Refactoring to Improve Modularity and Error Handling

Example Code

```rust
use std::{env, fs, io, num::ParseIntError};

// --- BEFORE REFACTORING ---
// This section demonstrates a less modular approach with poor error handling.
// - All logic is within `main`.
// - Uses `expect()` extensively, leading to panics on errors.
// - No custom error types, making error messages generic.
// - Hard to test individual components.
// - Poor separation of concerns.
/*
fn main() {
    let args: Vec<String> = env::args().collect();
    if args.len() < 2 {
        eprintln!("Usage: {} <config_file_path>", args[0]);
        // Unclean exit for missing argument. panics if run without args.
        panic!("Missing config file path");
    }
    let file_path = &args[1];

    let content = fs::read_to_string(file_path)
        .expect(&format!("Failed to read file: {}", file_path)); // panics on file error

    let lines: Vec<&str> = content.lines().collect();
    if lines.is_empty() {
        panic!("Config file is empty!"); // panics on empty file
    }

    let mut config_value: Option<u32> = None;
    for line in lines {
        let parts: Vec<&str> = line.split('=').collect();
        if parts.len() == 2 && parts[0].trim() == "my_setting" {
            config_value = Some(parts[1].trim().parse().expect("Failed to parse my_setting value")); // panics on parse error
            break;
        }
    }

    let setting = config_value.expect("my_setting not found in config file"); // panics if not found

    println!("Configuration 'my_setting' value: {}", setting);
    println!("Performing operation with setting: {}", setting * 2);
}
*/


// --- AFTER REFACTORING ---
// This section demonstrates improved modularity and robust error handling.
// - Logic is broken into dedicated functions.
// - Custom error types provide specific, actionable error information.
// - Uses `Result` and `?` operator for explicit and graceful error propagation.
// - Clear separation of concerns.
// - Easier to test and maintain.

// --- 1. Define custom error types for better error handling ---
#[derive(Debug)]
enum ConfigError {
    Io(io::Error),
    Parse(ParseIntError),
    NotFound(String),
    EmptyFile,
    InvalidFormat(String),
}

// Implement From traits to easily convert standard library errors into our custom error.
// This allows using the `?` operator with standard errors that occur during config processing.
impl From<io::Error> for ConfigError {
    fn from(err: io::Error) -> Self {
        ConfigError::Io(err)
    }
}

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

// A top-level application error enum that can wrap `ConfigError` or other potential errors
#[derive(Debug)]
enum AppError {
    Config(ConfigError),
    ProcessingFailure(String),
    MissingArguments,
}

// Allow converting `ConfigError` into `AppError` easily
impl From<ConfigError> for AppError {
    fn from(err: ConfigError) -> Self {
        AppError::Config(err)
    }
}

// --- 2. Define a struct for our configuration to improve modularity ---
// This encapsulates all configuration parameters into a single, self-contained unit.
#[derive(Debug)]
struct AppConfig {
    my_setting: u32,
    // Potentially other settings here
}

// --- 3. Refactor functions for specific concerns and proper error handling ---

/// Reads the content of a file from the given path.
/// Returns `Result<String, ConfigError>` to explicitly handle I/O errors.
fn read_file_content(path: &str) -> Result<String, ConfigError> {
    fs::read_to_string(path).map_err(ConfigError::Io) // Convert io::Error to ConfigError::Io
}

/// Parses the string content into an `AppConfig` struct.
/// Returns `Result<AppConfig, ConfigError>` to handle parsing-specific errors.
fn parse_config_content(content: &str) -> Result<AppConfig, ConfigError> {
    if content.trim().is_empty() {
        return Err(ConfigError::EmptyFile);
    }

    let mut my_setting_found = false;
    let mut parsed_setting = 0;

    for line in content.lines() {
        let line = line.trim();
        if line.is_empty() || line.starts_with('#') { // Ignore empty lines and comments
            continue;
        }

        let parts: Vec<&str> = line.splitn(2, '=').collect();
        if parts.len() != 2 {
            return Err(ConfigError::InvalidFormat(format!("Line malformed: {}", line)));
        }

        let key = parts[0].trim();
        let value = parts[1].trim();

        match key {
            "my_setting" => {
                parsed_setting = value.parse()?; // Uses `?` operator for ParseIntError conversion via `From` trait
                my_setting_found = true;
            }
            // Handle other settings here with their own validation/parsing logic
            _ => {
                eprintln!("Warning: Unknown config key '{}' in config file.", key);
            }
        }
    }

    if !my_setting_found {
        return Err(ConfigError::NotFound("my_setting".to_string()));
    }

    Ok(AppConfig { my_setting: parsed_setting })
}

/// Performs the main application operation using the parsed configuration.
/// Returns `Result<(), AppError>` to handle potential operational failures.
fn perform_application_operation(config: &AppConfig) -> Result<(), AppError> {
    println!("Configuration 'my_setting' value: {}", config.my_setting);
    println!("Performing operation with setting: {}", config.my_setting * 2);

    // Simulate a potential failure in the operation based on config values
    if config.my_setting == 0 {
        return Err(AppError::ProcessingFailure("Setting cannot be zero for this operation.".to_string()));
    }

    // More complex operations would go here...

    Ok(())
}

// --- 4. Main function orchestrates and handles top-level errors gracefully ---
// The `main` function now returns `Result<(), AppError>`, allowing the application to
// gracefully exit and print detailed error messages without panicking.
fn main() -> Result<(), AppError> {
    let args: Vec<String> = env::args().collect();
    if args.len() < 2 {
        // Instead of panic!, return a specific error that `main` can handle gracefully.
        return Err(AppError::MissingArguments);
    }
    let file_path = &args[1];

    // Each step is now a function call returning a Result.
    // The `?` operator propagates errors up to `main`.
    let content = read_file_content(file_path)?; // Handles file I/O errors
    let config = parse_config_content(&content)?; // Handles config parsing errors

    perform_application_operation(&config)?; // Handles operation-specific errors

    println!("Application finished successfully.");
    Ok(())
}

/*
Example `config.ini` file content for testing:
# This is a comment
my_setting = 123
other_setting = hello_world
*/

// To run this code:
// 1. Save it as `main.rs`.
// 2. Create a file named `config.ini` in the same directory with content like above.
// 3. Compile: `rustc main.rs`
// 4. Run: `./main config.ini`
// 5. Test error cases:
//    - `./main` (MissingArguments)
//    - `./main non_existent.ini` (Io error)
//    - `./main empty.ini` (EmptyFile, if `empty.ini` is empty)
//    - `./main bad_format.ini` (InvalidFormat, if `bad_format.ini` has `key value`)
//    - `./main no_my_setting.ini` (NotFound, if `no_my_setting.ini` lacks 'my_setting')
//    - `./main bad_value.ini` (Parse, if 'my_setting = abc')
//    - (To trigger ProcessingFailure, modify `my_setting` in `config.ini` to 0 and re-run.)
```