Design domain-specific error types

// Instead of using anyhow or generic errors:
fn process() -> anyhow::Result<()> {
    if cancelled {
        bail!("operation cancelled"); // Generic error
    }
    Ok(())
}

// Create domain-specific error types:
#[derive(Error, Debug)]
pub enum ProcessError {
    #[error("operation cancelled")]
    Cancelled,
    #[error("failed to write to {path}: {source}")]
    IoError { path: String, source: std::io::Error },
    #[error("invalid configuration: {0}")]
    Config(String),
}

fn process() -> Result<(), ProcessError> {
    if cancelled {
        return Err(ProcessError::Cancelled);
    }
    Ok(())
}

// Proper error mapping when crossing domain boundaries:
fn handle_process() -> Result<(), ApiError> {
    process().map_err(|e| match e {
        ProcessError::Cancelled => ApiError::ServiceUnavailable,
        ProcessError::IoError { .. } => ApiError::InternalError,
        ProcessError::Config(_) => ApiError::BadRequest,
    })?;
    Ok(())
}