RULE: Errors to stderr, data to stdout
WHY: Pipeable output, scriptability
RUST: eprintln! for errors, println! for data

RULE: CLI args > env vars > config file > defaults
WHY: User expectation, override capability
RUST: Layered config with clap + figment/config

RULE: Return non-zero on any error
WHY: Script integration, automation
RUST: main() -> Result<(), Error> or explicit exit()

"Need argument parsing"
    ↓ m05-type-driven: Derive structs for args
    ↓ clap: #[derive(Parser)]
 
"Need config layering"
    ↓ m09-domain: Config as domain object
    ↓ figment/config: Layer sources
 
"Need progress display"
    ↓ m12-lifecycle: Progress bar as RAII
    ↓ indicatif: ProgressBar

use clap::{Parser, Subcommand};
 
#[derive(Parser)]
#[command(name = "myapp", about = "My CLI tool")]
struct Cli {
    /// Enable verbose output
    #[arg(short, long)]
    verbose: bool,
 
    #[command(subcommand)]
    command: Commands,
}
 
#[derive(Subcommand)]
enum Commands {
    /// Initialize a new project
    Init { name: String },
    /// Run the application
    Run {
        #[arg(short, long)]
        port: Option<u16>,
    },
}
 
fn main() -> anyhow::Result<()> {
    let cli = Cli::parse();
    match cli.command {
        Commands::Init { name } => init_project(&name)?,
        Commands::Run { port } => run_server(port.unwrap_or(8080))?,
    }
    Ok(())
}