A KDL file format parser with great error reporting and convenient derive macros.

About KDL

To give you some background on the KDL format. Here is a small example:

foo 1 key="val" "three" {
    bar
    (role)baz 1 2
}

Here is what are annotations for all the datum as described by the specification and this guide:

foo 1 "three" key="val" {                           ╮
─┬─ ┬ ───┬─── ────┬────                             │
 │  │    │        ╰───── property (can be multiple) │
 │  │    │                                          │
 │  ╰────┴────────────── arguments                  │
 │                                                  │
 └── node name                                      ├─ node "foo", with
                                                    │  "bar" and "baz"
    bar                                             │  being children
    (role)baz 1 2                                   │
     ──┬─                                           │
       └────── type name for node named "baz"       │
}                                                   ╯

(note, the order of properties doesn't matter as well as the order of properties with respect to arguments, so I've moved arguments to have less intersections for the arrows)

Usage

Most common usage of this library is using derive and [parse] function:

#[derive(knuffel::Decode)]
enum TopLevelNode {
    Route(Route),
    Plugin(Plugin),
}

#[derive(knuffel::Decode)]
struct Route {
    #[knuffel(argument)]
    path: String,
    #[knuffel(children(name="route"))]
    subroutes: Vec<Route>,
}

#[derive(knuffel::Decode)]
struct Plugin {
    #[knuffel(argument)]
    name: String,
    #[knuffel(property)]
    url: String,
}

# fn main() -> miette::Result<()> {
let config = knuffel::parse::<Vec<TopLevelNode>>("example.kdl", r#"
    route "/api" {
        route "/api/v1"
    }
    plugin "http" url="https://example.org/http"
"#)?;
# Ok(())
# }

This parses into a vector of nodes as enums TopLevelNode, but you also use some node as a root of the document if there is no properties and arguments declared:

#[derive(knuffel::Decode)]
struct Document {
    #[knuffel(child, unwrap(argument))]
    version: Option<String>,
    #[knuffel(children(name="route"))]
    routes: Vec<Route>,
    #[knuffel(children(name="plugin"))]
    plugins: Vec<Plugin>,
}

let config = parse::<Document>("example.kdl", r#"
    version "2.0"
    route "/api" {
        route "/api/v1"
    }
    plugin "http" url="https://example.org/http"
"#)?;

See description of Decode and DecodeScalar for the full reference on allowed attributes and parse modes.

Errors

This crate publishes nice errors, like this:

<img width="50%" src="https://raw.githubusercontent.com/tailhook/knuffel/main/images/error.png" alt=" Screenshot of error. Here is how narratable printer would print the error: Error: single char expected after `Alt+` Diagnostic severity: error \ Begin snippet for test.kdl starting at line 17, column 1 \ snippet line 17: } snippet line 18: key &quot;Alt+&quot; mode=&quot;normal&quot; { label starting at line 18, column 10: invalid value snippet line 19: move-focus &quot;left&quot; ">

To make them working, miette's "fancy" feature must be enabled in the final application's Cargo.toml:

[dependencies]
miette = { version="4.3.0", features=["fancy"] }

And the error returned from parser should be converted to [miette::Report] and printed with debugging handler. The most manual way to do that is:

# #[derive(knuffel::Decode, Debug)]
# struct Config {}
# let file_name = "1.kdl";
# let text = "";
let config = match knuffel::parse::<Config>(file_name, text) {
    Ok(config) => config,
    Err(e) => {
         println!("{:?}", miette::Report::new(e));
         std::process::exit(1);
    }
};

But usually function that returns miette::Result is good enough:

# use std::fs;
# #[derive(knuffel::Decode)]
# struct Config {}
use miette::{IntoDiagnostic, Context};

fn parse_config(path: &str) -> miette::Result<Config> {
    let text = fs::read_to_string(path).into_diagnostic()
        .wrap_err_with(|| format!("cannot read {:?}", path))?;
    Ok(knuffel::parse(path, &text)?)
}
fn main() -> miette::Result<()> {
    let config = parse_config("my.kdl")?;
    # Ok(())
}

See miette guide for other ways of configuring error output.

The Name

KDL is pronounced as cuddle. "Knuffel" means the same as cuddle in Dutch.

License

Licensed under either of

• Apache License, Version 2.0, (./LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (./LICENSE-MIT or http://opensource.org/licenses/MIT) at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.