mfmt

Meta formatter library in Rust.

mfmt is a language formatter library written in Rust inspired by go fmt. It's designed to be configuration-free and generous about styling. It simply focuses on aligning indentations.

This library is used in the following projects.

• schemat, the Scheme formatter
• Pen programming language

Install

cargo add mfmt

Examples

use bumpalo::Bump;
use indoc::indoc;
use mfmt::{Builder, format, FormatOptions, line};

let allocator = Bump::new();
let builder = Builder::new(&allocator);

let mut string = String::new();

format(
    &builder.sequence([
        "{".into(),
        builder.indent(builder.sequence([line(), "foo".into(), line(), "bar".into()])),
        line(),
        "}".into(),
    ]),
    &mut string,
    FormatOptions::new(4),
)
.unwrap();

assert_eq!(
    string,
    indoc!(
        "
        {
            foo
            bar
        }
        "
    )
    .trim(),
);

Technical notes

Unlike the Wadler's algorithm or some other formatters like prettier, mfmt does not search the best format given source codes. For example, we do not have any "group" combinator. Instead, we rather give a formatter information to reconstruct the "best" format that is available in the original source codes.

References

• A prettier printer by Philip Wadler

License

The Unlicense