Crate knuffel

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,
}

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

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:

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:

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 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")?;
}

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.

Re-exports

• pub use traits::Decode;
• pub use traits::DecodeScalar;
• pub use traits::DecodeChildren;
• pub use errors::Error;

Modules

• ast
  Structures that represent abstract syntax tree (AST) of the KDL document
• decode
  Decode support stuff
• errors
  Error types for the knuffel library
• span
  Knuffel supports to kinds of the span for parsing
• traits
  Traits used for the library

Functions

• parse
  Parse KDL text and decode Rust object
• parse_ast
  Parse KDL text and return AST
• parse_with_context
  Parse KDL text and decode Rust object providing extra context for the decoder

Derive Macros

• Decode
  The derive is the most interesting part of the knuffel libary.
• DecodeScalar
  Currently DecodeScalar derive is only implemented for enums