# Error Handling: Library vs Application

## Library Error Design

### Principles
1. **Define specific error types** - Don't use `anyhow` in libraries
2. **Implement std::error::Error** - For compatibility
3. **Provide error variants** - Let users match on errors
4. **Include source errors** - Enable error chains
5. **Be `Send + Sync`** - For async compatibility

### Example: Library Error Type
```rust
// lib.rs
use thiserror::Error;

#[derive(Error, Debug)]
pub enum DatabaseError {
    #[error("connection failed: {host}:{port}")]
    ConnectionFailed {
        host: String,
        port: u16,
        #[source]
        source: std::io::Error,
    },

    #[error("query failed: {query}")]
    QueryFailed {
        query: String,
        #[source]
        source: SqlError,
    },

    #[error("record not found: {table}.{id}")]
    NotFound { table: String, id: String },

    #[error("constraint violation: {0}")]
    ConstraintViolation(String),
}

// Public Result alias
pub type Result<T> = std::result::Result<T, DatabaseError>;

// Library functions
pub fn connect(host: &str, port: u16) -> Result<Connection> {
    // ...
}

pub fn query(conn: &Connection, sql: &str) -> Result<Rows> {
    // ...
}
```

### Library Usage of Errors
```rust
impl Database {
    pub fn get_user(&self, id: &str) -> Result<User> {
        let rows = self.query(&format!("SELECT * FROM users WHERE id = '{}'", id))?;

        rows.first()
            .cloned()
            .ok_or_else(|| DatabaseError::NotFound {
                table: "users".to_string(),
                id: id.to_string(),
            })
    }
}
```

---

## Application Error Design

### Principles
1. **Use anyhow for convenience** - Or custom unified error
2. **Add context liberally** - Help debugging
3. **Log at boundaries** - Don't log in libraries
4. **Convert to user-friendly messages** - For display

### Example: Application Error Handling
```rust
// main.rs
use anyhow::{Context, Result};
use tracing::{error, info};

async fn run_server() -> Result<()> {
    let config = load_config()
        .context("failed to load configuration")?;

    let db = Database::connect(&config.db_url)
        .await
        .context("failed to connect to database")?;

    let server = Server::new(config.port)
        .context("failed to create server")?;

    info!("Server starting on port {}", config.port);

    server.run(db).await
        .context("server error")?;

    Ok(())
}

#[tokio::main]
async fn main() {
    tracing_subscriber::init();

    if let Err(e) = run_server().await {
        error!("Application error: {:#}", e);
        std::process::exit(1);
    }
}
```

### Converting Library Errors
```rust
use mylib::DatabaseError;

async fn get_user_handler(id: &str) -> Result<Response> {
    match db.get_user(id).await {
        Ok(user) => Ok(Response::json(user)),

        Err(DatabaseError::NotFound { .. }) => {
            Ok(Response::not_found("User not found"))
        }

        Err(DatabaseError::ConnectionFailed { .. }) => {
            error!("Database connection failed");
            Ok(Response::internal_error("Service unavailable"))
        }

        Err(e) => {
            error!("Database error: {}", e);
            Err(e.into())  // Convert to anyhow::Error
        }
    }
}
```

---

## Error Handling Layers

```
┌─────────────────────────────────────┐
│           Application Layer          │
│  - Use anyhow or unified error       │
│  - Add context at boundaries         │
│  - Log errors                        │
│  - Convert to user messages          │
└─────────────────────────────────────┘
                 │
                 │ calls
                 ▼
┌─────────────────────────────────────┐
│           Service Layer              │
│  - Map between error types           │
│  - Add business context              │
│  - Handle recoverable errors         │
└─────────────────────────────────────┘
                 │
                 │ calls
                 ▼
┌─────────────────────────────────────┐
│           Library Layer              │
│  - Define specific error types       │
│  - Use thiserror                     │
│  - Include source errors             │
│  - No logging                        │
└─────────────────────────────────────┘
```

---

## Practical Examples

### HTTP API Error Response
```rust
use axum::{response::IntoResponse, http::StatusCode};
use serde::Serialize;

#[derive(Serialize)]
struct ErrorResponse {
    error: String,
    code: String,
}

enum AppError {
    NotFound(String),
    BadRequest(String),
    Internal(anyhow::Error),
}

impl IntoResponse for AppError {
    fn into_response(self) -> axum::response::Response {
        let (status, error, code) = match self {
            AppError::NotFound(msg) => {
                (StatusCode::NOT_FOUND, msg, "NOT_FOUND")
            }
            AppError::BadRequest(msg) => {
                (StatusCode::BAD_REQUEST, msg, "BAD_REQUEST")
            }
            AppError::Internal(e) => {
                tracing::error!("Internal error: {:#}", e);
                (
                    StatusCode::INTERNAL_SERVER_ERROR,
                    "Internal server error".to_string(),
                    "INTERNAL_ERROR",
                )
            }
        };

        let body = ErrorResponse {
            error,
            code: code.to_string(),
        };

        (status, axum::Json(body)).into_response()
    }
}
```

### CLI Error Handling
```rust
use anyhow::{Context, Result};
use clap::Parser;

#[derive(Parser)]
struct Args {
    #[arg(short, long)]
    config: String,
}

fn main() {
    if let Err(e) = run() {
        eprintln!("Error: {:#}", e);
        std::process::exit(1);
    }
}

fn run() -> Result<()> {
    let args = Args::parse();

    let config = std::fs::read_to_string(&args.config)
        .context(format!("Failed to read config file: {}", args.config))?;

    let parsed: Config = toml::from_str(&config)
        .context("Failed to parse config file")?;

    process(parsed)?;

    println!("Done!");
    Ok(())
}
```

---

## Testing Error Handling

### Testing Error Cases
```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_not_found_error() {
        let result = db.get_user("nonexistent");

        assert!(matches!(
            result,
            Err(DatabaseError::NotFound { table, id })
            if table == "users" && id == "nonexistent"
        ));
    }

    #[test]
    fn test_error_message() {
        let err = DatabaseError::NotFound {
            table: "users".to_string(),
            id: "123".to_string(),
        };

        assert_eq!(err.to_string(), "record not found: users.123");
    }

    #[test]
    fn test_error_chain() {
        let io_err = std::io::Error::new(
            std::io::ErrorKind::ConnectionRefused,
            "connection refused"
        );

        let err = DatabaseError::ConnectionFailed {
            host: "localhost".to_string(),
            port: 5432,
            source: io_err,
        };

        // Check source is preserved
        assert!(err.source().is_some());
    }
}
```

### Testing with anyhow
```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_with_context() -> anyhow::Result<()> {
        let result = process("valid input")?;
        assert_eq!(result, expected);
        Ok(())
    }

    #[test]
    fn test_error_context() {
        let err = process("invalid")
            .context("processing failed")
            .unwrap_err();

        // Check error chain contains expected text
        let chain = format!("{:#}", err);
        assert!(chain.contains("processing failed"));
    }
}
```

# Error Handling Patterns

## The ? Operator

### Basic Usage
```rust
fn read_config() -> Result<Config, io::Error> {
    let content = std::fs::read_to_string("config.toml")?;
    let config: Config = toml::from_str(&content)?;  // needs From impl
    Ok(config)
}
```

### With Different Error Types
```rust
use std::error::Error;

// Box<dyn Error> for quick prototyping
fn process() -> Result<(), Box<dyn Error>> {
    let file = std::fs::read_to_string("data.txt")?;
    let num: i32 = file.trim().parse()?;  // different error type
    Ok(())
}
```

### Custom Conversion with From
```rust
#[derive(Debug)]
enum MyError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
}

impl From<std::io::Error> for MyError {
    fn from(err: std::io::Error) -> Self {
        MyError::Io(err)
    }
}

impl From<std::num::ParseIntError> for MyError {
    fn from(err: std::num::ParseIntError) -> Self {
        MyError::Parse(err)
    }
}

fn process() -> Result<i32, MyError> {
    let content = std::fs::read_to_string("num.txt")?;  // auto-converts
    let num: i32 = content.trim().parse()?;  // auto-converts
    Ok(num)
}
```

---

## Error Type Design

### Simple Enum Error
```rust
#[derive(Debug, Clone, PartialEq)]
pub enum ConfigError {
    NotFound,
    InvalidFormat,
    MissingField(String),
}

impl std::fmt::Display for ConfigError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ConfigError::NotFound => write!(f, "configuration file not found"),
            ConfigError::InvalidFormat => write!(f, "invalid configuration format"),
            ConfigError::MissingField(field) => write!(f, "missing field: {}", field),
        }
    }
}

impl std::error::Error for ConfigError {}
```

### Error with Source (Wrapping)
```rust
#[derive(Debug)]
pub struct AppError {
    kind: AppErrorKind,
    source: Option<Box<dyn std::error::Error + Send + Sync>>,
}

#[derive(Debug, Clone, Copy)]
pub enum AppErrorKind {
    Config,
    Database,
    Network,
}

impl std::fmt::Display for AppError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self.kind {
            AppErrorKind::Config => write!(f, "configuration error"),
            AppErrorKind::Database => write!(f, "database error"),
            AppErrorKind::Network => write!(f, "network error"),
        }
    }
}

impl std::error::Error for AppError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        self.source.as_ref().map(|e| e.as_ref() as _)
    }
}
```

---

## Using thiserror

### Basic Usage
```rust
use thiserror::Error;

#[derive(Error, Debug)]
pub enum DataError {
    #[error("file not found: {path}")]
    NotFound { path: String },

    #[error("invalid data format")]
    InvalidFormat,

    #[error("IO error")]
    Io(#[from] std::io::Error),

    #[error("parse error: {0}")]
    Parse(#[from] std::num::ParseIntError),
}

// Usage
fn load_data(path: &str) -> Result<Data, DataError> {
    let content = std::fs::read_to_string(path)
        .map_err(|_| DataError::NotFound { path: path.to_string() })?;
    let num: i32 = content.trim().parse()?;  // auto-converts with #[from]
    Ok(Data { value: num })
}
```

### Transparent Wrapper
```rust
use thiserror::Error;

#[derive(Error, Debug)]
#[error(transparent)]
pub struct MyError(#[from] InnerError);

// Useful for newtype error wrappers
```

---

## Using anyhow

### For Applications
```rust
use anyhow::{Context, Result, bail, ensure};

fn process_file(path: &str) -> Result<Data> {
    let content = std::fs::read_to_string(path)
        .context("failed to read config file")?;

    ensure!(!content.is_empty(), "config file is empty");

    let data: Data = serde_json::from_str(&content)
        .context("failed to parse JSON")?;

    if data.version < 1 {
        bail!("unsupported config version: {}", data.version);
    }

    Ok(data)
}

fn main() -> Result<()> {
    let data = process_file("config.json")
        .context("failed to load configuration")?;
    Ok(())
}
```

### Error Chain
```rust
use anyhow::{Context, Result};

fn deep_function() -> Result<()> {
    std::fs::read_to_string("missing.txt")
        .context("failed to read file")?;
    Ok(())
}

fn middle_function() -> Result<()> {
    deep_function()
        .context("failed in deep function")?;
    Ok(())
}

fn top_function() -> Result<()> {
    middle_function()
        .context("failed in middle function")?;
    Ok(())
}

// Error output shows full chain:
// Error: failed in middle function
// Caused by:
//     0: failed in deep function
//     1: failed to read file
//     2: No such file or directory (os error 2)
```

---

## Option Handling

### Converting Option to Result
```rust
fn find_user(id: u32) -> Option<User> { ... }

// Using ok_or for static error
fn get_user(id: u32) -> Result<User, &'static str> {
    find_user(id).ok_or("user not found")
}

// Using ok_or_else for dynamic error
fn get_user(id: u32) -> Result<User, String> {
    find_user(id).ok_or_else(|| format!("user {} not found", id))
}
```

### Chaining Options
```rust
fn get_nested_value(data: &Data) -> Option<&str> {
    data.config
        .as_ref()?
        .nested
        .as_ref()?
        .value
        .as_deref()
}

// Equivalent with and_then
fn get_nested_value(data: &Data) -> Option<&str> {
    data.config
        .as_ref()
        .and_then(|c| c.nested.as_ref())
        .and_then(|n| n.value.as_deref())
}
```

---

## Pattern: Result Combinators

### map and map_err
```rust
fn parse_port(s: &str) -> Result<u16, ParseError> {
    s.parse::<u16>()
        .map_err(|e| ParseError::InvalidPort(e))
}

fn get_url(config: &Config) -> Result<String, Error> {
    config.url()
        .map(|u| format!("https://{}", u))
}
```

### and_then (flatMap)
```rust
fn validate_and_save(input: &str) -> Result<(), Error> {
    validate(input)
        .and_then(|valid| save(valid))
        .and_then(|saved| notify(saved))
}
```

### unwrap_or and unwrap_or_else
```rust
// Default value
let port = config.port().unwrap_or(8080);

// Computed default
let port = config.port().unwrap_or_else(|| find_free_port());

// Default for Result
let data = load_data().unwrap_or_default();
```

---

## Pattern: Early Return vs Combinators

### Early Return Style
```rust
fn process(input: &str) -> Result<Output, Error> {
    let step1 = validate(input)?;
    if !step1.is_valid {
        return Err(Error::Invalid);
    }

    let step2 = transform(step1)?;
    let step3 = save(step2)?;

    Ok(step3)
}
```

### Combinator Style
```rust
fn process(input: &str) -> Result<Output, Error> {
    validate(input)
        .and_then(|s| {
            if s.is_valid {
                Ok(s)
            } else {
                Err(Error::Invalid)
            }
        })
        .and_then(transform)
        .and_then(save)
}
```

### When to Use Which

| Style | Best For |
|-------|----------|
| Early return (`?`) | Most cases, clearer flow |
| Combinators | Functional pipelines, one-liners |
| Match | Complex branching on errors |

---

## Panic vs Result

### When to Panic
```rust
// 1. Unrecoverable programmer error
fn get_config() -> &'static Config {
    CONFIG.get().expect("config must be initialized")
}

// 2. In tests
#[test]
fn test_parsing() {
    let result = parse("valid").unwrap();  // OK in tests
    assert_eq!(result, expected);
}

// 3. Prototype/examples
fn main() {
    let data = load().unwrap();  // OK for quick examples
}
```

### When to Return Result
```rust
// 1. Any I/O operation
fn read_file(path: &str) -> Result<String, io::Error>

// 2. User input validation
fn parse_port(s: &str) -> Result<u16, ParseError>

// 3. Network operations
async fn fetch(url: &str) -> Result<Response, Error>

// 4. Anything that can fail at runtime
fn connect(addr: &str) -> Result<Connection, Error>
```

---

## Error Context Best Practices

### Add Context at Boundaries
```rust
fn load_user_config(user_id: u64) -> Result<Config, Error> {
    let path = format!("/home/{}/config.toml", user_id);

    std::fs::read_to_string(&path)
        .context(format!("failed to read config for user {}", user_id))?
        // NOT: .context("failed to read file")  // too generic

    // ...
}
```

### Include Relevant Data
```rust
// Good: includes the problematic value
fn parse_age(s: &str) -> Result<u8, Error> {
    s.parse()
        .context(format!("invalid age value: '{}'", s))
}

// Bad: no context about what failed
fn parse_age(s: &str) -> Result<u8, Error> {
    s.parse()
        .context("parse error")
}
```