# Lexopt

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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 [`OsString`](https://doc.rust-lang.org/std/ffi/struct.OsString.html)s, 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
```rust
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`](https://doc.rust-lang.org/std/ffi/struct.OsString.html).
  - For convenience, `use lexopt::prelude::*` adds a `.parse()` method, analogous to [`str::parse`](https://doc.rust-lang.org/std/primitive.str.html#method.parse).
  - 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`](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`)
- 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()`](https://docs.rs/lexopt/latest/lexopt/struct.Parser.html#method.optional_value))
- Options with multiple arguments ([`Parser::values()`](https://docs.rs/lexopt/latest/lexopt/struct.Parser.html#method.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()`](https://docs.rs/lexopt/latest/lexopt/struct.Parser.html#method.raw_args) and [`Parser::try_raw_args()`](https://docs.rs/lexopt/latest/lexopt/struct.Parser.html#method.try_raw_args) offer an escape hatch for consuming the original command line. See [`examples/nonstandard.rs`](examples/nonstandard.rs) for an example of parsing non-standard option syntax.

## 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`](https://doc.rust-lang.org/std/string/struct.String.html#method.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 `OsString`s 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](https://github.com/rosetta-rs/argparse-rosetta-rs).
- libc's [`getopt`](https://en.wikipedia.org/wiki/Getopt#Examples).
- Plan 9's [*arg(3)* macros](https://9fans.github.io/plan9port/man/man3/arg.html).