kdl-org.github.io/src/_includes/partials/faq.md

FAQ

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 (r"foo"), 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 <entry name="foo" value="x" /> 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 <element><child>1</child><child>2</child>(etc)</element>. 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.