use std::fmt;
use std::error::Error;
 
#[derive(Debug)]
enum DataStoreError {
    NotFound(String),
    PermissionDenied { resource: String, user: String },
    ConnectionFailed(String),
}
 
impl fmt::Display for DataStoreError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            DataStoreError::NotFound(key) => write!(f, "Item '{}' not found", key),
            DataStoreError::PermissionDenied { resource, user } => {
                write!(f, "User '{}' cannot access '{}'", user, resource)
            },
            DataStoreError::ConnectionFailed(msg) => write!(f, "Connection failed: {}", msg),
        }
    }
}
 
impl Error for DataStoreError {}
 
fn lookup_user(user_id: &str) -> Result<String, DataStoreError> {
    if user_id.is_empty() {
        return Err(DataStoreError::NotFound("empty user id".to_string()));
    }
    if user_id == "forbidden" {
        return Err(DataStoreError::PermissionDenied {
            resource: "user database".to_string(),
            user: user_id.to_string(),
        });
    }
    Ok(format!("User #{}", user_id))
}
 
fn main() {
    match lookup_user("forbidden") {
        Ok(name) => println!("Found: {}", name),
        Err(e) => eprintln!("Error: {}", e),
    }
}