--- no_site_title: true title: "Preserves: Text Syntax" --- Tony Garnock-Jones {{ site.version_date }}. Version {{ site.version }}. [sexp.txt]: http://people.csail.mit.edu/rivest/Sexp.txt [abnf]: https://tools.ietf.org/html/rfc7405 *Preserves* is a data model, with associated serialization formats. This document defines one of those formats: a textual syntax for `Value`s from the [Preserves data model](preserves.html) that is easy for people to read and write. An [equivalent machine-oriented binary syntax](preserves-binary.html) also exists. ## Preliminaries The definition uses [case-sensitive ABNF][abnf]. ABNF allows easy definition of US-ASCII-based languages. However, Preserves is a Unicode-based language. Therefore, we reinterpret ABNF as a grammar for recognising sequences of Unicode scalar values. **Encoding.** Textual syntax for a `Value` *SHOULD* be encoded using UTF-8 where possible. **Whitespace.** Whitespace `ws` is defined as any number of spaces, tabs, carriage returns, or line feeds. ws = *(%x20 / %x09 / CR / LF) **Commas.** In some positions inside compound terms, commas are permitted and ignored. commas = *(ws ",") ws **Delimiters.** Some tokens (`Boolean`, `SymbolOrNumber`) *MUST* be followed by a `delimiter` or by the end of the input.[^delimiters-lookahead] delimiter = ws / "<" / ">" / "[" / "]" / "{" / "}" / "#" / ":" / DQUOTE / "|" / "@" / ";" / "," [^delimiters-lookahead]: The addition of this constraint means that implementations must now use some kind of lookahead to make sure a delimiter follows a `Boolean`; this should not be onerous, as something similar is required to read `SymbolOrNumber`s correctly. ## Grammar Standalone documents may have trailing whitespace. Document = Value ws Any `Value` may be preceded by whitespace. Value = ws (Record / Collection / Atom / Embedded) Collection = Sequence / Set / Dictionary Atom = Boolean / String / ByteString / QuotedSymbol / SymbolOrNumber Each `Record` is an angle-bracket enclosed grouping of its label-`Value` followed by its field-`Value`s. Record = "<" Value *Value ws ">" `Sequence`s are enclosed in square brackets. `Set`s are written as values enclosed by the tokens `#{` and `}`. `Dictionary` values are curly-brace-enclosed colon-separated pairs of values.[^printing-collections] It is an error for a set to contain duplicate elements or for a dictionary to contain duplicate keys. When printing sets and dictionaries, implementations *SHOULD* order elements resp. keys with respect to the [total order over `Value`s](preserves.html#total-order).[^rationale-print-ordering] Sequence = "[" *(commas Value) commas "]" Set = "#{" *(commas Value) commas "}" Dictionary = "{" *(commas Value ws ":" Value) commas "}" [^printing-collections]: **Implementation note.** When implementing printing of `Value`s using the textual syntax, consider supporting (a) optional pretty-printing with indentation, (b) optional JSON-compatible print mode for that subset of `Value` that is compatible with JSON, and (c) optional submodes for no commas, commas separating, and commas terminating elements or key/value pairs within a collection. [^rationale-print-ordering]: **Rationale.** Consistently printing the elements of unordered collections in some arbitrary but stable order helps, for example, keep diffs small and somewhat meaningful when Preserves values are pretty-printed to text documents under source control. `Boolean`s are the simple literal strings `#t` and `#f` for true and false, respectively. Boolean = %s"#t" / %s"#f" `String`s are, [as in JSON](https://tools.ietf.org/html/rfc8259#section-7), possibly escaped text surrounded by double quotes. The escaping rules are the same as for JSON,[^string-json-correspondence] [^escaping-surrogate-pairs] [except](https://tools.ietf.org/html/rfc8259#section-8.2) that unpaired [surrogate code points](https://unicode.org/glossary/#surrogate_code_point) *MUST NOT* be generated or accepted.[^unpaired-surrogates] String = DQUOTE *char DQUOTE char = / escaped / "\" DQUOTE escaped = "\\" / "\/" / %s"\b" / %s"\f" / %s"\n" / %s"\r" / %s"\t" / %s"\u" 4HEXDIG [^string-json-correspondence]: The grammar for `String` has the same effect as the [JSON](https://tools.ietf.org/html/rfc8259#section-7) grammar for `string`. [^escaping-surrogate-pairs]: In particular, note JSON's rules around the use of surrogate pairs for scalar values not in the Basic Multilingual Plane. We encourage implementations to avoid using `\u` escapes when producing output, and instead to rely on the UTF-8 encoding of the entire document to handle scalar values outside the ASCII range correctly. [^unpaired-surrogates]: Because Preserves forbids unpaired surrogates in its text syntax, any valid JSON text including an unpaired [surrogate code point](https://unicode.org/glossary/#surrogate_code_point) will not be parseable using the Preserves text syntax rules. A `ByteString` may be written in any of three different forms.[^rationale-bytestring] [^rationale-bytestring]: **Rationale.** While the [machine-oriented syntax](preserves-binary.html) defines just one representation for binary data, the text syntax is intended primarily for humans to use, and so it defines many. Different usages of binary data will be more naturally expressed in text as hexadecimal, Base 64, or almost-ASCII. Accepting multiple syntax variations improves the ergonomics of the text syntax. The first is similar to a `String`, but prepended with a hash sign `#`. Many bytes map directly to printable 7-bit ASCII; the remainder must be escaped, either as `\x` followed by a two-digit hexadecimal number, or following the usual rules for double quote and backslash. ByteString = "#" DQUOTE *binchar DQUOTE binchar = / "\" ("\" / "/" / %s"b" / %s"f" / %s"n" / %s"r" / %s"t") / %s"\x" 2HEXDIG / "\" DQUOTE The second is pairs of hexadecimal digits interleaved with whitespace and surrounded by `#x"` and `"`. ByteString =/ %s"#x" DQUOTE *(ws 2HEXDIG) ws DQUOTE The third is a sequence of [Base64](https://tools.ietf.org/html/rfc4648) characters, interleaved with whitespace and surrounded by `#[` and `]`. [Plain](https://datatracker.ietf.org/doc/html/rfc4648#section-4) (`+`,`/`) and [URL-safe](https://datatracker.ietf.org/doc/html/rfc4648#section-5) (`-`,`_`) Base64 characters are accepted; [URL-safe](https://datatracker.ietf.org/doc/html/rfc4648#section-5) (`-`,`_`) characters *SHOULD* be generated by default. Padding characters (`=`) may be omitted. ByteString =/ "#[" *(ws base64char) ws "]" base64char = ALPHA / DIGIT / "+" / "/" / "-" / "_" / "=" A `Symbol` may be written in either of two forms. The first is a quoted form, much the same as the syntax for `String`s, including embedded escape syntax, except using a bar or pipe character (`|`) instead of a double quote mark. QuotedSymbol = "|" *symchar "|" symchar = / escaped / "\|" Alternatively, a `Symbol` may be written in a “bare” form[^cf-sexp-token]. The grammar for numeric data is a subset of the grammar for bare `Symbol`s, so if a `SymbolOrNumber` also matches the grammar for `Double` or `SignedInteger` then it must be interpreted as one of those, and otherwise it must be interpreted as a bare `Symbol`. SymbolOrNumber = 1*(ALPHA / DIGIT / sympunct / symuchar) sympunct = "~" / "!" / "$" / "%" / "^" / "&" / "*" / "?" / "_" / "=" / "+" / "-" / "/" / "." symuchar = [^cf-sexp-token]: Compare with the [SPKI S-expression][sexp.txt] definition of “token representation”, and with the [R6RS definition of identifiers](http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-7.html#node_sec_4.2.4). Numeric data follow the [JSON grammar](https://tools.ietf.org/html/rfc8259#section-6) except that leading zeros are permitted and an optional leading `+` sign is allowed. `Double`s always have either a fractional part or an exponent part, where `SignedInteger`s never have either.[^reading-and-writing-floats-accurately] [^arbitrary-precision-signedinteger] Double = flt SignedInteger = int nat = 1*DIGIT int = ["-"/"+"] nat frac = "." 1*DIGIT exp = %i"e" ["-"/"+"] 1*DIGIT flt = int (frac exp / frac / exp) [^reading-and-writing-floats-accurately]: **Implementation note.** Your language's standard library likely has a good routine for converting between decimal notation and IEEE 754 floating-point. However, if not, or if you are interested in the challenges of accurately reading and writing floating point numbers, see the excellent matched pair of 1990 papers by Clinger and Steele & White, and a recent follow-up by Jaffer: Clinger, William D. ‘How to Read Floating Point Numbers Accurately’. In Proc. PLDI. White Plains, New York, 1990. . Steele, Guy L., Jr., and Jon L. White. ‘How to Print Floating-Point Numbers Accurately’. In Proc. PLDI. White Plains, New York, 1990. . Jaffer, Aubrey. ‘Easy Accurate Reading and Writing of Floating-Point Numbers’. ArXiv:1310.8121 [Cs], 27 October 2013. . [^arbitrary-precision-signedinteger]: **Implementation note.** Be aware when implementing reading and writing of `SignedInteger`s that the data model *requires* arbitrary-precision integers. Your implementation may (but, ideally, should not) truncate precision when reading or writing a `SignedInteger`; however, if it does so, it should (a) signal its client that truncation has occurred, and (b) make it clear to the client that comparing such truncated values for equality or ordering will not yield results that match the expected semantics of the data model. Some valid IEEE 754 `Double`s are not covered by the grammar above, namely, the several million NaNs and the two infinities. These are represented as raw hexadecimal strings similar to hexadecimal `ByteString`s. Implementations are free to use hexadecimal floating-point syntax whereever convenient, even for values representable using the grammar above.[^rationale-no-general-machine-syntax] Double =/ "#xd" DQUOTE 8(ws 2HEXDIG) ws DQUOTE [^rationale-no-general-machine-syntax]: **Rationale.** Previous versions of this specification included an escape to the [machine-oriented binary syntax](preserves-binary.html) by prefixing a `ByteString` containing the binary representation of a `Value` with `#=`. The only true need for this feature was to represent otherwise-unrepresentable floating-point values. Instead, this specification allows such floating-point values to be written directly. Removing the `#=` syntax simplifies implementations (there is no longer any need to support the machine-oriented syntax) and avoids complications around treatment of annotations potentially contained within machine-encoded values. Finally, an `Embedded` is written as a `Value` chosen to represent the denoted object, prefixed with `#:`. Embedded = "#:" Value ## Annotations and Comments When written down, a `Value` may have an associated sequence of *annotations* carrying “out-of-band” contextual metadata about the value. Each annotation is, in turn, a `Value`, and may itself have annotations. The ordering of annotations attached to a `Value` is significant. Value =/ ws Annotation Value Annotation = "@" Value Each annotation is preceded by `@`; the underlying annotated value follows its annotations. Here we extend only the syntactic nonterminal named “`Value`” without altering the semantic class of `Value`s. **Comments.** Strings annotating a `Value` are conventionally interpreted as comments associated with that value. Comments are sufficiently common that special syntax exists for them. Annotation =/ "#" [(%x20 / %x09) linecomment] (CR / LF) linecomment = * When written this way, everything between the hash-space or hash-tab and the end of the line is included in the string annotating the `Value`. Comments that are just hash `#` followed immediately by newline yield an empty-string annotation. **Equivalence.** Annotations appear within syntax denoting a `Value`; however, the annotations are not part of the denoted value. They are only part of the syntax. Annotations do not play a part in equivalences and orderings of `Value`s. Reflective tools such as debuggers, user interfaces, and message routers and relays---tools which process `Value`s generically---may use annotated inputs to tailor their operation, or may insert annotations in their outputs. By contrast, in ordinary programs, as a rule of thumb, the presence, absence or content of an annotation should not change the control flow or output of the program. Annotations are data *describing* `Value`s, and are not in the domain of any specific application of `Value`s. That is, an annotation will almost never cause a non-reflective program to do anything observably different. ## Security Considerations **Whitespace.** The textual format allows arbitrary whitespace in many positions. Consider optional restrictions on the amount of consecutive whitespace that may appear. **Annotations.** Similarly, in modes where a `Value` is being read while annotations are skipped, an endless sequence of annotations may give an illusion of progress. ## Acknowledgements The text syntax for `Boolean`s, `Symbol`s, and `ByteString`s is directly inspired by [Racket](https://racket-lang.org/)'s lexical syntax. ## Appendix. Regular expressions for bare symbols and numbers When parsing, if a token matches both `SymbolOrNumber` and `Number`, it's a number; use `Double` and `SignedInteger` to disambiguate. If it matches `SymbolOrNumber` but not `Number`, it's a "bare" `Symbol`. SymbolOrNumber: ^[-a-zA-Z0-9~!$%^&*?_=+/.]+$ Number: ^([-+]?\d+)((\.\d+([eE][-+]?\d+)?)|([eE][-+]?\d+))?$ Double: ^([-+]?\d+)((\.\d+([eE][-+]?\d+)?)|([eE][-+]?\d+))$ SignedInteger: ^([-+]?\d+)$ When printing, if a symbol matches both `SymbolOrNumber` and `Number` or neither `SymbolOrNumber` nor `Number`, it must be quoted (`|...|`). If it matches `SymbolOrNumber` but not `Number`, it may be printed as a "bare" `Symbol`. ## Notes