error-handling（错误处理）

Error Handling with wellcrafted trySync and tryAsync

When to Apply This Skill

Use this pattern when you need to:

• Replace recoverable try-catch blocks with trySync or tryAsync.
• Handle fallback success paths via Ok(...) and propagate failures with Err(...).
• Wrap caught exceptions as cause for typed domain error constructors.
• Refactor nested error branches into immediate-return linear control flow.
• Convert handler failures into HTTP status responses with explicit guards.

References

Load these on demand based on what you're working on:

• If working with wrapping boundaries, minimal vs extended wrapping, or immediate-return control flow, read references/wrapping-patterns.md
• If working with toast notifications for errors (toastOnError, extractErrorMessage in UI), read references/toast-on-error.md
• If working with real-world codebase examples and wrapping scenario guidelines, read references/real-world-examples.md
• If working with HTTP route handlers and status-response error conversion, read references/http-handlers.md

Use trySync/tryAsync Instead of try-catch for Graceful Error Handling

When handling errors that can be gracefully recovered from, use trySync (for synchronous code) or tryAsync (for asynchronous code) from wellcrafted instead of traditional try-catch blocks. This provides better type safety and explicit error handling.

Related Skills: See services-layer skill for defineErrors patterns and service architecture. See query-layer skill for error transformation to WhisperingError.

The Pattern

import { trySync, tryAsync, Ok, Err } from 'wellcrafted/result';

// SYNCHRONOUS: Use trySync for sync operations
const { data, error } = trySync({
	try: () => {
		const parsed = JSON.parse(jsonString);
		return validateData(parsed); // Automatically wrapped in Ok()
	},
	catch: (e) => {
		// Gracefully handle parsing/validation errors
		console.log('Using default configuration');
		return Ok(defaultConfig); // Return Ok with fallback
	},
});

// ASYNCHRONOUS: Use tryAsync for async operations
await tryAsync({
	try: async () => {
		const child = new Child(session.pid);
		await child.kill();
		console.log(`Process killed successfully`);
	},
	catch: (e) => {
		// Gracefully handle the error
		console.log(`Process was already terminated`);
		return Ok(undefined); // Return Ok(undefined) for void functions
	},
});

// Both support the same catch patterns
const syncResult = trySync({
	try: () => riskyOperation(),
	catch: (error) => {
		// For recoverable errors, return Ok with fallback value
		return Ok('fallback-value');
		// For unrecoverable errors, pass the raw cause — the constructor handles extractErrorMessage
		return CompletionError.ConnectionFailed({ cause: error });
	},
});

Key Rules

1. Choose the right function - Use trySync for synchronous code, tryAsync for asynchronous code
2. Always await tryAsync - Unlike try-catch, tryAsync returns a Promise and must be awaited
3. trySync returns immediately - No await needed for synchronous operations
4. Match return types - If the try block returns T, the catch should return Ok<T> for graceful handling
5. Use Ok(undefined) for void - When the function returns void, use Ok(undefined) in the catch
6. Return Err for propagation - Use custom error constructors that return Err when you want to propagate the error
7. Transform cause in the constructor, not the call site - When wrapping a caught error, pass the raw error as cause: unknown and let the defineErrors constructor call extractErrorMessage(cause) inside its message template. Don't call extractErrorMessage at the call site. This centralizes message extraction where the message is composed:

// ✅ GOOD: cause: error at call site, extractErrorMessage in constructor
catch: (error) => CompletionError.ConnectionFailed({ cause: error })

// ❌ BAD: extractErrorMessage at call site, string passed to constructor
catch: (error) => CompletionError.ConnectionFailed({ underlyingError: extractErrorMessage(error) })

1. CRITICAL: Wrap destructured errors with Err() - When you destructure {data, error} from tryAsync/trySync, the error variable is the raw error value, NOT wrapped in Err. You must wrap it before returning:

// WRONG - error is just the raw error value, not a Result
const { data, error } = await tryAsync({...});
if (error) return error; // TYPE ERROR: Returns raw error, not Result

// CORRECT - wrap with Err() to return a proper Result
const { data, error } = await tryAsync({...});
if (error) return Err(error); // Returns Err<CustomError>

This is different from returning the entire result object:

// This is also correct - userResult is already a Result type
const userResult = await tryAsync({...});
if (userResult.error) return userResult; // Returns the full Result

Examples

// SYNCHRONOUS: JSON parsing with fallback
const { data: config } = trySync({
	try: () => JSON.parse(configString),
	catch: (e) => {
		console.log('Invalid config, using defaults');
		return Ok({ theme: 'dark', autoSave: true });
	},
});

// SYNCHRONOUS: File system check
const { data: exists } = trySync({
	try: () => fs.existsSync(path),
	catch: () => Ok(false), // Assume doesn't exist if check fails
});

// ASYNCHRONOUS: Graceful process termination
await tryAsync({
	try: async () => {
		await process.kill();
	},
	catch: (e) => {
		console.log('Process already dead, continuing...');
		return Ok(undefined);
	},
});

// ASYNCHRONOUS: File operations with fallback
const { data: content } = await tryAsync({
	try: () => readFile(path),
	catch: (e) => {
		console.log('File not found, using default');
		return Ok('default content');
	},
});

// EITHER: Error propagation (works with both)
// Pass the raw caught error as cause — the defineErrors constructor calls extractErrorMessage
const { data, error } = await tryAsync({
	try: () => criticalOperation(),
	catch: (error) =>
		CompletionError.ConnectionFailed({ cause: error }),
});
if (error) return Err(error);

When to Use trySync vs tryAsync vs try-catch

• Use trySync when:

- Working with synchronous operations (JSON parsing, validation, calculations) - You need immediate Result types without promises - Handling errors in synchronous utility functions - Working with filesystem sync operations

• Use tryAsync when:

- Working with async/await operations - Making network requests or database calls - Reading/writing files asynchronously - Any operation that returns a Promise

• Use traditional try-catch when:

- In module-level initialization code where you can't await - For simple fire-and-forget operations - When you're outside of a function context - When integrating with code that expects thrown exceptions