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 stdoutWHY: Pipeable output, scriptabilityRUST: eprintln! for errors, println! for dataConfiguration Priority
RULE: CLI args > env vars > config file > defaultsWHY: User expectation, override capabilityRUST: Layered config with clap + figment/configExit Codes
RULE: Return non-zero on any errorWHY: Script integration, automationRUST: 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: ProgressBarKey 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
rust
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 |
