Module clap::_derive::_tutorial::chapter_3

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§Validation

  1. Enumerated values
  2. Validated values
  3. Argument Relations
  4. Custom Validation

An appropriate default parser/validator will be selected for the field’s type. See value_parser! for more details.

§Enumerated values

For example, if you have arguments of specific values you want to test for, you can derive ValueEnum (any PossibleValue builder function can be used as a #[value] attribute on enum variants).

This allows you specify the valid values for that argument. If the user does not use one of those specific values, they will receive a graceful exit with error message informing them of the mistake, and what the possible valid values are

use clap::{Parser, ValueEnum};

#[derive(Parser)]
#[command(version, about, long_about = None)]
struct Cli {
    /// What mode to run the program in
    #[arg(value_enum)]
    mode: Mode,
}

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum)]
enum Mode {
    /// Run swiftly
    Fast,
    /// Crawl slowly but steadily
    ///
    /// This paragraph is ignored because there is no long help text for possible values.
    Slow,
}

fn main() {
    let cli = Cli::parse();

    match cli.mode {
        Mode::Fast => {
            println!("Hare");
        }
        Mode::Slow => {
            println!("Tortoise");
        }
    }
}
$ 04_01_enum_derive --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_01_enum_derive[EXE] <MODE>

Arguments:
  <MODE>
          What mode to run the program in

          Possible values:
          - fast: Run swiftly
          - slow: Crawl slowly but steadily

Options:
  -h, --help
          Print help (see a summary with '-h')

  -V, --version
          Print version

$ 04_01_enum_derive -h
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_01_enum_derive[EXE] <MODE>

Arguments:
  <MODE>  What mode to run the program in [possible values: fast, slow]

Options:
  -h, --help     Print help (see more with '--help')
  -V, --version  Print version

$ 04_01_enum_derive fast
Hare

$ 04_01_enum_derive slow
Tortoise

$ 04_01_enum_derive medium
? failed
error: invalid value 'medium' for '<MODE>'
  [possible values: fast, slow]

For more information, try '--help'.

§Validated values

More generally, you can validate and parse into any data type with Arg::value_parser.

use clap::Parser;

#[derive(Parser)]
#[command(version, about, long_about = None)]
struct Cli {
    /// Network port to use
    #[arg(value_parser = clap::value_parser!(u16).range(1..))]
    port: u16,
}

fn main() {
    let cli = Cli::parse();

    println!("PORT = {}", cli.port);
}
$ 04_02_parse_derive --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_02_parse_derive[EXE] <PORT>

Arguments:
  <PORT>  Network port to use

Options:
  -h, --help     Print help
  -V, --version  Print version

$ 04_02_parse_derive 22
PORT = 22

$ 04_02_parse_derive foobar
? failed
error: invalid value 'foobar' for '<PORT>': invalid digit found in string

For more information, try '--help'.

$ 04_02_parse_derive 0
? failed
error: invalid value '0' for '<PORT>': 0 is not in 1..=65535

For more information, try '--help'.

A custom parser can be used to improve the error messages or provide additional validation:

use std::ops::RangeInclusive;

use clap::Parser;

#[derive(Parser)]
#[command(version, about, long_about = None)]
struct Cli {
    /// Network port to use
    #[arg(value_parser = port_in_range)]
    port: u16,
}

fn main() {
    let cli = Cli::parse();

    println!("PORT = {}", cli.port);
}

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 port number"))?;
    if PORT_RANGE.contains(&port) {
        Ok(port as u16)
    } else {
        Err(format!(
            "port not in range {}-{}",
            PORT_RANGE.start(),
            PORT_RANGE.end()
        ))
    }
}
$ 04_02_validate_derive --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_02_validate_derive[EXE] <PORT>

Arguments:
  <PORT>  Network port to use

Options:
  -h, --help     Print help
  -V, --version  Print version

$ 04_02_validate_derive 22
PORT = 22

$ 04_02_validate_derive foobar
? failed
error: invalid value 'foobar' for '<PORT>': `foobar` isn't a port number

For more information, try '--help'.

$ 04_02_validate_derive 0
? failed
error: invalid value '0' for '<PORT>': port not in range 1-65535

For more information, try '--help'.

See Arg::value_parser for more details.

§Argument Relations

You can declare dependencies or conflicts between Args or even ArgGroups.

ArgGroups make it easier to declare relations instead of having to list each individually, or when you want a rule to apply “any but not all” arguments.

Perhaps the most common use of ArgGroups is to require one and only one argument to be present out of a given set. Imagine that you had multiple arguments, and you want one of them to be required, but making all of them required isn’t feasible because perhaps they conflict with each other.

ArgGroups are automatically created for a struct with its ArgGroup::id being the struct’s name.

use clap::{Args, Parser};

#[derive(Parser)]
#[command(version, about, long_about = None)]
struct Cli {
    #[command(flatten)]
    vers: Vers,

    /// some regular input
    #[arg(group = "input")]
    input_file: Option<String>,

    /// some special input argument
    #[arg(long, group = "input")]
    spec_in: Option<String>,

    #[arg(short, requires = "input")]
    config: Option<String>,
}

#[derive(Args)]
#[group(required = true, multiple = false)]
struct Vers {
    /// set version manually
    #[arg(long, value_name = "VER")]
    set_ver: Option<String>,

    /// auto inc major
    #[arg(long)]
    major: bool,

    /// auto inc minor
    #[arg(long)]
    minor: bool,

    /// auto inc patch
    #[arg(long)]
    patch: bool,
}

fn main() {
    let cli = Cli::parse();

    // Let's assume the old version 1.2.3
    let mut major = 1;
    let mut minor = 2;
    let mut patch = 3;

    // See if --set_ver was used to set the version manually
    let vers = &cli.vers;
    let version = if let Some(ver) = vers.set_ver.as_deref() {
        ver.to_string()
    } else {
        // Increment the one requested (in a real program, we'd reset the lower numbers)
        let (maj, min, pat) = (vers.major, vers.minor, vers.patch);
        match (maj, min, pat) {
            (true, _, _) => major += 1,
            (_, true, _) => minor += 1,
            (_, _, true) => patch += 1,
            _ => unreachable!(),
        };
        format!("{major}.{minor}.{patch}")
    };

    println!("Version: {version}");

    // Check for usage of -c
    if let Some(config) = cli.config.as_deref() {
        let input = cli
            .input_file
            .as_deref()
            .unwrap_or_else(|| cli.spec_in.as_deref().unwrap());
        println!("Doing work using input {input} and config {config}");
    }
}
$ 04_03_relations_derive --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_03_relations_derive[EXE] [OPTIONS] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

Arguments:
  [INPUT_FILE]  some regular input

Options:
      --set-ver <VER>      set version manually
      --major              auto inc major
      --minor              auto inc minor
      --patch              auto inc patch
      --spec-in <SPEC_IN>  some special input argument
  -c <CONFIG>              
  -h, --help               Print help
  -V, --version            Print version

$ 04_03_relations_derive
? failed
error: the following required arguments were not provided:
  <--set-ver <VER>|--major|--minor|--patch>

Usage: 04_03_relations_derive[EXE] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

For more information, try '--help'.

$ 04_03_relations_derive --major
Version: 2.2.3

$ 04_03_relations_derive --major --minor
? failed
error: the argument '--major' cannot be used with '--minor'

Usage: 04_03_relations_derive[EXE] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

For more information, try '--help'.

$ 04_03_relations_derive --major -c config.toml
? failed
error: the following required arguments were not provided:
  <INPUT_FILE|--spec-in <SPEC_IN>>

Usage: 04_03_relations_derive[EXE] -c <CONFIG> <--set-ver <VER>|--major|--minor|--patch> <INPUT_FILE|--spec-in <SPEC_IN>>

For more information, try '--help'.

$ 04_03_relations_derive --major -c config.toml --spec-in input.txt
Version: 2.2.3
Doing work using input input.txt and config config.toml

§Custom Validation

As a last resort, you can create custom errors with the basics of clap’s formatting.

use clap::error::ErrorKind;
use clap::{CommandFactory, Parser};

#[derive(Parser)]
#[command(version, about, long_about = None)]
struct Cli {
    /// set version manually
    #[arg(long, value_name = "VER")]
    set_ver: Option<String>,

    /// auto inc major
    #[arg(long)]
    major: bool,

    /// auto inc minor
    #[arg(long)]
    minor: bool,

    /// auto inc patch
    #[arg(long)]
    patch: bool,

    /// some regular input
    input_file: Option<String>,

    /// some special input argument
    #[arg(long)]
    spec_in: Option<String>,

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

fn main() {
    let cli = Cli::parse();

    // Let's assume the old version 1.2.3
    let mut major = 1;
    let mut minor = 2;
    let mut patch = 3;

    // See if --set-ver was used to set the version manually
    let version = if let Some(ver) = cli.set_ver.as_deref() {
        if cli.major || cli.minor || cli.patch {
            let mut cmd = Cli::command();
            cmd.error(
                ErrorKind::ArgumentConflict,
                "Can't do relative and absolute version change",
            )
            .exit();
        }
        ver.to_string()
    } else {
        // Increment the one requested (in a real program, we'd reset the lower numbers)
        let (maj, min, pat) = (cli.major, cli.minor, cli.patch);
        match (maj, min, pat) {
            (true, false, false) => major += 1,
            (false, true, false) => minor += 1,
            (false, false, true) => patch += 1,
            _ => {
                let mut cmd = Cli::command();
                cmd.error(
                    ErrorKind::ArgumentConflict,
                    "Can only modify one version field",
                )
                .exit();
            }
        };
        format!("{major}.{minor}.{patch}")
    };

    println!("Version: {version}");

    // Check for usage of -c
    if let Some(config) = cli.config.as_deref() {
        let input = cli
            .input_file
            .as_deref()
            // 'or' is preferred to 'or_else' here since `Option::as_deref` is 'const'
            .or(cli.spec_in.as_deref())
            .unwrap_or_else(|| {
                let mut cmd = Cli::command();
                cmd.error(
                    ErrorKind::MissingRequiredArgument,
                    "INPUT_FILE or --spec-in is required when using --config",
                )
                .exit()
            });
        println!("Doing work using input {input} and config {config}");
    }
}
$ 04_04_custom_derive --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_04_custom_derive[EXE] [OPTIONS] [INPUT_FILE]

Arguments:
  [INPUT_FILE]  some regular input

Options:
      --set-ver <VER>      set version manually
      --major              auto inc major
      --minor              auto inc minor
      --patch              auto inc patch
      --spec-in <SPEC_IN>  some special input argument
  -c <CONFIG>              
  -h, --help               Print help
  -V, --version            Print version

$ 04_04_custom_derive
? failed
error: Can only modify one version field

Usage: clap [OPTIONS] [INPUT_FILE]

For more information, try '--help'.

$ 04_04_custom_derive --major
Version: 2.2.3

$ 04_04_custom_derive --major --minor
? failed
error: Can only modify one version field

Usage: clap [OPTIONS] [INPUT_FILE]

For more information, try '--help'.

$ 04_04_custom_derive --major -c config.toml
? failed
Version: 2.2.3
error: INPUT_FILE or --spec-in is required when using --config

Usage: clap [OPTIONS] [INPUT_FILE]

For more information, try '--help'.

$ 04_04_custom_derive --major -c config.toml --spec-in input.txt
Version: 2.2.3
Doing work using input input.txt and config config.toml

Re-exports§