Sentinel Errors

Pattern

Define package-level errors as exported variables for expected error conditions.

CORRECT

package user

import "errors"

// Sentinel errors - define at package level
var (
    ErrNotFound      = errors.New("user not found")
    ErrInvalidEmail  = errors.New("invalid email format")
    ErrDuplicate     = errors.New("user already exists")
)

func Get(id int) (*User, error) {
    u, ok := db[id]
    if !ok {
        return nil, ErrNotFound
    }
    return u, nil
}

WRONG

// Bad: Creates new error each time (can't use errors.Is)
func Get(id int) (*User, error) {
    if _, ok := db[id]; !ok {
        return nil, errors.New("user not found")
    }
    return db[id], nil
}

// Bad: String comparison is fragile
if err.Error() == "user not found" { }

Naming Convention

• Prefix with Err
• Use camel case: ErrNotFound, not ERR_NOT_FOUND
• Be specific: ErrInvalidEmail, not ErrBadInput

When to Use

Use sentinel errors for:

• Expected business logic errors
• Public API boundaries
• Conditions callers need to handle differently

Avoid for:

• Unexpected/rare conditions
• Internal implementation details