# KDL Spec This is the semi-formal specification for KDL, including the intended data model and the grammar. This document describes KDL version `2.0.0-draft.2`. It was released on 2024-02-06. ## Introduction KDL is a node-oriented document language. Its niche and purpose overlaps with XML, and as do many of its semantics. You can use KDL both as a configuration language, and a data exchange or storage format, if you so choose. The bulk of this document is dedicated to a long-form description of all [Components](#components) of a KDL document. There is also a much more terse [Grammar](#full-grammar) at the end of the document that covers most of the rules, with some semantic exceptions involving the data model. KDL is designed to be easy to read _and_ easy to implement. In this document, references to "left" or "right" refer to directions in the *data stream* towards the beginning or end, respectively; in other words, the directions if the data stream were only ASCII text. They do not refer to the writing direction of text, which can flow in either direction, depending on the characters used. ## Components ### Document The toplevel concept of KDL is a Document. A Document is composed of zero or more [Nodes](#node), separated by newlines and whitespace, and eventually terminated by an EOF. All KDL documents should be UTF-8 encoded and conform to the specifications in this document. #### Example The following is a document composed of two toplevel nodes: ```kdl foo { bar } baz ``` ### Node Being a node-oriented language means that the real core component of any KDL document is the "node". Every node must have a name, which must be a [String](#string). The name may be preceded by a [Type Annotation](#type-annotation) to further clarify its type, particularly in relation to its parent node. (For example, clarifying that a particular `date` child node is for the _publication_ date, rather than the last-modified date, with `(published)date`.) Following the name are zero or more [Arguments](#argument) or [Properties](#property), separated by either [whitespace](#whitespace) or [a slash-escaped line continuation](#line-continuation). Arguments and Properties may be interspersed in any order, much like is common with positional arguments vs options in command line tools. [Children](#children-block) can be placed after the name and the optional Arguments and Properties, possibly separated by either whitespace or a slash-escaped line continuation. Arguments are ordered relative to each other (but not relative to Properties) and that order must be preserved in order to maintain the semantics. By contrast, Property order _SHOULD NOT_ matter to implementations. [Children](#children-block) should be used if an order-sensitive key/value data structure must be represented in KDL. Nodes _MAY_ be prefixed with [Slashdash](#slashdash-comments) to "comment out" the entire node, including its properties, arguments, and children, and make it act as plain whitespace, even if it spreads across multiple lines. Finally, a node is terminated by either a [Newline](#newline), a semicolon (`;`) or the end of the file/stream (an `EOF`). #### Example ```kdl foo 1 key=val 3 { bar (role)baz 1 2 } ``` ### Line Continuation Line continuations allow [Nodes](#node) to be spread across multiple lines. A line continuation is a `\` character followed by zero or more whitespace items (including multiline comments) and an optional single-line comment. It must be terminated by a [Newline](#newline) (including the Newline that is part of single-line comments). Following a line continuation, processing of a Node can continue as usual. #### Example ```kdl my-node 1 2 \ // comments are ok after \ 3 4 // This is the actual end of the Node. ``` ### Property A Property is a key/value pair attached to a [Node](#node). A Property is composed of a [String](#string), followed immediately by an [equals sign](#equals-sign), and then a [Value](#value). Properties should be interpreted left-to-right, with rightmost properties with identical names overriding earlier properties. That is: ```kdl node a=1 a=2 ``` In this example, the node's `a` value must be `2`, not `1`. No other guarantees about order should be expected by implementers. Deserialized representations may iterate over properties in any order and still be spec-compliant. Properties _MAY_ be prefixed with `/-` to "comment out" the entire token and make it act as plain whitespace, even if it spreads across multiple lines. #### Equals Sign Any of the following characters may be used as equals signs in properties: | Name | Character | Code Point | |----|-----|----| | EQUALS SIGN | `=` | `U+003D` | | SMALL EQUALS SIGN | `﹦` | `U+FE66` | | FULLWIDTH EQUALS SIGN | `＝` | `U+FF1D` | | HEAVY EQUALS SIGN | `🟰` | `U+1F7F0` | ### Argument An Argument is a bare [Value](#value) attached to a [Node](#node), with no associated key. It shares the same space as [Properties](#properties), and may be interleaved with them. A Node may have any number of Arguments, which should be evaluated left to right. KDL implementations _MUST_ preserve the order of Arguments relative to each other (not counting Properties). Arguments _MAY_ be prefixed with `/-` to "comment out" the entire token and make it act as plain whitespace, even if it spreads across multiple lines. #### Example ```kdl my-node 1 2 3 a b c ``` ### Children Block A children block is a block of [Nodes](#node), surrounded by `{` and `}`. They are an optional part of nodes, and create a hierarchy of KDL nodes. Regular node termination rules apply, which means multiple nodes can be included in a single-line children block, as long as they're all terminated by `;`. #### Example ```kdl parent { child1 child2 } parent { child1; child2; } ``` ### Value A value is either: a [String](#string), a [Number](#number), a [Boolean](#boolean), or [Null](#null). Values _MUST_ be either [Arguments](#argument) or values of [Properties](#property). Only [String](#string) values may be used as [Node](#node) names or [Property](#property) keys. Values (both as arguments and as properties) _MAY_ be prefixed by a single [Type Annotation](#type-annotation). ### Type Annotation A type annotation is a prefix to any [Node Name](#node) or [Value](#value) that includes a _suggestion_ of what type the value is _intended_ to be treated as, or as a _context-specific elaboration_ of the more generic type the node name indicates. Type annotations are written as a set of `(` and `)` with a single [String](#string) in it. It may contain Whitespace after the `(` and before the `)`, and may be separated from its target by Whitespace. KDL does not specify any restrictions on what implementations might do with these annotations. They are free to ignore them, or use them to make decisions about how to interpret a value. Additionally, the following type annotations MAY be recognized by KDL parsers and, if used, SHOULD interpret these types as follows: #### Reserved Type Annotations for Numbers Without Decimals: Signed integers of various sizes (the number is the bit size): * `i8` * `i16` * `i32` * `i64` Unsigned integers of various sizes (the number is the bit size): * `u8` * `u16` * `u32` * `u64` Platform-dependent integer types, both signed and unsigned: * `isize` * `usize` #### Reserved Type Annotations for Numbers With Decimals: IEEE 754 floating point numbers, both single (32) and double (64) precision: * `f32` * `f64` IEEE 754-2008 decimal floating point numbers * `decimal64` * `decimal128` #### Reserved Type Annotations for Strings: * `date-time`: ISO8601 date/time format. * `time`: "Time" section of ISO8601. * `date`: "Date" section of ISO8601. * `duration`: ISO8601 duration format. * `decimal`: IEEE 754-2008 decimal string format. * `currency`: ISO 4217 currency code. * `country-2`: ISO 3166-1 alpha-2 country code. * `country-3`: ISO 3166-1 alpha-3 country code. * `country-subdivision`: ISO 3166-2 country subdivision code. * `email`: RFC5322 email address. * `idn-email`: RFC6531 internationalized email address. * `hostname`: RFC1132 internet hostname (only ASCII segments) * `idn-hostname`: RFC5890 internationalized internet hostname (only `xn--`-prefixed ASCII "punycode" segments, or non-ASCII segments) * `ipv4`: RFC2673 dotted-quad IPv4 address. * `ipv6`: RFC2373 IPv6 address. * `url`: RFC3986 URI. * `url-reference`: RFC3986 URI Reference. * `irl`: RFC3987 Internationalized Resource Identifier. * `irl-reference`: RFC3987 Internationalized Resource Identifier Reference. * `url-template`: RFC6570 URI Template. * `uuid`: RFC4122 UUID. * `regex`: Regular expression. Specific patterns may be implementation-dependent. * `base64`: A Base64-encoded string, denoting arbitrary binary data. #### Examples ```kdl node (u8)123 node prop=(regex).* (published)date "1970-01-01" (contributor)person name="Foo McBar" ``` ### String Strings in KDL represent textual UTF-8 [Values](#value). A String is either an [Identifier String](#identifier-string) (like `foo`), a [Quoted String](#quoted-string) (like `"foo"`) or a [Raw String](#raw-string) (like `#"foo"#`). Identifier Strings let you write short, "single-word" strings with a minimum of syntax; Quoted Strings let you write strings with whitespace (including newlines!) or escapes; Raw Strings let you write strings with whitespace *but without escapes*, allowing you to not worry about the string's content containing anything that might look like an escape. Strings _MUST_ be represented as UTF-8 values. Strings _MUST NOT_ include the code points for [disallowed literal code points](#disallowed-literal-code-points) directly. Quoted Strings may include these code points as _values_ by representing them with their corresponding `\u{...}` escape. ### Identifier String An Identifier String (sometimes referred to as just an "identifier") is composed of any [Unicode Scalar Value](https://unicode.org/glossary/#unicode_scalar_value) other than [non-initial characters](#non-initial-characters), followed by any number of Unicode Scalar Values other than [non-identifier characters](#non-identifier-characters). A handful of patterns are disallowed, to avoid confusion with other values: * idents that appear to start with a [Number](#number) (like `1.0v2` or `-1em`) or the "almost a number" pattern of a decimal point without a leading digit (like `.1`) * idents that are the language keywords (`true`, `false`, and `null`) without their leading `#` Identifiers that match these patterns _MUST_ be treated as a syntax error; such values can only be written as quoted or raw strings. The precise details of the identifier syntax is specified in the [Full Grammar](#full-grammar) below. Identifier Strings are terminated by [Whitespace](#whitespace) or [Newlines](#newline). #### Non-initial characters The following characters cannot be the first character in an [Identifier String](#identifier-string): * Any decimal digit (0-9) * Any [non-identifier characters](#non-identifier-characters) Additionally, the `-` character can only be used as an initial character if the second character is *not* a digit. This allows identifiers to look like `--this`, and removes the ambiguity of having an identifier look like a negative number. #### Non-identifier characters The following characters cannot be used anywhere in a [Identifier String](#identifier-string): * Any of `(){}[]/\"#;` * Any [Equals Sign](#equals-sign) * Any [Whitespace](#whitespace) or [Newline](#newline). * Any [disallowed literal code points](#disallowed-literal-code-points) in KDL documents. ### Quoted String A Quoted String is delimited by `"` on either side of any number of literal string characters except unescaped `"` and `\`. This includes literal [Newline](#newline) characters, which means a String Value can encompass multiple lines without behaving like a Newline for [Node](#node) parsing purposes. Like Identifier Strings, Quoted Strings _MUST NOT_ include any of the [disallowed literal code-points](#disallowed-literal-code-points) as code points in their body. Quoted Strings also follow the Multi-line rules specified in [Multi-line String](#multi-line-strings). #### Escapes In addition to literal code points, a number of "escapes" are supported in Quoted Strings. "Escapes" are the character `\` followed by another character, and are interpreted as described in the following table: | Name | Escape | Code Pt | |-------------------------------|--------|----------| | Line Feed | `\n` | `U+000A` | | Carriage Return | `\r` | `U+000D` | | Character Tabulation (Tab) | `\t` | `U+0009` | | Reverse Solidus (Backslash) | `\\` | `U+005C` | | Quotation Mark (Double Quote) | `\"` | `U+0022` | | Backspace | `\b` | `U+0008` | | Form Feed | `\f` | `U+000C` | | Space | `\s` | `U+0020` | | Unicode Escape | `\u{(1-6 hex chars)}` | Code point described by hex characters, as long as it represents a [Unicode Scalar Value](https://unicode.org/glossary/#unicode_scalar_value) | | Whitespace Escape | See below | N/A | ##### Escaped Whitespace In addition to escaping individual characters, `\` can also escape whitespace. When a `\` is followed by one or more literal whitespace characters, the `\` and all of that whitespace are discarded. For example, `"Hello World"` and `"Hello \ World"` are semantically identical. See [whitespace](#whitespace) and [newlines](#newlines) for how whitespace is defined. Note that only literal whitespace is escaped; whitespace escapes (`\n` and such) are retained. For example, these strings are all semantically identical: ```kdl "Hello\ \nWorld" "Hello\n\ World" "Hello\nWorld" " Hello World " ``` ##### Invalid escapes Except as described in the escapes table, above, `\` *MUST NOT* precede any other characters in a string. ### Raw String Raw Strings in KDL are much like [Quoted Strings](#quoted-string), except they do not support `\`-escapes. They otherwise share the same properties as far as literal [Newline](#newline) characters go, multi-line rules, and the requirement of UTF-8 representation. Raw String literals are represented with one or more `#` characters, followed by `"`, followed by any number of UTF-8 literals. The string is then closed by a `"` followed by a _matching_ number of `#` characters. This means that the string sequence `"` or `"#` and such must not match the closing `"` with the same or more `#` characters as the opening `#`, in the body of the string. Like other Strings, Raw Strings _MUST NOT_ include any of the [disallowed literal code-points](#disallowed-literal-code-points) as code points in their body. Unlike with Quoted Strings, these cannot simply be escaped, and are thus unrepresentable when using Raw Strings. #### Example ```kdl just-escapes #"\n will be literal"# ``` The string contains the literal characters `\n will be literal`. ```kdl quotes-and-escapes ##"hello\n\r\asd"#world"## ``` The string contains the literal characters `hello\n\r\asd"#world` ### Multi-line Strings When a Quoted or Raw String spans multiple lines with literal, non-escaped Newlines, it follows a special multi-line syntax that automatically "dedents" the string, allowing its value to be indented to a visually matching level if desired. A Multi-line string _MUST_ start with a [Newline](#newline) immediately following its opening `"`. Its final line, preceding the closing `"`, _MUST_ contain only whitespace. All in-between lines that contain non-whitespace characters _MUST_ start with the exact same whitespace as the final line (precisely matching codepoints, not merely counting characters). The value of the Multi-line String omits the first and last Newline, the Whitespace of the last line, the matching Whitespace prefix on all intermediate lines, and all Whitespace on intermediate Whitespace-only lines. The first and last Newline can be the same character (that is, empty multi-line strings are legal). Strings with literal Newlines that do not immediately start with a Newline and whose final `"` is not preceeded by optional whitespace and a Newline are illegal. In other words, the final line specifies the whitespace prefix that will be removed from all other lines. #### Example ```kdl multi-line " foo This is the base indentation bar " ``` The last example's string value will be: ``` foo This is the base indentation bar ``` Equivalent to `" foo\nThis is the base indentation\n bar"`. --------- If the last line wasn't indented as far, it won't dedent the rest of the lines as much: ```kdl multi-line " foo This is no longer on the left edge bar " ``` This example's string value will be: ``` foo This is no longer on the left edge bar ``` Equivalent to `" foo\n This is no longer on the left edge\n bar"`. ----------- Empty lines can contain any whitespace, or none at all, and will be reflected as empty in the value: ```kdl multi-line " Indented a bit A second indented paragraph. " ``` This example's string value will be: ``` Indented a bit. A second indented paragraph. ``` Equivalent to `"Indented a bit.\n\nA second indented paragraph."` ### Number Numbers in KDL represent numerical [Values](#value). There is no logical distinction in KDL between real numbers, integers, and floating point numbers. It's up to individual implementations to determine how to represent KDL numbers. There are four syntaxes for Numbers: Decimal, Hexadecimal, Octal, and Binary. * All numbers may optionally start with one of `-` or `+`, which determine whether they'll be positive or negative. * Binary numbers start with `0b` and only allow `0` and `1` as digits, which may be separated by `_`. They represent numbers in radix 2. * Octal numbers start with `0o` and only allow digits between `0` and `7`, which may be separated by `_`. They represent numbers in radix 8. * Hexadecimal numbers start with `0x` and allow digits between `0` and `9`, as well as letters `A` through `F`, in either lower or upper case, which may be separated by `_`. They represent numbers in radix 16. * Decimal numbers are a bit more special: * They have no radix prefix. * They use digits `0` through `9`, which may be separated by `_`. * They may optionally include a decimal separator `.`, followed by more digits, which may again be separated by `_`. * They may optionally be followed by `E` or `e`, an optional `-` or `+`, and more digits, to represent an exponent value. Note that, similar to JSON and some other languages, numbers without an integer digit (such as `.1`) are illegal. They must be written with at least one integer digit, like `0.1`. (These patterns are also disallowed from [Identifier Strings](#identifier-string), to avoid confusion.) ### Boolean A boolean [Value](#value) is either the symbol `#true` or `#false`. These _SHOULD_ be represented by implementation as boolean logical values, or some approximation thereof. #### Example ```kdl my-node true value=#false ``` ### Null The symbol `#null` represents a null [Value](#value). It's up to the implementation to decide how to represent this, but it generally signals the "absence" of a value. #### Example ```kdl my-node #null key=#null ``` ### Whitespace The following characters should be treated as non-[Newline](#newline) [white space](https://www.unicode.org/Public/UCD/latest/ucd/PropList.txt): | Name | Code Pt | |----------------------|---------| | Character Tabulation | `U+0009` | | Line Tabulation | `U+000B` | | Space | `U+0020` | | No-Break Space | `U+00A0` | | Ogham Space Mark | `U+1680` | | En Quad | `U+2000` | | Em Quad | `U+2001` | | En Space | `U+2002` | | Em Space | `U+2003` | | Three-Per-Em Space | `U+2004` | | Four-Per-Em Space | `U+2005` | | Six-Per-Em Space | `U+2006` | | Figure Space | `U+2007` | | Punctuation Space | `U+2008` | | Thin Space | `U+2009` | | Hair Space | `U+200A` | | Narrow No-Break Space| `U+202F` | | Medium Mathematical Space | `U+205F` | | Ideographic Space | `U+3000` | #### Single-line comments Any text after `//`, until the next literal [Newline](#newline) is "commented out", and is considered to be [Whitespace](#whitespace). #### Multi-line comments In addition to single-line comments using `//`, comments can also be started with `/*` and ended with `*/`. These comments can span multiple lines. They are allowed in all positions where [Whitespace](#whitespace) is allowed and can be nested. #### Slashdash comments Finally, a special kind of comment called a "slashdash", denoted by `/-`, can be used to comment out entire _components_ of a KDL document logically, and have those elements be treated as whitespace. Slashdash comments can be used before: * A [Node](#node) name (or its type annotation): the entire Node is treated as Whitespace, including all props, args, and children. * A node [Argument](#argument) (or its type annotation), in which case the Argument value is treated as Whitespace. * A [Property](#property) key, in which case the entire property, both key and value, is treated as Whitespace. * A [Children Block](#children-block), in which case the entire block, including all children within, is treated as Whitespace. ### Newline The following characters [should be treated as new lines](https://www.unicode.org/versions/Unicode13.0.0/ch05.pdf): | Acronym | Name | Code Pt | |---------|-----------------|---------| | CR | Carriage Return | `U+000D` | | LF | Line Feed | `U+000A` | | CRLF | Carriage Return and Line Feed | `U+000D` + `U+000A` | | NEL | Next Line | `U+0085` | | FF | Form Feed | `U+000C` | | LS | Line Separator | `U+2028` | | PS | Paragraph Separator | `U+2029` | Note that for the purpose of new lines, CRLF is considered _a single newline_. ### Disallowed Literal Code Points The following code points may not appear literally anywhere in the document. They may be represented in Strings (but not Raw Strings) using `\u{}`. * The codepoints `U+0000-0009`, the codepoint `U+000B`, or the codepoints `U+000E-001F` (various control characters). * `U+007F` (the Delete control character). * Any codepoint that is not a [Unicode Scalar Value](https://unicode.org/glossary/#unicode_scalar_value). * `U+2066-2069`, `U+202A-202E`, `U+200E`, and `U+200F`, the [unicode "direction control" characters](https://www.w3.org/International/questions/qa-bidi-unicode-controls) ## Full Grammar This is the full official grammar for KDL and should be considered authoritative if something seems to disagree with the text above. The [grammar language syntax](#grammar-language) is defined below. ``` document := bom? nodes nodes := (line-space* node)* line-space* plain-line-space := newline | ws | single-line-comment plain-node-space := ws* escline ws* | ws+ line-space := plain-line-space+ ('/-' plain-node-space* node)? node-space := plain-node-space+ ('/-' plain-node-space* (node-prop-or-arg | node-children))? required-node-space := node-space* plain-node-space+ optional-node-space := node-space* base-node := type? optional-node-space string (required-node-space node-prop-or-arg)* (required-node-space node-children)? node := base-node optional-node-space node-terminator final-node := base-node optional-node-space node-terminator? node-prop-or-arg := prop | value node-children := '{' nodes final-node? '}' node-terminator := single-line-comment | newline | ';' | eof prop := string optional-node-space equals-sign optional-node-space value value := type? optional-node-space (string | number | keyword) type := '(' optional-node-space string optional-node-space ')' equals-sign := See Table (Equals Sign) string := identifier-string | quoted-string | raw-string identifier-string := unambiguous-ident | signed-ident | dotted-ident unambiguous-ident := ((identifier-char - digit - sign - '.') identifier-char*) - 'true' - 'false' - 'null' signed-ident := sign ((identifier-char - digit - '.') identifier-char*)? dotted-ident := sign? '.' ((identifier-char - digit) identifier-char*)? identifier-char := unicode - line-space - [\\/(){};\[\]="#] - disallowed-literal-code-points quoted-string := '"' (single-line-string-body | newline multi-line-string-body newline ws*) '"' single-line-string-body := (string-character - newline)* multi-line-string-body := string-character* string-character := '\' escape | [^\\"] - disallowed-literal-code-points escape := ["\\bfnrt] | 'u{' hex-digit{1, 6} '}' | (unicode-space | newline)+ hex-digit := [0-9a-fA-F] raw-string := '#' raw-string-quotes '#' | '#' raw-string '#' raw-string-quotes := '"' (single-line-raw-string-body | newline multi-line-raw-string-body newline ws*) '"' single-line-raw-string-body := (unicode - newline - disallowed-literal-code-points)* multi-line-raw-string-body := (unicode - disallowed-literal-code-points)* number := hex | octal | binary | decimal decimal := sign? integer ('.' integer)? exponent? exponent := ('e' | 'E') sign? integer integer := digit (digit | '_')* digit := [0-9] sign := '+' | '-' hex := sign? '0x' hex-digit (hex-digit | '_')* octal := sign? '0o' [0-7] [0-7_]* binary := sign? '0b' ('0' | '1') ('0' | '1' | '_')* keyword := boolean | '#null' boolean := '#true' | '#false' escline := '\\' ws* (single-line-comment | newline | eof) newline := See Table (All line-break white_space) ws := unicode-space | multi-line-comment bom := '\u{FEFF}' disallowed-literal-code-points := See Table (Disallowed Literal Code Points) unicode-space := See Table (All White_Space unicode characters which are not `newline`) single-line-comment := '//' ^newline* (newline | eof) multi-line-comment := '/*' commented-block commented-block := '*/' | (multi-line-comment | '*' | '/' | [^*/]+) commented-block ``` ### Grammar language The grammar language syntax is a combination of ABNF with some regex spice thrown in. Specifically: * Single quotes (`'`) are used to denote literal text. `\` within a literal string is used for escaping other single-quotes, for initiating unicode characters using hex values (`\u{FEFF}`), and for escaping `\` itself (`\\`). * `*` is used for "zero or more", `+` is used for "one or more", and `?` is used for "zero or one". * `()` can be used to group matches that must be matched together. * `a | b` means `a or b`, whichever matches first. If multipe items are before a `|`, they are a single group. `a b c | d` is equivalent to `(a b c) | d`. * `[]` are used for regex-style character matches, where any character between the brackets will be a single match. `\` is used to escape `\`, `[`, and `]`. They also support character ranges (`0-9`), and negation (`^`) * `-` is used for "except for" or "minus" whatever follows it. For example, `a - `'x'` means "any `a`, except something that matches the literal `'x'`". * The prefix `^` means "something that does not match" whatever follows it. For example, `^foo` means "must not match `foo`".