Agent skill

clap-patterns

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

View SKILL.md on GitHub Repository
Stars 163
Forks 31

Install this agent skill to your Project

npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/clap-patterns

SKILL.md

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
rust
#[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
rust
fn build_cli() -> Command {
    Command::new("app")
        .arg(Arg::new("input").short('i'))
}

Common Patterns

Global Options with Subcommands

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

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

Argument Groups for Mutual Exclusivity

rust
#[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

rust
fn 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

rust
#[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

rust
#[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

rust
#[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

rust
#[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)

rust
#[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

rust
#[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

rust
#[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

rust
fn 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

Didn't find tool you were looking for?

Be as detailed as possible for better results