Lexopt

Lexopt is an argument parser for Rust. It tries to have the simplest possible design that's still correct. It's so simple that it's a bit tedious to use.

Lexopt is: - Small: one file, no dependencies, no macros. Easy to audit or vendor. - Correct: standard conventions are supported and ambiguity is avoided. Tested and fuzzed. - Pedantic: arguments are returned as OsStrings, forcing you to convert them explicitly. This lets you handle badly-encoded filenames. - Imperative: options are returned as they are found, nothing is declared ahead of time. - Minimalist: only basic functionality is provided. - Unhelpful: there is no help generation and error messages often lack context.

Example

struct Args {
    thing: String,
    number: u32,
    shout: bool,
}

fn parse_args() -> Result<Args, lexopt::Error> {
    use lexopt::prelude::*;

    let mut thing = None;
    let mut number = 1;
    let mut shout = false;
    let mut parser = lexopt::Parser::from_env();
    while let Some(arg) = parser.next()? {
        match arg {
            Short('n') | Long("number") => {
                number = parser.value()?.parse()?;
            }
            Long("shout") => {
                shout = true;
            }
            Value(val) if thing.is_none() => {
                thing = Some(val.string()?);
            }
            Long("help") => {
                println!("Usage: hello [-n|--number=NUM] [--shout] THING");
                std::process::exit(0);
            }
            _ => return Err(arg.unexpected()),
        }
    }

    Ok(Args {
        thing: thing.ok_or("missing argument THING")?,
        number,
        shout,
    })
}

fn main() -> Result<(), lexopt::Error> {
    let args = parse_args()?;
    let mut message = format!("Hello {}", args.thing);
    if args.shout {
        message = message.to_uppercase();
    }
    for _ in 0..args.number {
        println!("{}", message);
    }
    Ok(())
}

Let's walk through this: - We start parsing with Parser::from_env(). - We call parser.next() in a loop to get all the arguments until they run out. - We match on arguments. Short and Long indicate an option. - To get the value that belongs to an option (like 10 in -n 10) we call parser.value(). - This returns a standard OsString. - For convenience, use lexopt::prelude::* adds a .parse() method, analogous to the one on &str. - Calling parser.value() is how we tell Parser that -n takes a value at all. - Value indicates a free-standing argument. - if thing.is_none() is a useful pattern for positional arguments. If we already found thing we pass it on to another case. - It also contains an OsString. - The .string() method decodes it into a plain String. - If we don't know what to do with an argument we use return Err(arg.unexpected()) to turn it into an error message. - Strings can be promoted to errors for custom error messages.

This covers most of the functionality in the library. Lexopt does very little for you.

For a larger example with useful patterns, see examples/cargo.rs.

Command line syntax

The following conventions are supported: - Short options (-q) - Long options (--verbose) - -- to mark the end of options - = to separate options from values (--option=value, -o=value) - The nonstandard -o=value syntax can be disabled. - Spaces to separate options from values (--option value, -o value) - Unseparated short options (-ovalue) - Combined short options (-abc to mean -a -b -c) - Options with optional arguments (like GNU sed's -i, which can be used standalone or as -iSUFFIX) (Parser::optional_value()) - Options with multiple arguments (Parser::values())

These are not supported out of the box: - Single-dash long options (like find's -name) - Abbreviated long options (GNU's getopt lets you write --num instead of --number if it can be expanded unambiguously)

Parser::raw_args() and Parser::try_raw_args() provide an escape hatch for consuming the original command line. This can be used for custom syntax, like treating -123 as a number instead of a string of options. See examples/nonstandard.rs for an example of this.

Unicode

This library supports unicode while tolerating non-unicode arguments.

Short options may be unicode, but only a single codepoint (a char).

Options can be combined with non-unicode arguments. That is, --option=��� will not cause an error or mangle the value.

Options themselves are patched as by String::from_utf8_lossy if they're not valid unicode. That typically means you'll raise an error later when they're not recognized.

Why?

For a particular application I was looking for a small parser that's pedantically correct. There are other compact argument parsing libraries, but I couldn't find one that handled OsStrings and implemented all the fiddly details of the argument syntax faithfully.

This library may also be useful if a lot of control is desired, like when the exact argument order matters or not all options are known ahead of time. It could be considered more of a lexer than a parser.

Why not?

This library may not be worth using if: - You don't care about non-unicode arguments - You don't care about exact compliance and correctness - You don't care about code size - You do care about great error messages - You hate boilerplate

See also

• Collected benchmarks of argument parsing crates.
• libc's getopt.
• Plan 9's arg(3) macros.