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ERROR HANDLING

Core Concept: Errors are values returned explicitly, not exceptions thrown implicitly.

Why It Matters

Forces explicit handling at each call site. No hidden control flow. Foundation of Go's reliability.

When to Use

✅ Return error when:

• Operation can fail
• Caller decides handling

❌ Don't return error when:

• Programming errors (use panic)
• Can handle internally

vs Java Exceptions

Java: try/catch (hidden control flow)
Go: Explicit if err != nil at each call

Go errors are values - can inspect, wrap, pass around.

WARNING

Wrapping without %w loses error chain. Use fmt.Errorf("context: %w", err) not %v. ```

Trade-offs

Pros: Explicit, clear flow, composable
Cons: Verbose, repetitive checks

Error wrapping builds on interfaces (error interface).

Quick Reference

// Basic pattern
result, err := operation()
if err != nil {
    return fmt.Errorf("operation failed: %w", err)
}

// Wrap errors
return fmt.Errorf("context: %w", err)

// Check specific error
if errors.Is(err, sql.ErrNoRows) { }

// Sentinel errors
var ErrNotFound = errors.New("not found")

Pattern	Use	Example
errors.New	Static error	errors.New("failed")
fmt.Errorf	Dynamic message	fmt.Errorf("id: %d", id)
%w	Wrap error	fmt.Errorf("save: %w", err)
errors.Is	Check type	errors.Is(err, ErrNotFound)

Examples

EXAMPLE

Error wrapping with context:

func SaveUser(user *User) error {
    if err := validate(user); err != nil {
        return fmt.Errorf("validation failed: %w", err)
    }
    
    if err := db.Insert(user); err != nil {
        return fmt.Errorf("failed to save user %s: %w", user.Name, err)
    }
    
    return nil
}

// Caller can check root cause
err := SaveUser(user)
if errors.Is(err, sql.ErrNoRows) {
    // Handle specific database error
}

Sentinel errors for control flow:

var (
    ErrNotFound    = errors.New("user not found")
    ErrInvalidData = errors.New("invalid user data")
)

func FindUser(id int) (*User, error) {
    if id < 0 {
        return nil, ErrInvalidData
    }
    
    user := db.Query(id)
    if user == nil {
        return nil, ErrNotFound
    }
    
    return user, nil
}

// Caller checks specific errors
user, err := FindUser(5)
if errors.Is(err, ErrNotFound) {
    return nil, status.Error(codes.NotFound, "user not found")
}
if errors.Is(err, ErrInvalidData) {
    return nil, status.Error(codes.InvalidArgument, "bad request")
}

```

References

• Go Blog: Error Handling