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<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(()) }
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