--- no_site_title: true title: "Preserves: an Expressive Data Language" --- Tony Garnock-Jones May 2021. Version 0.6.0. [sexp.txt]: http://people.csail.mit.edu/rivest/Sexp.txt [spki]: http://world.std.com/~cme/html/spki.html [varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints [LEB128]: https://en.wikipedia.org/wiki/LEB128 [erlang-map]: http://erlang.org/doc/reference_manual/data_types.html#map [abnf]: https://tools.ietf.org/html/rfc7405 [canonical]: canonical-binary.html This document proposes a data model and serialization format called *Preserves*. Preserves supports *records* with user-defined *labels*, embedded *references*, and the usual suite of atomic and compound data types, including *binary* data as a distinct type from text strings. Its *annotations* allow separation of data from metadata such as [comments](conventions.html#comments), trace information, and provenance information. Preserves departs from many other data languages in defining how to *compare* two values. Comparison is based on the data model, not on syntax or on data structures of any particular implementation language. ## Starting with Semantics Taking inspiration from functional programming, we start with a definition of the *values* that we want to work with and give them meaning independent of their syntax. Our `Value`s fall into two broad categories: *atomic* and *compound* data. Every `Value` is finite and non-cyclic. Embedded values, called `Embedded`s, are a third, special-case category. Value = Atom | Compound | Embedded Atom = Boolean | Float | Double | SignedInteger | String | ByteString | Symbol Compound = Record | Sequence | Set | Dictionary **Total order.** As we go, we will incrementally specify a total order over `Value`s. Two values of the same kind are compared using kind-specific rules. The ordering among values of different kinds is essentially arbitrary, but having a total order is convenient for many tasks, so we define it as follows: (Values) Atom < Compound < Embedded (Compounds) Record < Sequence < Set < Dictionary (Atoms) Boolean < Float < Double < SignedInteger < String < ByteString < Symbol **Equivalence.** Two `Value`s are equal if neither is less than the other according to the total order. ### Signed integers. A `SignedInteger` is a signed integer of arbitrary width. `SignedInteger`s are compared as mathematical integers. ### Unicode strings. A `String` is a sequence of Unicode [code-point](http://www.unicode.org/glossary/#code_point)s. `String`s are compared lexicographically, code-point by code-point.[^utf8-is-awesome] [^utf8-is-awesome]: Happily, the design of UTF-8 is such that this gives the same result as a lexicographic byte-by-byte comparison of the UTF-8 encoding of a string! ### Binary data. A `ByteString` is a sequence of octets. `ByteString`s are compared lexicographically. ### Symbols. Programming languages like Lisp and Prolog frequently use string-like values called *symbols*. Here, a `Symbol` is, like a `String`, a sequence of Unicode code-points representing an identifier of some kind. `Symbol`s are also compared lexicographically by code-point. ### Booleans. There are two `Boolean`s, “false” and “true”. The “false” value is less-than the “true” value. ### IEEE floating-point values. `Float`s and `Double`s are single- and double-precision IEEE 754 floating-point values, respectively. `Float`s, `Double`s and `SignedInteger`s are disjoint; by the rules [above](#total-order), every `Float` is less than every `Double`, and every `SignedInteger` is greater than both. Two `Float`s or two `Double`s are to be ordered by the `totalOrder` predicate defined in section 5.10 of [IEEE Std 754-2008](https://dx.doi.org/10.1109/IEEESTD.2008.4610935). ### Records. A `Record` is a *labelled* tuple of `Value`s, the record's *fields*. A label can be any `Value`, but is usually a `Symbol`.[^extensibility] [^iri-labels] `Record`s are compared lexicographically: first by label, then by field sequence. [^extensibility]: The [Racket](https://racket-lang.org/) programming language defines “[prefab](http://docs.racket-lang.org/guide/define-struct.html#(part._prefab-struct))” structure types, which map well to our `Record`s. Racket supports record extensibility by encoding record supertypes into record labels as specially-formatted lists. [^iri-labels]: It is occasionally (but seldom) necessary to interpret such `Symbol` labels as UTF-8 encoded IRIs. Where a label can be read as a relative IRI, it is notionally interpreted with respect to the IRI `urn:uuid:6bf094a6-20f1-4887-ada7-46834a9b5b34`; where a label can be read as an absolute IRI, it stands for that IRI; and otherwise, it cannot be read as an IRI at all, and so the label simply stands for itself—for its own `Value`. ### Sequences. A `Sequence` is a sequence of `Value`s. `Sequence`s are compared lexicographically. ### Sets. A `Set` is an unordered finite set of `Value`s. It contains no duplicate values, following the [equivalence relation](#equivalence) induced by the total order on `Value`s. Two `Set`s are compared by sorting their elements ascending using the [total order](#total-order) and comparing the resulting `Sequence`s. ### Dictionaries. A `Dictionary` is an unordered finite collection of pairs of `Value`s. Each pair comprises a *key* and a *value*. Keys in a `Dictionary` are pairwise distinct. Instances of `Dictionary` are compared by lexicographic comparison of the sequences resulting from ordering each `Dictionary`'s pairs in ascending order by key. ### Embeddeds. An `Embedded` allows inclusion of *domain-specific*, potentially *stateful* or *located* data into a `Value`.[^embedded-rationale] `Embedded`s may be used to denote stateful objects, network services, object capabilities, file descriptors, Unix processes, or other possibly-stateful things. Because each `Embedded` is a domain-specific datum, comparison of two `Embedded`s is done according to domain-specific rules. [^embedded-rationale]: **Rationale.** Why include `Embedded`s as a special class, distinct from, say, a specially-labeled `Record`? First, a `Record` can only hold other `Value`s: in order to embed values such as live pointers to Java objects, some means of "escaping" from the `Value` data type must be provided. Second, `Embedded`s are meant to be able to denote stateful entities, for which comparison by address is appropriate; however, we do not wish to place restrictions on the *nature* of these entities: if we had used `Record`s instead of distinct `Embedded`s, users would have to invent an encoding of domain data into `Record`s that reflected domain ordering into `Value` ordering. This is often difficult and may not always be possible. Finally, because `Embedded`s are intended to be able to represent network and memory *locations*, they must be able to be rewritten at network and process boundaries. Having a distinct class allows generic `Embedded` rewriting without the quotation-related complications of encoding references as, say, `Record`s. *Examples.* In a Java or Python implementation, an `Embedded` may denote a reference to a Java or Python object; comparison would be done via the language's own rules for equivalence and ordering. In a Unix application, an `Embedded` may denote an open file descriptor or a process ID. In an HTTP-based application, each `Embedded` might be a URL, compared according to [RFC 6943](https://tools.ietf.org/html/rfc6943#section-3.3). When a `Value` is serialized for storage or transfer, `Embedded`s will usually be represented as ordinary `Value`s, in which case the ordinary rules for comparing `Value`s will apply. ## Textual Syntax Now we have discussed `Value`s and their meanings, we may turn to techniques for *representing* `Value`s for communication or storage. In this section, we use [case-sensitive ABNF][abnf] to define a textual syntax that is easy for people to read and write.[^json-superset] Most of the examples in this document are written using this syntax. In the following section, we will define an equivalent compact machine-readable syntax. [^json-superset]: The grammar of the textual syntax is a superset of JSON, with the slightly unusual feature that `true`, `false`, and `null` are all read as `Symbol`s, and that `SignedInteger`s are never read as `Double`s. ### Character set. [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 code points. Textual syntax for a `Value` *SHOULD* be encoded using UTF-8 where possible. ### Whitespace. Whitespace is defined as any number of spaces, tabs, carriage returns, line feeds, or commas. ws = *(%x20 / %x09 / newline / ",") newline = CR / LF ### Grammar. Standalone documents may have trailing whitespace. Document = Value ws Any `Value` may be preceded by whitespace. Value = ws (Record / Collection / Atom / Embedded / Compact) Collection = Sequence / Dictionary / Set Atom = Boolean / Float / Double / SignedInteger / String / ByteString / Symbol 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. `Dictionary` values are curly-brace-enclosed colon-separated pairs of values. `Set`s are written as values enclosed by the tokens `#{` and `}`.[^printing-collections] It is an error for a set to contain duplicate elements or for a dictionary to contain duplicate keys. Sequence = "[" *Value ws "]" Dictionary = "{" *(Value ws ":" Value) ws "}" Set = "#{" *Value ws "}" [^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. `Boolean`s are the simple literal strings `#t` and `#f` for true and false, respectively. Boolean = %s"#t" / %s"#f" Numeric data follow the [JSON grammar](https://tools.ietf.org/html/rfc8259#section-6), with the addition of a trailing “f” distinguishing `Float` from `Double` values. `Float`s and `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] Float = flt %i"f" Double = flt SignedInteger = int digit1-9 = %x31-39 nat = %x30 / ( digit1-9 *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. `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] String = %x22 *char %x22 char = unescaped / %x7C / escape (escaped / %x22 / %s"u" 4HEXDIG) unescaped = %x20-21 / %x23-5B / %x5D-7B / %x7D-10FFFF escape = %x5C ; \ escaped = ( %x5C / ; \ reverse solidus U+005C %x2F / ; / solidus U+002F %x62 / ; b backspace U+0008 %x66 / ; f form feed U+000C %x6E / ; n line feed U+000A %x72 / ; r carriage return U+000D %x74 ) ; t tab U+0009 [^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`. Some auxiliary definitions (e.g. `escaped`) are lifted largely unmodified from the text of RFC 8259. [^escaping-surrogate-pairs]: In particular, note JSON's rules around the use of surrogate pairs for code points 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 non-ASCII codepoints correctly. A `ByteString` may be written in any of three different forms. The first is similar to a `String`, but prepended with a hash sign `#`. In addition, only Unicode code points overlapping with printable 7-bit ASCII are permitted unescaped inside such a `ByteString`; other byte values must be escaped by prepending a two-digit hexadecimal value with `\x`. ByteString = "#" %x22 *binchar %x22 binchar = binunescaped / escape (escaped / %x22 / %s"x" 2HEXDIG) binunescaped = %x20-21 / %x23-5B / %x5D-7E The second is as a sequence of pairs of hexadecimal digits interleaved with whitespace and surrounded by `#x"` and `"`. ByteString =/ %s"#x" %x22 *(ws / 2HEXDIG) ws %x22 The third is as a sequence of [Base64](https://tools.ietf.org/html/rfc4648) characters, interleaved with whitespace and surrounded by `#[` and `]`. Plain and URL-safe Base64 characters are allowed. ByteString =/ "#[" *(ws / base64char) ws "]" / base64char = %x41-5A / %x61-7A / %x30-39 / "+" / "/" / "-" / "_" / "=" A `Symbol` may be written in a “bare” form[^cf-sexp-token] so long as it conforms to certain restrictions on the characters appearing in the symbol. Alternatively, it may be written in a quoted form. The quoted form is 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. Symbol = symstart *symcont / "|" *symchar "|" symstart = ALPHA / sympunct / symustart symcont = ALPHA / sympunct / symustart / symucont / DIGIT / "-" sympunct = "~" / "!" / "$" / "%" / "^" / "&" / "*" / "?" / "_" / "=" / "+" / "/" / "." symchar = unescaped / %x22 / escape (escaped / %x7C / %s"u" 4HEXDIG) symustart = symucont = [^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). An `Embedded` is written as a `Value` chosen to represent the denoted object, prefixed with `#!`. Embedded = "#!" Value Finally, any `Value` may be represented by escaping from the textual syntax to the [compact binary syntax](#compact-binary-syntax) by prefixing a `ByteString` containing the binary representation of the `Value` with `#=`.[^rationale-switch-to-binary] [^no-literal-binary-in-text] [^compact-value-annotations] Compact = "#=" ws ByteString [^rationale-switch-to-binary]: **Rationale.** The textual syntax cannot express every `Value`: specifically, it cannot express the several million floating-point NaNs, or the two floating-point Infinities. Since the compact binary format for `Value`s expresses each `Value` with precision, embedding binary `Value`s solves the problem. [^no-literal-binary-in-text]: Every text is ultimately physically stored as bytes; therefore, it might seem possible to escape to the raw binary form of compact binary encoding from within a pieces of textual syntax. However, while bytes must be involved in any *representation* of text, the text *itself* is logically a sequence of *code points* and is not *intrinsically* a binary structure at all. It would be incoherent to expect to be able to access the representation of the text from within the text itself. [^compact-value-annotations]: Any text-syntax annotations preceding the `#` are prepended to any binary-syntax annotations yielded by decoding the `ByteString`. ### Annotations. **Syntax.** 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. Value =/ ws "@" Value 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. Value =/ ws ";" *(%x00-09 / %x0B-0C / %x0E-%x10FFFF) newline Value When written this way, everything between the `;` and the newline is included in the string annotating the `Value`. **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. ## Compact Binary Syntax A `Repr` is a binary-syntax encoding, or representation, of a `Value`. For a value `v`, we write `«v»` for the `Repr` of v. ### Type and Length representation. Each `Repr` starts with a tag byte, describing the kind of information represented. Depending on the tag, a length indicator, further encoded information, and/or an ending tag may follow. tag (simple atomic data and small integers) tag ++ binarydata (most integers) tag ++ length ++ binarydata (large integers, strings, symbols, and binary) tag ++ repr ++ ... ++ endtag (compound data) The unique end tag is byte value `0x84`. If present after a tag, the length of a following piece of binary data is formatted as a [base 128 varint][varint].[^see-also-leb128] We write `varint(m)` for the varint-encoding of `m`. Quoting the [Google Protocol Buffers][varint] definition, [^see-also-leb128]: Also known as [LEB128][] encoding, for unsigned integers. Varints and LEB128-encoded integers differ only for signed integers, which are not used in Preserves. > Each byte in a varint, except the last byte, has the most > significant bit (msb) set – this indicates that there are further > bytes to come. The lower 7 bits of each byte are used to store the > two's complement representation of the number in groups of 7 bits, > least significant group first. The following table illustrates varint-encoding. | Number, `m` | `m` in binary, grouped into 7-bit chunks | `varint(m)` bytes | | ------ | ------------------- | ------------ | | 15 | `0001111` | 15 | | 300 | `0000010 0101100` | 172 2 | | 1000000000 | `0000011 1011100 1101011 0010100 0000000` | 128 148 235 220 3 | It is an error for a varint-encoded `m` in a `Repr` to be anything other than the unique shortest encoding for that `m`. That is, a varint-encoding of `m` *MUST NOT* end in `0` unless `m`=0. ### Records, Sequences, Sets and Dictionaries. «» = [0xB4] ++ «L» ++ «F_1» ++...++ «F_m» ++ [0x84] «[X_1...X_m]» = [0xB5] ++ «X_1» ++...++ «X_m» ++ [0x84] «#{E_1...E_m}» = [0xB6] ++ «E_1» ++...++ «E_m» ++ [0x84] «{K_1:V_1...K_m:V_m}» = [0xB7] ++ «K_1» ++ «V_1» ++...++ «K_m» ++ «V_m» ++ [0x84] There is *no* ordering requirement on the `E_i` elements or `K_i`/`V_i` pairs.[^no-sorting-rationale] They may appear in any order. However, the `E_i` and `K_i` *MUST* be pairwise distinct. In addition, implementations *SHOULD* default to writing set elements and dictionary key/value pairs in order sorted lexicographically by their `Repr`s[^not-sorted-semantically], and *MAY* offer the option of serializing in some other implementation-defined order. [^no-sorting-rationale]: In the BitTorrent encoding format, [bencoding](http://www.bittorrent.org/beps/bep_0003.html#bencoding), dictionary key/value pairs must be sorted by key. This is a necessary step for ensuring serialization of `Value`s is canonical. We do not require that key/value pairs (or set elements) be in sorted order for serialized `Value`s; however, a [canonical form][canonical] for `Repr`s does exist where a sorted ordering is required. [^not-sorted-semantically]: It's important to note that the sort ordering for writing out set elements and dictionary key/value pairs is *not* the same as the sort ordering implied by the semantic ordering of those elements or keys. For example, the `Repr` of a negative number very far from zero will start with byte that is *greater* than the byte which starts the `Repr` of zero, making it sort lexicographically later by `Repr`, despite being semantically *less than* zero. **Rationale**. This is for ease-of-implementation reasons: not all languages can easily represent sorted sets or sorted dictionaries, but encoding and then sorting byte strings is much more likely to be within easy reach. ### SignedIntegers. «x» when x ∈ SignedInteger = [0xB0] ++ varint(m) ++ intbytes(x) if ¬(-3≤x≤12) ∧ m>16 ([0xA0] + m - 1) ++ intbytes(x) if ¬(-3≤x≤12) ∧ m≤16 ([0xA0] + x) if (-3≤x≤-1) ([0x90] + x) if ( 0≤x≤12) where m = |intbytes(x)| Integers in the range [-3,12] are compactly represented with tags between `0x90` and `0x9F` because they are so frequently used. Integers up to 16 bytes long are represented with a single-byte tag encoding the length of the integer. Larger integers are represented with an explicit varint length. Every `SignedInteger` *MUST* be represented with its shortest possible encoding. The function `intbytes(x)` gives the big-endian two's-complement binary representation of `x`, taking exactly as many whole bytes as needed to unambiguously identify the value and its sign, and `m = |intbytes(x)|`. The most-significant bit in the first byte in `intbytes(x)` is the sign bit.[^zero-intbytes] For example, «87112285931760246646623899502532662132736» = B0 12 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 «-257» = A1 FE FF «-3» = 9D «128» = A1 00 80 «-256» = A1 FF 00 «-2» = 9E «255» = A1 00 FF «-255» = A1 FF 01 «-1» = 9F «256» = A1 01 00 «-254» = A1 FF 02 «0» = 90 «32767» = A1 7F FF «-129» = A1 FF 7F «1» = 91 «32768» = A2 00 80 00 «-128» = A0 80 «12» = 9C «65535» = A2 00 FF FF «-127» = A0 81 «13» = A0 0D «65536» = A2 01 00 00 «-4» = A0 FC «127» = A0 7F «131072» = A2 02 00 00 [^zero-intbytes]: The value 0 needs zero bytes to identify the value, so `intbytes(0)` is the empty byte string. Non-zero values need at least one byte. ### Strings, ByteStrings and Symbols. Syntax for these three types varies only in the tag used. For `String` and `Symbol`, the data following the tag is a UTF-8 encoding of the `Value`'s code points, while for `ByteString` it is the raw data contained within the `Value` unmodified. «S» = [0xB1] ++ varint(|utf8(S)|) ++ utf8(S) if S ∈ String [0xB2] ++ varint(|S|) ++ S if S ∈ ByteString [0xB3] ++ varint(|utf8(S)|) ++ utf8(S) if S ∈ Symbol ### Booleans. «#f» = [0x80] «#t» = [0x81] ### Floats and Doubles. «F» when F ∈ Float = [0x82] ++ binary32(F) «D» when D ∈ Double = [0x83] ++ binary64(D) The functions `binary32(F)` and `binary64(D)` yield big-endian 4- and 8-byte IEEE 754 binary representations of `F` and `D`, respectively. ### Embeddeds. The `Repr` of an `Embedded` is the `Repr` of a `Value` chosen to represent the denoted object, prefixed with `[0x86]`. «#!V» = [0x86] ++ «V» ### Annotations. To annotate a `Repr` `r` with some `Value` `v`, prepend `r` with `[0x85] ++ «v»`. For example, the `Repr` corresponding to textual syntax `@a@b[]`, i.e. an empty sequence annotated with two symbols, `a` and `b`, is «@a @b []» = [0x85] ++ «a» ++ [0x85] ++ «b» ++ «[]» = [0x85, 0xB3, 0x01, 0x61, 0x85, 0xB3, 0x01, 0x62, 0xB5, 0x84] ## Examples ### Ordering. The total ordering specified [above](#total-order) means that the following statements are true: "bzz" < "c" < "caa" < #!"a" #t < 3.0f < 3.0 < 3 < "3" < |3| < [] < #!#t ### Simple examples. | Value | Encoded byte sequence | |-----------------------------|---------------------------------------------------------------------------------| | `>` | B4 B3 07 'c' 'a' 'p' 't' 'u' 'r' 'e' B4 B3 07 'd' 'i' 's' 'c' 'a' 'r' 'd' 84 84 | | `[1 2 3 4]` | B5 91 92 93 94 84 | | `[-2 -1 0 1]` | B5 9E 9F 90 91 84 | | `"hello"` (format B) | B1 05 'h' 'e' 'l' 'l' 'o' | | `["a" b #"c" [] #{} #t #f]` | B5 B1 01 'a' B3 01 'b' B2 01 'c' B5 84 B6 84 81 80 84 | | `-257` | A1 FE FF | | `-1` | 9F | | `0` | 90 | | `1` | 91 | | `255` | A1 00 FF | | `1.0f` | 82 3F 80 00 00 | | `1.0` | 83 3F F0 00 00 00 00 00 00 | | `-1.202e300` | 83 FE 3C B7 B7 59 BF 04 26 | The next example uses a non-`Symbol` label for a record.[^extensibility2] The `Record` <[titled person 2 thing 1] 101 "Blackwell" "Dr"> encodes to B4 ;; Record B5 ;; Sequence B3 06 74 69 74 6C 65 64 ;; Symbol, "titled" B3 06 70 65 72 73 6F 6E ;; Symbol, "person" 92 ;; SignedInteger, "2" B3 05 74 68 69 6E 67 ;; Symbol, "thing" 91 ;; SignedInteger, "1" 84 ;; End (sequence) A0 65 ;; SignedInteger, "101" B1 09 42 6C 61 63 6B 77 65 6C 6C ;; String, "Blackwell" B4 ;; Record B3 04 64 61 74 65 ;; Symbol, "date" A1 07 1D ;; SignedInteger, "1821" 92 ;; SignedInteger, "2" 93 ;; SignedInteger, "3" 84 ;; End (record) B1 02 44 72 ;; String, "Dr" 84 ;; End (record) [^extensibility2]: It happens to line up with Racket's representation of a record label for an inheritance hierarchy where `titled` extends `person` extends `thing`: (struct date (year month day) #:prefab) (struct thing (id) #:prefab) (struct person thing (name date-of-birth) #:prefab) (struct titled person (title) #:prefab) For more detail on Racket's representations of record labels, see [the Racket documentation for `make-prefab-struct`](http://docs.racket-lang.org/reference/structutils.html#%28def._%28%28quote._~23~25kernel%29._make-prefab-struct%29%29). --- ### JSON examples. The examples from [RFC 8259](https://tools.ietf.org/html/rfc8259#section-13) read as valid Preserves, though the JSON literals `true`, `false` and `null` read as `Symbol`s. The first example: { "Image": { "Width": 800, "Height": 600, "Title": "View from 15th Floor", "Thumbnail": { "Url": "http://www.example.com/image/481989943", "Height": 125, "Width": 100 }, "Animated" : false, "IDs": [116, 943, 234, 38793] } } encodes to binary as follows: B7 B1 05 "Image" B7 B1 03 "IDs" B5 A0 74 A1 03 AF A1 00 EA A2 00 97 89 84 B1 05 "Title" B1 14 "View from 15th Floor" B1 05 "Width" A1 03 20 B1 06 "Height" A1 02 58 B1 08 "Animated" B3 05 "false" B1 09 "Thumbnail" B7 B1 03 "Url" B1 26 "http://www.example.com/image/481989943" B1 05 "Width" A0 64 B1 06 "Height" A0 7D 84 84 84 and the second example: [ { "precision": "zip", "Latitude": 37.7668, "Longitude": -122.3959, "Address": "", "City": "SAN FRANCISCO", "State": "CA", "Zip": "94107", "Country": "US" }, { "precision": "zip", "Latitude": 37.371991, "Longitude": -122.026020, "Address": "", "City": "SUNNYVALE", "State": "CA", "Zip": "94085", "Country": "US" } ] encodes to binary as follows: B5 B7 B1 03 "Zip" B1 05 "94107" B1 04 "City" B1 0D "SAN FRANCISCO" B1 05 "State" B1 02 "CA" B1 07 "Address" B1 00 B1 07 "Country" B1 02 "US" B1 08 "Latitude" 83 40 42 E2 26 80 9D 49 52 B1 09 "Longitude" 83 C0 5E 99 56 6C F4 1F 21 B1 09 "precision" B1 03 "zip" 84 B7 B1 03 "Zip" B1 05 "94085" B1 04 "City" B1 09 "SUNNYVALE" B1 05 "State" B1 02 "CA" B1 07 "Address" B1 00 B1 07 "Country" B1 02 "US" B1 08 "Latitude" 83 40 42 AF 9D 66 AD B4 03 B1 09 "Longitude" 83 C0 5E 81 AA 4F CA 42 AF B1 09 "precision" B1 03 "zip" 84 84 ## 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. **Canonical form for cryptographic hashing and signing.** No canonical textual encoding of a `Value` is specified. A [canonical form][canonical] exists for binary encoded `Value`s, and implementations *SHOULD* produce canonical binary encodings by default; however, an implementation *MAY* permit two serializations of the same `Value` to yield different binary `Repr`s. ## Acknowledgements The treatment of commas as whitespace in the text syntax is inspired by the same feature of [EDN](https://github.com/edn-format/edn). The text syntax for `Boolean`s, `Symbol`s, and `ByteString`s is directly inspired by [Racket](https://racket-lang.org/)'s lexical syntax. ## Appendix. Autodetection of textual or binary syntax Every tag byte in a binary Preserves `Document` falls within the range [`0x80`, `0xBF`]. These bytes, interpreted as UTF-8, are *continuation bytes*, and will never occur as the first byte of a UTF-8 encoded code point. This means no binary-encoded document can be misinterpreted as valid UTF-8. Conversely, a UTF-8 document must start with a valid codepoint, meaning in particular that it must not start with a byte in the range [`0x80`, `0xBF`]. This means that no UTF-8 encoded textual-syntax Preserves document can be misinterpreted as a binary-syntax document. Examination of the top two bits of the first byte of a document gives its syntax: if the top two bits are `10`, it should be interpreted as a binary-syntax document; otherwise, it should be interpreted as text. ## Appendix. Table of tag values 80 - False 81 - True 82 - Float 83 - Double 84 - End marker 85 - Annotation 86 - Embedded (8x) RESERVED 87-8F 9x - Small integers 0..12,-3..-1 An - Small integers, (n+1) bytes long B0 - Small integers, variable length B1 - String B2 - ByteString B3 - Symbol B4 - Record B5 - Sequence B6 - Set B7 - Dictionary ## Appendix. Binary SignedInteger representation Languages that provide fixed-width machine word types may find the following table useful in encoding and decoding binary `SignedInteger` values. | Integer range | Bytes required | Encoding (hex) | | --- | --- | --- | | -3 ≤ n ≤ 12 | 1 | `3X` | | -27 ≤ n < 27 (i8) | 2 | `A0` `XX` | | -215 ≤ n < 215 (i16) | 3 | `A1` `XX` `XX` | | -223 ≤ n < 223 (i24) | 4 | `A2` `XX` `XX` `XX` | | -231 ≤ n < 231 (i32) | 5 | `A3` `XX` `XX` `XX` `XX` | | -239 ≤ n < 239 (i40) | 6 | `A4` `XX` `XX` `XX` `XX` `XX` | | -247 ≤ n < 247 (i48) | 7 | `A5` `XX` `XX` `XX` `XX` `XX` `XX` | | -255 ≤ n < 255 (i56) | 8 | `A6` `XX` `XX` `XX` `XX` `XX` `XX` `XX` | | -263 ≤ n < 263 (i64) | 9 | `A7` `XX` `XX` `XX` `XX` `XX` `XX` `XX` `XX` | ## Notes