---

name: clap-patterns description: Common Clap patterns and idioms for argument parsing, validation, and CLI design. Use when implementing CLI arguments with Clap v4+.

Clap Patterns Skill

Common patterns and idioms for using Clap v4+ effectively in Rust CLI applications.

Derive API vs Builder API

When to Use Derive API

• CLI structure known at compile time
• Want type safety and compile-time validation
• Prefer declarative style
• Standard CLI patterns are sufficient

Copy#[derive(Parser)]
#[command(version, about)]
struct Cli {
    #[arg(short, long)]
    input: PathBuf,
}

When to Use Builder API

• CLI needs to be built dynamically at runtime
• Building plugin systems
• Arguments depend on configuration
• Need maximum flexibility

Copyfn build_cli() -> Command {
    Command::new("app")
        .arg(Arg::new("input").short('i'))
}

Common Patterns

Global Options with Subcommands

Copy#[derive(Parser)]
struct Cli {
    #[arg(short, long, global = true, action = ArgAction::Count)]
    verbose: u8,

    #[command(subcommand)]
    command: Commands,
}

Argument Groups for Mutual Exclusivity

Copy#[derive(Parser)]
#[command(group(
    ArgGroup::new("format")
        .required(true)
        .args(&["json", "yaml", "toml"])
))]
struct Cli {
    #[arg(long)]
    json: bool,
    #[arg(long)]
    yaml: bool,
    #[arg(long)]
    toml: bool,
}

Custom Value Parsers

Copyfn parse_port(s: &str) -> Result<u16, String> {
    let port: u16 = s.parse()
        .map_err(|_| format!("`{s}` isn't a valid port"))?;
    if (1024..=65535).contains(&port) {
        Ok(port)
    } else {
        Err(format!("port not in range 1024-65535"))
    }
}

#[derive(Parser)]
struct Cli {
    #[arg(long, value_parser = parse_port)]
    port: u16,
}

Environment Variable Fallbacks

Copy#[derive(Parser)]
struct Cli {
    #[arg(long, env = "API_TOKEN")]
    token: String,

    #[arg(long, env = "API_ENDPOINT", default_value = "https://api.example.com")]
    endpoint: String,
}

Flattening Shared Options

Copy#[derive(Args)]
struct CommonOpts {
    #[arg(short, long)]
    verbose: bool,

    #[arg(short, long)]
    config: Option<PathBuf>,
}

#[derive(Parser)]
struct Cli {
    #[command(flatten)]
    common: CommonOpts,

    #[command(subcommand)]
    command: Commands,
}

Multiple Values

Copy#[derive(Parser)]
struct Cli {
    /// Tags (can be specified multiple times)
    #[arg(short, long)]
    tag: Vec<String>,

    /// Files to process
    files: Vec<PathBuf>,
}
// Usage: myapp --tag rust --tag cli file1.txt file2.txt

Subcommand with Shared Arguments

Copy#[derive(Parser)]
struct Cli {
    #[command(subcommand)]
    command: Commands,
}

#[derive(Subcommand)]
enum Commands {
    Build(BuildArgs),
    Test(TestArgs),
}

#[derive(Args)]
struct BuildArgs {
    #[command(flatten)]
    common: CommonOpts,

    #[arg(short, long)]
    release: bool,
}

Argument Counting (Verbosity Levels)

Copy#[derive(Parser)]
struct Cli {
    /// Verbosity (-v, -vv, -vvv)
    #[arg(short, long, action = ArgAction::Count)]
    verbose: u8,
}
// Usage: -v (1), -vv (2), -vvv (3)

Help Template Customization

Copy#[derive(Parser)]
#[command(
    after_help = "EXAMPLES:\n  \
        myapp --input file.txt\n  \
        myapp -i file.txt -vv\n\n\
        For more info: https://example.com"
)]
struct Cli {
    // ...
}

Value Hints

Copy#[derive(Parser)]
struct Cli {
    #[arg(short, long, value_name = "FILE", value_hint = ValueHint::FilePath)]
    input: PathBuf,

    #[arg(short, long, value_name = "DIR", value_hint = ValueHint::DirPath)]
    output: PathBuf,

    #[arg(short, long, value_name = "URL", value_hint = ValueHint::Url)]
    endpoint: String,
}

Default Values with Functions

Copyfn default_config_path() -> PathBuf {
    dirs::config_dir()
        .unwrap()
        .join("myapp")
        .join("config.toml")
}

#[derive(Parser)]
struct Cli {
    #[arg(long, default_value_os_t = default_config_path())]
    config: PathBuf,
}

Best Practices

1. Use value_name for clearer help text
2. Provide both short and long flags where appropriate
3. Add help text to all arguments
4. Use ValueEnum for fixed set of choices
5. Validate early with custom parsers
6. Support environment variables for sensitive data
7. Use argument groups for mutually exclusive options
8. Document with examples in after_help
9. Use semantic types (PathBuf, not String for paths)
10. Test CLI parsing with integration tests

References

• Clap Documentation
• Clap Derive Reference
• Clap Examples