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

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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 |

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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 |

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Related Skills

| When | See | |------|-----| | Error handling | m06-error-handling | | Type-driven args | m05-type-driven | | Progress lifecycle | m12-lifecycle | | Async CLI | m07-concurrency |