CLI Domain

Layer 3: Domain Constraints

Domain Constraints → Design Implications

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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

Design Patterns

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

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Trace to Layer 1

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Related Skills