CLI Domain

Layer 3: Domain Constraints

Domain Constraints → Design Implications Domain Rule Design Constraint Rust Implication User ergonomics Clear help, errors clap derive macros Config precedence CLI > env > file Layered config loading Exit codes Non-zero on error Proper Result handling Stdout/stderr Data vs errors eprintln! for errors Interruptible Handle Ctrl+C Signal handling Critical Constraints User Communication RULE: Errors to stderr, data to stdout WHY: Pipeable output, scriptability RUST: eprintln! for errors, println! for data

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

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

Trace Down ↓

From constraints to design (Layer 2):

"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

Key Crates Purpose Crate Argument parsing clap Interactive prompts dialoguer Progress bars indicatif Colored output colored Terminal UI ratatui Terminal control crossterm Console utilities console Design Patterns Pattern Purpose Implementation Args struct Type-safe args #[derive(Parser)] Subcommands Command hierarchy #[derive(Subcommand)] Config layers Override precedence CLI > env > file Progress User feedback ProgressBar::new(len) Code Pattern: CLI Structure 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, }, }

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(()) }

Common Mistakes Mistake Domain Violation Fix Errors to stdout Breaks piping eprintln! No help text Poor UX #[arg(help = "...")] Panic on error Bad exit code Result + proper handling No progress for long ops User uncertainty indicatif Trace to Layer 1 Constraint Layer 2 Pattern Layer 1 Implementation Type-safe args Derive macros clap Parser Error handling Result propagation anyhow + exit codes User feedback Progress RAII indicatif ProgressBar Config precedence Builder pattern Layered sources Related Skills When See Error handling m06-error-handling Type-driven args m05-type-driven Progress lifecycle m12-lifecycle Async CLI m07-concurrency