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

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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()

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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<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(())
}

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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