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The KDL Document Language

KDL is a document language with xml-like semantics that looks like you're invoking a bunch of CLI commands! It's meant to be used both as a serialization format and a configuration language, much like JSON, YAML, or XML.

There's a living specification, as well as various implementations. You can also check out the FAQ to answer all your burning questions!

The language is based on SDLang, with a number of modifications and clarifications on its syntax and behavior.

Design and Discussion

KDL is still extremely new, and discussion about the format should happen over on the discussions page. Feel free to jump in and give us your 2 cents!


Editor Support



A KDL node is a node name, followed by zero or more "arguments", and children.

title "Hello, World"

You can also have multiple values in a single node!

bookmarks 12 15 188 1234

Nodes can have properties.

author "Alex Monad" email="[email protected]" active=true

And they can have nested child nodes, too!

contents {
  section "First section" {
    paragraph "This is the first paragraph"
    paragraph "This is the second paragraph"

Nodes without children are terminated by a newline, a semicolon, or the end of a file stream:

node1; node2; node3;


KDL supports 4 data types:

  • Strings:
    "hello world"
  • Numbers:
  • Booleans:
  • Null:


It supports two different formats for string input: escaped and raw.

node "this\nhas\tescapes"
other r"C:\Users\zkat\"

Both types of string can be multiline as-is, without a different syntax:

string "my

And for raw strings, you can add any number of # after the r and the last " to disambiguate literal " characters:

other-raw r#"hello"world"#


There's 4 ways to represent numbers in KDL. KDL does not prescribe any representation for these numbers, and it's entirely up to individual implementations whether to represent all numbers with a single type, or to have different representations for different forms.

KDL has regular decimal-radix numbers, with optional decimal part, as well as an optional exponent.

num 1.234e-42

And using the appropriate prefix, you can also enter hexadecimal, octal, and binary literals:

my-hex 0xdeadbeef
my-octal 0o755
my-binary 0b10101101

Finally, all numbers can have underscores to help readability:

bignum 1_000_000


KDL supports C-style comments, both line-based and multiline. Multiline comments can be nested.

// C style

/* C style multiline */

tag /foo=true/ bar=false

// hello //

On top of that, KDL supports

"slashdash" comments, which can be used to comment out individual nodes, arguments, or children:
// This entire node and its children are all commented out.
/-mynode "foo" key=1 {

mynode /-"commented" "not commented" /-key="value" /-{ a b }

More Details

// Nodes can be separated into multiple lines
title \
  "Some title"

// Files must be utf8 encoded! smile "😁"

// Instead of anonymous nodes, nodes and properties can be wrapped // in "" for arbitrary node names. "[email protected]#[email protected]$%Q#$%[email protected]!40" "1.2.3" "!!!!!"=true

// The following is a legal bare identifier: [email protected]#$%^&*.:'|/?+ "weeee"

// And you can also use unicode! ノード お名前="☜(゚ヮ゚☜)"

// kdl specifically allows properties and values to be // interspersed with each other, much like CLI commands. foo bar=true "baz" quux=false 1 2 3

Design Principles

  1. Maintainability
  2. Flexibility
  3. Cognitive simplicity and Learnability
  4. Ease of de/serialization
  5. Ease of implementation


How do you pronounce KDL?

Same as "cuddle".

Why yet another document language?

Because nothing out there felt quite right. The closest one I found was SDLang, but that had some design choices I disagreed with.

Ok, then, why not SDLang?

SDLang is designed for use cases that are not interesting to me, but are very relevant to the D-lang community. KDL is very similar in many ways, but is different in the following ways:

  • The grammar and expected semantics are well-defined and specified.
  • There is only one "number" type. KDL does not prescribe representations.
  • Slashdash (
    ) comments are great and useful!
  • I am not interested in having first-class date types, and SDLang's are very non-standard.
  • Values and properties can be interspersed with each other, rather than one having to follow the other.
  • KDL does not have a first-class binary data type. Just use strings with base64.
  • All strings in KDL are multi-line, and raw strings are written with Rust-style syntax (
    ), instead of backticks.
  • KDL identifiers can use UTF-8 and are much more lax about symbols than SDLang.
  • KDL does not support "anonymous" nodes.
  • Instead, KDL supports arbitrary identifiers for node names and attribute names, meaning you can use arbitrary strings for those:
    "123" "value"=1
    is a valid node, for example. This makes it easier to use KDL for representing arbitrary key/value pairs.

Have you seen that one XKCD comic about standards?

Yes. I have. Please stop linking me to it.

What about YAML?

YAML is a great, widespread language. Unlike KDL, which is node-based (like XML or HTML), it's based on map and array data structures, which can provide an easier serialization experience in some cases.

At the same time, YAML can be ambiguous about what types the data written into it is. There's also a persistent issue where very large YAML files become unmanageable, especially due to the significant indentation feature.

KDL is designed to avoid these particular pitfalls by always being explicit about types, and having clearly-delimited scope (and the ability to auto-indent/auto-format). Syntax errors are easier to catch, and large files are (hopefully!) much more manageable.

What about JSON?

JSON is a great serialization language, but it can be very difficult to use as a human configuration language. This is largely due to its very specific, very strict syntax, as well as its lack of support for comments.

KDL, on the other hand, has great comment support, and has a much more forgiving syntax without being so flexible as to allow certain classes of unfortunate mistakes. It also has much more flexibility around how to represent data.

What about TOML?

It nests very poorly. It doesn't fare well with large files.

What about XML?

XML is actually pretty fantastic, and has long been a standard for data exchange across many industries. At the same time, XML is known to be very verbose, and editing it involves writing (and updating) matching tags. Another large pitfall with XML is its lack of direct support for arbitrary string key/value pairs, so what would be a simple

foo: x
in some languages has to be represented as
 or something similar. XML
also functions great as a markup language. That is, it is easy to
intersperse with text, like HTML.

KDL, just like XML, is a node/element-based language, but with much more lightweight syntax. It also adds the ability to apply anonymous values directly to a node, rather than as children. That is,

nodename 1 2 3
instead of
. This can make it much more manageable and readable as a human configuration language, and is also less verbose when exchanging documents across APIs!

Finally, KDL is not a markup language. XML or HTML do a much better job of "marking up" a text document with special tags, although KDL can still be useful for templating engines that want to be more strict about text fragments.


Creative Commons License
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

This license applies to the text and assets in this repository. Implementations of this specification are not "derivative works", and thus are not bound by the restrictions of CC-BY-SA.

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