Error Types Explained

This document explains the two custom error types defined in the project: RepositoryError and AppError.

RepositoryError

Defined in src/todo_item.rs:12-18, RepositoryError handles errors that can occur during data persistence operations:

#[derive(Debug, thiserror::Error)]
pub enum RepositoryError {
    #[error("IO Error: {0}")]
    Io(#[from] std::io::Error),
    #[error("JSON Error: {0}")]
    Json(#[from] serde_json::Error),
}

Purpose: Wraps low-level storage errors that can occur when reading from or writing to the file system.

Variants:

• Io(std::io::Error) - Covers file not found, permission denied, disk full, etc. Uses the #[from] attribute for automatic conversion.
• Json(serde_json::Error) - Covers JSON serialization/deserialization errors (e.g., corrupted data, invalid format).

Used by: The TodoRepository trait and JsonFileRepository implementation.

---

AppError

Defined in src/todo_item.rs:20-26, AppError represents application-level errors:

#[derive(Debug, thiserror::Error)]
pub enum AppError {
    #[error("Repository Error: {0}")]
    Repository(#[from] RepositoryError),
    #[error("Task not found")]
    NotFound,
}

Purpose: Represents high-level business logic errors that the application can encounter.

Variants:

• Repository(RepositoryError) - Wraps any repository error (IO or JSON). Uses #[from] for automatic conversion from RepositoryError.
• NotFound - Indicates a requested task with a specific ID does not exist.

Used by: Public functions like add_task and mark_done to provide meaningful error messages to callers.

---

Error Handling Pattern

The project uses a layered error approach:

1. Repository layer handles low-level I/O and serialization issues via RepositoryError
2. Application layer handles business logic issues via AppError
3. The #[from] attribute enables automatic error propagation using the ? operator

This design allows callers to handle errors at an appropriate level - they can either handle AppError for user-facing messages, or let RepositoryError bubble up for debugging/storage issues.