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#40 in Command-line interface

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xflags provides a procedural macro for parsing command line arguments.

It is intended for use in development tools, so it emphasizes fast compile times and convenience at the expense of features.

Rough decision tree for picking an argument parsing library:

  • if you need all of the features and don't care about minimalism, use clap
  • if you want to be maximally minimal, need only basic features (eg, no help generation), and want to be pedantically correct, use lexopt
  • if you want to get things done fast (eg, you want auto help, but not at the cost of waiting for syn to compile), consider this crate.

The secret sauce of xflags is that it is the opposite of a derive macro. Rather than generating a command line grammar from a Rust struct, xflags generates Rust structs based on input grammar. The grammar definition is both shorter and simpler to write, and is lighter on compile times.

Here's a complete example of parse_or_exit! macro which parses arguments into an "anonymous" struct:

use std::path::PathBuf;

fn main() {
    let flags = xflags::parse_or_exit! {
        /// Remove directories and their contents recursively.
        optional -r,--recursive
        /// File or directory to remove
        required path: PathBuf
    };

    println!(
        "removing {}{}",
        flags.path.display(),
        if flags.recursive { "recursively" } else { "" },
    )
}

The above program, when run with --help argument, generates the following help:

Usage:  <path> [-r] [-h]
Arguments:
  <path>               File or directory to remove

Options:
  -r, --recursive      Remove directories and their contents recursively.
  -h, --help           Prints help

Commands:
  help                 Print this message or the help of the given subcommand(s)

For larger programs, you'd typically want to use xflags! macro, which generates named structs for you. Unlike a typical macro, xflags writes generated code into the source file, to make it easy to understand the rust types at a glance.

mod flags {
    use std::path::PathBuf;

    xflags::xflags! {
        src "./examples/basic.rs"

        cmd my-command {
            required path: PathBuf
            optional -v, --verbose
        }
    }

    // generated start
    // The following code is generated by `xflags` macro.
    // Run `env UPDATE_XFLAGS=1 cargo build` to regenerate.
    #[derive(Debug)]
    pub struct MyCommand {
        pub path: PathBuf,
        pub verbose: bool,
    }

    impl MyCommand {
        pub fn from_env_or_exit() -> Self {
            Self::from_env_or_exit_()
        }
        pub fn from_env() -> xflags::Result<Self> {
            Self::from_env_()
        }
        pub fn from_vec(args: Vec<std::ffi::OsString>) -> xflags::Result<Self> {
            Self::from_vec_(args)
        }
    }
   // generated end
}

fn main() {
    let flags = flags::MyCommand::from_env();
    println!("{:#?}", flags);
}

If you'd rather use a typical proc-macro which generates hidden code, just omit the src attribute.

xflags correctly handles non-utf8 arguments.

Syntax Reference

The xflags! macro uses cmd keyword to introduce a command or subcommand that accepts positional arguments and switches.

xflags::xflags! {
    cmd command-name { }
}

Switches are specified inside the curly braces. Long names (--switch) are mandatory, short names (-s) are optional. Each switch can be optional, required, or repeated. Dashes are allowed in switch names.

xflags::xflags! {
    cmd switches {
        optional -q,--quiet
        required --pass-me
        repeated --verbose
    }
}

Switches can also take values. If the value type is OsString or PathBuf, it is created directly from the underlying argument. Otherwise, FromStr is used for parsing

use std::{path::PathBuf, ffi::OsString};

xflags::xflags! {
    cmd switches-with-values {
        optional --config path: PathBuf
        repeated --data val: OsString
        optional -j, --jobs n: u32
    }
}

Arguments without -- in then are are positional.

use std::{path::PathBuf, ffi::OsString};

xflags::xflags! {
    cmd positional-arguments {
        required program: PathBuf
        repeated args: OsString
    }
}

You can create aliases if desired, which is as simple as adding extra names to the cmd definition. In this case, run can be called as run, r and exec:

xflags::xflags! {
    cmd run r exec {}
}

Nesting cmd is allowed. xflag automatically generates boilerplate enums for subcommands:

xflags::xflags! {
    src "./examples/subcommands.rs"
    cmd app {
        repeated -v, --verbose
        cmd foo { optional -s, --switch }
        cmd bar {}
    }
}

// generated start
// The following code is generated by `xflags` macro.
// Run `env UPDATE_XFLAGS=1 cargo build` to regenerate.
#[derive(Debug)]
pub struct App {
    pub verbose: u32,
    pub subcommand: AppCmd,
}

#[derive(Debug)]
pub enum AppCmd {
    Foo(Foo),
    Bar(Bar),
}

#[derive(Debug)]
pub struct Foo {
    pub switch: bool,
}

#[derive(Debug)]
pub struct Bar {
}

impl App {
    pub fn from_env_or_exit() -> Self {
        Self::from_env_or_exit_()
    }
    pub fn from_env() -> xflags::Result<Self> {
        Self::from_env_()
    }
    pub fn from_vec(args: Vec<std::ffi::OsString>) -> xflags::Result<Self> {
        Self::from_vec_(args)
    }
}
// generated end

Switches are always "inherited". Both app -v foo and app foo -v produce the same result.

To make subcommand name optional use the default keyword to mark a subcommand to select if no subcommand name is passed. The name of the default subcommand affects only the name of the generated Rust struct, it can't be specified explicitly on the command line.

xflags::xflags! {
    cmd app {
        repeated -v, --verbose
        default cmd foo { optional -s, --switch }
        cmd bar {}
    }
}

Commands, arguments, and switches can be documented. Doc comments become a part of generated help:

mod flags {
    use std::path::PathBuf;

    xflags::xflags! {
        /// Run basic system diagnostics.
        cmd healthck {
            /// Optional configuration file.
            optional config: PathBuf
            /// Verbosity level, can be repeated multiple times.
            repeated -v, --verbose
        }
    }
}

fn main() {
    match flags::Healthck::from_env() {
        Ok(flags) => {
            run_checks(flags.config, flags.verbose);
        }
        Err(err) => err.exit()
    }
}

The src keyword controls how the code generation works. If it is absent, xflags acts as a typical procedure macro, which generates a bunch of structs and impls.

If the src keyword is present, it should specify the path to the file with xflags! invocation. The path should be relative to the directory with Cargo.toml. The macro then will avoid generating the structs. Instead, if the UPDATE_XFLAGS environmental variable is set, the macro will write them directly to the specified file.

By convention, xflag! macro should be invoked from the flags submodule. The flags:: prefix should be used to refer to command names. Additional validation logic can go to the flags module:

mod flags {
    xflags::xflags! {
        cmd my-command {
            repeated -v, --verbose
            optional -q, --quiet
        }
    }

    impl MyCommand {
        fn validate(&self) -> xflags::Result<()> {
            if self.quiet && self.verbose > 0 {
                return Err(xflags::Error::new(
                    "`-q` and `-v` can't be specified at the same time"
                ));
            }
            Ok(())
        }
    }
}

The parse_or_exit! macro is a syntactic sure for xflags!, which immediately parses the argument, exiting the process if needed. parse_or_exit only supports single top-level command and doesn't need the cmd keyword.

Limitations

xflags follows Fuchsia conventions for command line arguments. GNU conventions such as grouping short-flags (-xyz) or gluing short flag and a value (-fVAL) are not supported.

xflags requires the command line interface to be fully static. It's impossible to include additional flags at runtime.

Implementation is not fully robust, there might be some residual bugs in edge cases.

Dependencies