debug3

A space effiecent replacement for [std::fmt::Debug]

The Pitch

Lets say you have data that looks like this:

let complex_structure = vec![
        vec![None, Some(2)],
        vec![Some(2), None],
        vec![Some(4), Some(777)],
        vec![None, Some(2)],
        vec![Some(2), None],
        vec![None, None, None, None, None],
];

And you want to format it as a string. You could use format!("{:?}", complex_structure) and get something like

[[None, Some(2)], [Some(2), None], [Some(4), Some(777)], [None, Some(2)], [Some(2), None], [None, None, None, None, None]]

But this is too much one one line, and is hard to read. And it gets worse for larger structures.

Fortunaly theirs an alternative format!("{:#?}", complex_structure), which gives

[
    [
        None,
        Some(
            2,
        ),
    ],
    [
        Some(
            2,
        ),
        None,
    ],
    [
        Some(
            4,
        ),
        Some(
            777,
        ),
    ],
    [
        None,
        Some(
            2,
        ),
    ],
    [
        Some(
            2,
        ),
        None,
    ],
    [
        None,
        None,
        None,
        None,
        None,
    ],
]

This has the oposite problem, where it uses too much space, even when the code could be packed denser.

debug3 provides a third option that is denser than :#? but more readable than :?. If you use debug3::pprint(complex_structure), you get

[
    [None, Some(2)],
    [Some(2), None],
    [Some(4), Some(777)],
    [None, Some(2)],
    [Some(2), None],
    [None, None, None, None, None],
]

Overview

The main entrypoint is the [Debug] trait, which is the equivalent to [std::fmt::Debug], and has a similar API.

This can be either #[derive]d, or implemented manually.

use debug3::{Debug, Formatter, pprint};

#[derive(Debug)]
struct MyStruct {
    a: i32,
    b: i32,
}

struct AnotherStruct {
    a: i32,
    b: i32,
}

impl Debug for AnotherStruct {
    fn fmt(&self, f: &mut Formatter) {
        f.debug_struct("AnotherStruct")
            .field("a", &self.a)
            .field("b", &self.b)
            .finish()
    }
}

assert_eq!(pprint(MyStruct { a: 1, b: 2 }), "MyStruct { a: 1, b: 2 }");
assert_eq!(pprint(AnotherStruct { a: 1, b: 2 }), "AnotherStruct { a: 1, b: 2 }");

Once your type implements [Debug], you have several options to format it

• [pprint]: Convert it to a [String]
• [dbg]: Print it to stderr

Comparison to std::fmt::Debug:

While the main advantage of debug3 is the superior output quality, it has several drawbacks compared to [std::fmt] that you should know

1. Commonness: Virtually every type in Rust implements [std::fmt::Debug], vitrualy no types outside of [std] implement [debug3::Debug][Debug].
2. Availibility: [std::fmt] is also availible as [core::fmt], which allows you to use it in no_std environments. debug3 requires several allocated data structures, so cannot support these environments.
3. Versitility: [std::fmt::Formatter] has many more API's for implementing [std::fmt::Debug]. In order to achive nice formatting, we cannot accept arbitrary strings, but must have items in the form of Structs, Tuples, Maps, Lists and Sets.
4. Configurabiliy: We dont suport stuff like format!("{:x?}, 1) to configure how numbers are printed.
5. Ease of use: We don't have a macro like [std::format] to easily create a string from several elements which implement [Debug]

Prior Art

debug3 would not be possible without all of the following excelent work

• core::fmt - The public API of [Formatter] and [builders]
• prettyplease - Most of the prety printing algorithm is lifted directly from this crate
• custom_debug - The derive macro for [Debug] is based on this crate.
• python's pprint - Inspiration for this type of formatting for debug output.

License