) => Effect.fail(new DatabaseConnectionError({ message: String(error) })) ) ); const program = pipe( fetchUserFromDB(1), // Try to fetch user with ID 1 Effect.tap((user) => Console.log(`Fetched user: ${user.name}`)), Effect.flatMap(() => fetchUserFromDB(2)), // Try to fetch user with ID 2 (will fail) Effect.tapError((error) => error._tag === 'UserNotFoundError' ? Console.error(`Error: User with ID ${error.userId} not found.`) : Console.error(`Critical Error: ${error.message}`) ), Effect.retry(Schedule.exponential(Duration.seconds(1), 3)), // Retry failed operations with exponential backoff Effect.matchEffect({ // Handle both success and failure paths explicitly onFailure: (error) => Console.error(`Final failure: ${error._tag}`), onSuccess: (user) => Console.log(`Final success (should not happen for user 2, unless retries succeed): ${user.name}`), }) ); // Run the Effect program Effect.runPromise(program).then(() => Console.log('Program finished.')).catch(console.error);