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. - Annoyingly minimalist: only the barest necessities are provided. - Unhelpful: there is no help generation and error messages often lack context.

Example

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

fn parse_args() -> Result { 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.into_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 = parseargs()?; let mut message = format!("Hello {}", args.thing); if args.shout { message = message.touppercase(); } 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 str::parse. - Value indicates a free-standing argument. In this case, a filename. - 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 standard .into_string() method can decode 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 almost all 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 long options from values (--option=value) - Spaces to separate options from values (--option value, -f value) - Unseparated short options (-fvalue) - Combined short options (-abc to mean -a -b -c)

These are not supported: - -f=value for short options - Options with optional arguments (like GNU sed's -i, which can be used standalone or as -iSUFFIX) - 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)

Unicode

This library supports unicode while tolerating non-unicode arguments.

Short options may be unicode, but only a single codepoint. (If you need whole grapheme clusters you can use a long option. If you need normalization you're on your own, but it can be done.)

Options can be combined with non-unicode arguments. That is, --option=�� will not cause an error or mangle the value. This is surprisingly tricky to support: see os_str_bytes.

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?