Clap Patterns

clap-patterns

Provides modern type-safe Rust CLI patterns using Clap 4.x with derive macros, Parser trait, Subcommand enums, custom validation, value parsers, and environment variable integration for building maintainable command-line applications.

Core Patterns

1. Basic Parser with Derive Macros

Use derive macros for automatic CLI parsing with type safety:

use clap::Parser;

#[derive(Parser)]
#[command(name = "myapp")]
#[command(author, version, about, long_about = None)]
struct Cli {
    /// Input file path
    #[arg(short, long, value_name = "FILE")]
    input: std::path::PathBuf,

    /// Optional output file
    #[arg(short, long)]
    output: Option<std::path::PathBuf>,

    /// Verbose mode
    #[arg(short, long)]
    verbose: bool,

    /// Number of items to process
    #[arg(short, long, default_value_t = 10)]
    count: usize,
}

fn main() {
    let cli = Cli::parse();
    if cli.verbose {
        println!("Processing: {:?}", cli.input);
    }
}

2. Subcommand Enums

Organize complex CLIs with nested subcommands:

use clap::{Parser, Subcommand};

#[derive(Parser)]
#[command(name = "git")]
struct Cli {
    #[command(subcommand)]
    command: Commands,
}

#[derive(Subcommand)]
enum Commands {
    /// Add files to staging
    Add {
        /// Files to add
        #[arg(value_name = "FILE")]
        files: Vec<String>,
    },
    /// Commit changes
    Commit {
        /// Commit message
        #[arg(short, long)]
        message: String,
    },
}

3. Value Parsers and Validation

Implement custom parsing and validation:

use clap::Parser;
use std::ops::RangeInclusive;

const PORT_RANGE: RangeInclusive<usize> = 1..=65535;

fn port_in_range(s: &str) -> Result<u16, String> {
    let port: usize = s
        .parse()
        .map_err(|_| format!("`{s}` isn't a valid port number"))?;
    if PORT_RANGE.contains(&port) {
        Ok(port as u16)
    } else {
        Err(format!("port not in range {}-{}", PORT_RANGE.start(), PORT_RANGE.end()))
    }
}

#[derive(Parser)]
struct Cli {
    /// Port to listen on
    #[arg(short, long, value_parser = port_in_range)]
    port: u16,
}

4. Environment Variable Integration

Support environment variables with fallback:

use clap::Parser;

#[derive(Parser)]
struct Cli {
    /// API key (or set API_KEY env var)
    #[arg(long, env = "API_KEY")]
    api_key: String,

    /// Database URL
    #[arg(long, env = "DATABASE_URL")]
    database_url: String,

    /// Optional log level
    #[arg(long, env = "LOG_LEVEL", default_value = "info")]
    log_level: String,
}

5. ValueEnum for Constrained Choices

Use ValueEnum for type-safe option selection:

use clap::{Parser, ValueEnum};

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum)]
enum Format {
    Json,
    Yaml,
    Toml,
}

#[derive(Parser)]
struct Cli {
    /// Output format
    #[arg(value_enum, short, long, default_value_t = Format::Json)]
    format: Format,
}

Available Templates

The following Rust templates demonstrate Clap patterns:

• basic-parser.rs: Simple CLI with Parser derive macro
• subcommands.rs: Multi-level subcommand structure
• value-parser.rs: Custom validation with value parsers
• env-variables.rs: Environment variable integration
• value-enum.rs: Type-safe enums for options
• builder-pattern.rs: Manual builder API (for complex cases)
• full-featured-cli.rs: Complete CLI with all patterns

Available Scripts

Helper scripts for Clap development:

• generate-completions.sh: Generate shell completions (bash, zsh, fish)
• validate-cargo.sh: Check Cargo.toml for correct Clap dependencies
• test-cli.sh: Test CLI with various argument combinations

Usage Instructions

1. Choose the appropriate template based on your CLI complexity:

   • Simple single-command → basic-parser.rs
   • Multiple subcommands → subcommands.rs
   • Need validation → value-parser.rs
   • Environment config → env-variables.rs

2. Add Clap to Cargo.toml:

   [dependencies]
   clap = { version = "4.5", features = ["derive", "env"] }
   

3. Implement your CLI using the selected template as a starting point

4. Generate completions using the provided script for better UX

Best Practices

• Use derive macros for most cases (cleaner, less boilerplate)
• Add help text with doc comments (shows in --help)
• Validate early with value parsers
• Use ValueEnum for constrained choices
• Support environment variables for sensitive data
• Provide sensible defaults with default_value_t
• Use PathBuf for file/directory arguments
• Add version and author metadata

Common Patterns

Multiple Values

#[arg(short, long, num_args = 1..)]
files: Vec<PathBuf>,

Required Unless Present

#[arg(long, required_unless_present = "config")]
database_url: Option<String>,

Conflicting Arguments

#[arg(long, conflicts_with = "json")]
yaml: bool,

Global Arguments (for subcommands)

#[arg(global = true, short, long)]
verbose: bool,

Testing Your CLI

Run the test script to validate your CLI:

bash scripts/test-cli.sh your-binary

This tests:

• Help output (--help)
• Version flag (--version)
• Invalid arguments
• Subcommand routing
• Environment variable precedence

References

• Templates: skills/clap-patterns/templates/
• Scripts: skills/clap-patterns/scripts/
• Examples: skills/clap-patterns/examples/
• Clap Documentation: https://docs.rs/clap/latest/clap/