Domain Cli

# 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

```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 |