forked from syndicate-lang/preserves
1418 lines
59 KiB
Markdown
1418 lines
59 KiB
Markdown
---
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---
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<title>Preserves: an Expressive Data Language</title>
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<link rel="stylesheet" href="preserves.css">
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# Preserves: an Expressive Data Language
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Tony Garnock-Jones <tonyg@leastfixedpoint.com>
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September 2018. Version 0.0.3.
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[sexp.txt]: http://people.csail.mit.edu/rivest/Sexp.txt
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[spki]: http://world.std.com/~cme/html/spki.html
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[varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
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[erlang-map]: http://erlang.org/doc/reference_manual/data_types.html#map
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[abnf]: https://tools.ietf.org/html/rfc7405
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This document proposes a data model and serialization format called
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*Preserves*.
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Preserves supports *records* with user-defined *labels*. This makes it
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more expressive[^macro-expressiveness] than most data languages in use
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on the web and allows it to easily represent the *labelled sums of
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products* as seen in many functional programming languages.
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Preserves also supports the usual suite of atomic and compound data
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types, in particular including *binary* data as a distinct type from
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text strings.
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Finally, Preserves defines precisely how to *compare* two values.
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Comparison is based on the data model, not on syntax or on data
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structures of any particular implementation language.
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[^macro-expressiveness]: By "expressive" I mean *macro-expressive*
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in the sense of Felleisen's 1991 paper, "On the Expressive Power
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of Programming Languages".
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Roughly speaking, there's no way in a JSON document to introduce a
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new kind of information (such as binary data, or a date-stamp, or
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a "person" object) in an *unambiguous way* without *global
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agreement* from every potential consumer of the document. With an
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extensible labelled record type, there is.
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Felleisen, Matthias. “On the Expressive Power of Programming
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Languages.” Science of Computer Programming 17, no. 1--3 (1991):
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35–75.
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## Starting with Semantics
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Taking inspiration from functional programming, we start with a
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definition of the *values* that we want to work with and give them
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meaning independent of their syntax. When we write examples of values,
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we will do so using the [textual syntax](#textual-syntax) defined
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later in this document.
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Our `Value`s fall into two broad categories: *atomic* and *compound*
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data.
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Value = Atom
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| Compound
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Atom = Boolean
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| Float
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| Double
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| SignedInteger
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| String
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| ByteString
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| Symbol
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Compound = Record
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| Sequence
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| Set
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| Dictionary
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**Total order.**<a name="total-order"></a> As we go, we will
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incrementally specify a total order over `Value`s. Two values of the
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same kind are compared using kind-specific rules. The ordering among
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values of different kinds is essentially arbitrary, but having a total
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order is convenient for many tasks, so we define it as
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follows:[^ordering-by-syntax]
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(Values) Atom < Compound
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(Compounds) Record < Sequence < Set < Dictionary
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(Atoms) Boolean < Float < Double < SignedInteger
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< String < ByteString < Symbol
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[^ordering-by-syntax]: The observant reader may note that the
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ordering here is (almost) the same as that implied by the tagging
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scheme used in the concrete binary syntax for `Value`s. (The
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exception is the syntax for small integers near zero.)
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**Equivalence.**<a name="equivalence"></a> Two `Value`s are equal if
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neither is less than the other according to the total order.
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### Signed integers.
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A `SignedInteger` is a signed integer of arbitrary width.
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`SignedInteger`s are compared as mathematical integers.
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**Examples.** 10; -6; 0.
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**Non-examples.** NaN (the clue is in the name!); ∞ (not finite); 0.2
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(not an integer); 1/7 (likewise); 2+*i*3 (likewise); √2 (likewise).
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### Unicode strings.
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A `String` is a sequence of Unicode
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[code-point](http://www.unicode.org/glossary/#code_point)s. `String`s
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are compared lexicographically, code-point by
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code-point.[^utf8-is-awesome]
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[^utf8-is-awesome]: Happily, the design of UTF-8 is such that this
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gives the same result as a lexicographic byte-by-byte comparison
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of the UTF-8 encoding of a string!
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**Examples.** `"Hello world"`, an eleven-code-point string; `"z水𝄞"`,
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the string containing the three Unicode code-points `z` (0x7A), `水`
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(0x6C34) and `𝄞` (0x1D11E); `""`, the empty string.
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### Binary data.
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A `ByteString` is an ordered sequence of zero or more eight-bit bytes.
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`ByteString`s are compared lexicographically.
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**Examples.** `#""`, the empty `ByteString`; `#"ABC"`, the
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`ByteString` containing the integers 65, 66 and 67 (corresponding to
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ASCII characters `A`, `B` and `C`). **N.B.** Despite appearances,
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these are *binary* data.
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### Symbols.
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Programming languages like Lisp and Prolog frequently use string-like
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values called *symbols*. Here, a `Symbol` is, like a `String`, a
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sequence of Unicode code-points representing an identifier of some
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kind. `Symbol`s are also compared lexicographically by code-point.
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**Examples.** `hello-world`; `utf8-string`; `exact-integer?`.
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### Booleans.
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There are exactly two `Boolean` values, “false” and “true”. The
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“false” value compares less-than the “true” value. We write `#false`
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for “false”, and `#true` for “true”.
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### IEEE floating-point values.
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A `Float` is a single-precision IEEE 754 floating-point value; a
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`Double` is a double-precision IEEE 754 floating-point value.
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`Float`s, `Double`s and `SignedInteger`s are considered disjoint, and
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so by the rules [above](#total-order), every `Float` is less than
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every `Double`, and every `SignedInteger` is greater than both. Two
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`Float`s or two `Double`s are to be ordered by the `totalOrder`
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predicate defined in section 5.10 of
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[IEEE Std 754-2008](https://dx.doi.org/10.1109/IEEESTD.2008.4610935).
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We write examples using a fractional part and/or an exponent to
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distinguish them from `SignedInteger`s. An additional suffix `f`
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distinguishes `Float`s from `Double`s.
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**Examples.** 10.0f; -6.0; 0.0f; 0.5; -1.202e300.
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**Non-examples.** 10, -6, and 0, because writing them this way
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indicates `SignedInteger`s, not `Float`s or `Double`s.
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### Records.
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A `Record` is a *labelled* tuple of zero or more `Value`s, called the
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record's *fields*. A record's label is itself a `Value`, though it
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will usually be a `Symbol`.[^extensibility] [^iri-labels] `Record`s
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are compared lexicographically as if they were just tuples; that is,
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first by their labels, and then by the remainder of their fields.
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[^extensibility]: The [Racket](https://racket-lang.org/) programming
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language defines
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“[prefab](http://docs.racket-lang.org/guide/define-struct.html#(part._prefab-struct))”
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structure types, which map well to our `Record`s. Racket supports
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record extensibility by encoding record supertypes into record
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labels as specially-formatted lists.
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[^iri-labels]: It is occasionally (but seldom) necessary to
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interpret such `Symbol` labels as UTF-8 encoded IRIs. Where a
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label can be read as a relative IRI, it is notionally interpreted
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with respect to the IRI
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`urn:uuid:6bf094a6-20f1-4887-ada7-46834a9b5b34`; where a label can
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be read as an absolute IRI, it stands for that IRI; and otherwise,
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it cannot be read as an IRI at all, and so the label simply stands
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for itself—for its own `Value`.
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**Examples.** `foo(1 2 3)`, a `Record` with label `foo` and fields 1,
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2 and 3; `void()`, a `Record` with label `void` and no fields.
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**Non-examples.** `()`, because it lacks a label; `void`, because it
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lacks even an empty tuple of fields.
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### Sequences.
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A `Sequence` is a general-purpose, variable-length ordered sequence of
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zero or more `Value`s. `Sequence`s are compared lexicographically.
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**Examples.** `[]`, the empty sequence; `[1 2 3]`, the sequence of
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`SignedInteger`s 1, 2 and 3.
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### Sets.
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A `Set` is an unordered finite set of `Value`s. It contains no
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duplicate values, following the [equivalence relation](#equivalence)
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induced by the total order on `Value`s. Two `Set`s are compared by
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sorting their elements ascending using the [total order](#total-order)
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and comparing the resulting `Sequence`s.
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**Examples.** `#set{}`, the empty set; `#set{#set{}}`, the set
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containing only the empty set; `{4 "hello" (void) 9.0f}`, the set
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containing 4, the string `"hello"`, the record with label `void` and
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no fields, and the `Float` denoting the number 9.0; `{1 1.0f}`, the
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set containing a `SignedInteger` and a `Float`; `{mime(application/xml
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#"<x/>") mime(application/xml #"<x />")}`, a set containing two
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different `mime` records.[^mime-xml-difference]
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[^mime-xml-difference]: The two XML documents `<x/>` and `<x />`
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differ by bytewise comparison, and thus yield different record
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values, even though under the semantics of XML they denote
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identical XML infoset.
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**Non-examples.** `{1 1}`, because it contains multiple equivalent
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`Value`s; `{}`, because without the `#set` marker, it denotes the
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empty dictionary.
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### Dictionaries.
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A `Dictionary` is an unordered finite collection of pairs of `Value`s.
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Each pair comprises a *key* and a *value*. Keys in a `Dictionary` must
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be pairwise distinct. Instances of `Dictionary` are compared by
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lexicographic comparison of the sequences resulting from ordering each
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`Dictionary`'s pairs in ascending order by key.
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**Examples.** `{}`, the empty dictionary; `{a: 1}`, the dictionary
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mapping the `Symbol` `a` to the `SignedInteger` 1; `{[1 2 3]: a}`,
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mapping `[1 2 3]` to `a`; `{"hi": 0, hi: 0, there: []}`, having a
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`String` and two `Symbol` keys, and `SignedInteger` and `Sequence`
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values.
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**Non-examples.** `{a:1 b:2 a:3}`, because it contains duplicate
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keys; `{[7 8]:[] [7 8]:99}`, for the same reason.
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## Textual Syntax
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Now we have discussed `Value`s and their meanings, we may turn to
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techniques for *representing* `Value`s for communication or storage.
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In this section, we use [case-sensitive ABNF][abnf] to define a
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textual syntax that is easy for people to read and
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write.[^json-superset] Most of the examples in this document are
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written using this syntax. In the following section, we will define an
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equivalent compact machine-readable syntax.
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[^json-superset]: The grammar of the textual syntax is a superset of
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JSON, with the slightly unusual feature that `true`, `false`, and
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`null` are all read as `Symbol`s, and that `SignedInteger`s are
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never read as `Double`s.
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### Character set
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[ABNF][abnf] allows easy definition of US-ASCII-based languages.
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However, Preserves is a Unicode-based language. Therefore, we
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reinterpret ABNF as a grammar for recognising sequences of Unicode
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code points.
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Textual syntax for a `Value` *SHOULD* be encoded using UTF-8 where
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possible.
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### Whitespace
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Whitespace is defined as any number of spaces, tabs, carriage returns,
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line feeds, comments, or commas. A comment is a semicolon followed by
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the unicode code points up to and including the next carriage return
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or line feed.
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ws = *(%x20 / %x09 / newline / comment / ",")
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newline = CR / LF
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comment = ";" *(WSP / nonnl) newline
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nonnl = <any Unicode code point except CR or LF>
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### Grammar
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Standalone documents containing textual representations of `Value`s may have trailing whitespace.
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Document = Value ws
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Any `Value` may be preceded by whitespace.
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Value = ws (Record / Collection / Atom / Compact)
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Collection = Sequence / Dictionary / Set
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Atom = Boolean / Float / Double / SignedInteger /
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String / ByteString / Symbol
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Each `Record` is its label-`Value` followed by a parenthesised
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grouping of its field-`Value`s. Whitespace is not permitted between
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the label and the open-parenthesis.
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Record = Value "(" *Value ws ")"
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`Sequence`s are enclosed in square brackets. `Dictionary` values are
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curly-brace-enclosed colon-separated pairs of values. `Set`s are
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written either as a simple curly-brace-enclosed non-empty sequence of
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values, or as a possibly-empty sequence of values enclosed by the
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tokens `#set{` and `}`.[^printing-collections]
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Sequence = "[" *Value ws "]"
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Dictionary = "{" *(Value ws ":" Value) ws "}"
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Set = %s"#set{" *Value ws "}" / "{" 1*Value ws "}"
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[^printing-collections]: **Implementation note.** When implementing
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printing of `Value`s using the textual syntax, consider supporting
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(a) optional pretty-printing with indentation, (b) optional
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JSON-compatible print mode for that subset of `Value` that is
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compatible with JSON, and (c) optional submodes for no commas,
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commas separating, and commas terminating elements or key/value
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pairs within a collection.
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The special cases of records with a single field, which is in turn a
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sequence or dictionary, may be written omitting the parentheses.
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Record =/ Value Sequence
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Record =/ Value Dictionary
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`Boolean`s are the simple literal strings `#true` and `#false`.
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Boolean = %s"#true" / %s"#false"
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Numeric data follow the
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[JSON grammar](https://tools.ietf.org/html/rfc8259#section-6), with
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the addition of a trailing "f" distinguishing `Float` from `Double`
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values. `Float`s and `Double`s always have either a fractional part or
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an exponent part, where `SignedInteger`s never have
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either.[^reading-and-writing-floats-accurately]
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[^arbitrary-precision-signedinteger]
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Float = flt %i"f"
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Double = flt
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SignedInteger = int
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digit1-9 = %x31-39
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nat = %x30 / ( digit1-9 *DIGIT )
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int = ["-"] nat
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frac = "." 1*DIGIT
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exp = %i"e" ["-"/"+"] 1*DIGIT
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flt = int (frac exp / frac / exp)
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[^reading-and-writing-floats-accurately]: **Implementation note.**
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Your language's standard library likely has a good routine for
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converting between decimal notation and IEEE 754 floating-point.
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However, if not, or if you are interested in the challenges of
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accurately reading and writing floating point numbers, see the
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excellent matched pair of 1990 papers by Clinger and Steele &
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White, and a recent follow-up by Jaffer:
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Clinger, William D. ‘How to Read Floating Point Numbers
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Accurately’. In Proc. PLDI. White Plains, New York, 1990.
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<https://doi.org/10.1145/93542.93557>.
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Steele, Guy L., Jr., and Jon L. White. ‘How to Print
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Floating-Point Numbers Accurately’. In Proc. PLDI. White Plains,
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New York, 1990. <https://doi.org/10.1145/93542.93559>.
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Jaffer, Aubrey. ‘Easy Accurate Reading and Writing of
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Floating-Point Numbers’. ArXiv:1310.8121 [Cs], 27 October 2013.
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<http://arxiv.org/abs/1310.8121>.
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[^arbitrary-precision-signedinteger]: **Implementation note.** Be
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aware when implementing reading and writing of `SignedInteger`s
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that the data model *requires* arbitrary-precision integers. Your
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I/O routines must not truncate precision either when reading or
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writing a `SignedInteger`.
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`String`s are,
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[as in JSON](https://tools.ietf.org/html/rfc8259#section-7), possibly
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escaped text surrounded by double quotes. The escaping rules are the
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same as for JSON.[^string-json-correspondence] [^escaping-surrogate-pairs]
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String = %x22 *char %x22
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char = unescaped / %x7C / escape (escaped / %x22 / %s"u" 4HEXDIG)
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unescaped = %x20-21 / %x23-5B / %x5D-7B / %x7D-10FFFF
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escape = %x5C ; \
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escaped = ( %x5C / ; \ reverse solidus U+005C
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%x2F / ; / solidus U+002F
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%x62 / ; b backspace U+0008
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%x66 / ; f form feed U+000C
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%x6E / ; n line feed U+000A
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%x72 / ; r carriage return U+000D
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%x74 ) ; t tab U+0009
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[^string-json-correspondence]: The grammar for `String` has the same
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effect as the
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[JSON](https://tools.ietf.org/html/rfc8259#section-7) grammar for
|
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`string`. Some auxiliary definitions (e.g. `escaped`) are lifted
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largely unmodified from the text of RFC 8259.
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[^escaping-surrogate-pairs]: In particular, note JSON's rules around
|
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the use of surrogate pairs for code points not in the Basic
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Multilingual Plane. We encourage implementations to avoid escaping
|
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such characters when producing output, and instead to rely on the
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UTF-8 encoding of the entire document to handle them correctly.
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A `ByteString` may be written in any of three different forms.
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The first is similar to a `String`, but prepended with a hash sign
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`#`. In addition, only Unicode code points overlapping with printable
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7-bit ASCII are permitted unescaped inside such a `ByteString`; other
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byte values must be escaped by prepending a two-digit hexadecimal
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value with `\x`.
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ByteString = "#" %x22 *binchar %x22
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binchar = binunescaped / escape (escaped / %x22 / %s"x" 2HEXDIG)
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binunescaped = %x20-21 / %x23-5B / %x5D-7E
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The second is as a sequence of pairs of hexadecimal digits interleaved
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with whitespace and surrounded by `#hex{` and `}`.
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ByteString =/ %s"#hex{" *(ws / 2HEXDIG) ws "}"
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The third is as a sequence of
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[Base64](https://tools.ietf.org/html/rfc4648) characters, interleaved
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with whitespace and surrounded by `#base64{` and `}`. Plain and
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URL-safe Base64 characters are allowed.
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ByteString =/ %s"#base64{" *(ws / base64char) ws "}" /
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base64char = %x41-5A / %x61-7A / %x30-39 / "+" / "/" / "-" / "_" / "="
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A `Symbol` may be written in a "bare" form[^cf-sexp-token] so long as
|
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it conforms to certain restrictions on the characters appearing in the
|
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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
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||
escape syntax, except using a bar or pipe character (`|`) instead of a
|
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double quote mark.
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||
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Symbol = symstart *symcont / "|" *symchar "|"
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symstart = ALPHA / sympunct / symunicode
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symcont = ALPHA / sympunct / symunicode / DIGIT / "-"
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sympunct = "~" / "!" / "$" / "%" / "^" / "&" / "*" /
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"?" / "_" / "=" / "+" / "<" / ">" / "/" / "."
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||
symchar = unescaped / %x22 / escape (escaped / %x7C / %s"u" 4HEXDIG)
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||
symunicode = <any code point greater than 127 whose Unicode
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category is Lu, Ll, Lt, Lm, Lo, Mn, Mc, Me, Nd,
|
||
Nl, No, Pd, Pc, Po, Sc, Sm, Sk, So, or Co>
|
||
|
||
[^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).
|
||
|
||
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 `#value`.[^rationale-switch-to-binary] [^no-literal-binary-in-text]
|
||
|
||
Compact = %s"#value" 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 Binary Syntax
|
||
|
||
A `Repr` is an encoding, or representation, of a specific `Value`.
|
||
Each `Repr` comprises one or more bytes describing first the kind of
|
||
represented `Value` and the length of the representation, and then the
|
||
encoded details of the `Value` itself.
|
||
|
||
For a value `v`, we write `[[v]]` for the `Repr` of v.
|
||
|
||
### Type and Length representation
|
||
|
||
Each `Repr` takes one of three possible forms:
|
||
|
||
- (A) a fixed-length form, used for simple values such as `Boolean`s
|
||
or `Float`s.
|
||
|
||
- (B) a variable-length form with length specified up-front, used for
|
||
almost all `Record`s as well as for most `Sequence`s and `String`s,
|
||
when their sizes are known at the time serialization begins.
|
||
|
||
- (C) a variable-length streaming form with unknown or unpredictable
|
||
length, used only seldom for `Record`s, since the number of fields
|
||
in a `Record` is usually statically known, but sometimes used for
|
||
`Sequence`s, `String`s etc., such as in cases when serialization
|
||
begins before the number of elements or bytes in the corresponding
|
||
`Value` is known.
|
||
|
||
Applications may choose between formats B and C depending on their
|
||
needs at serialization time.
|
||
|
||
#### The lead byte
|
||
|
||
Every `Repr` starts with a *lead byte*, constructed by
|
||
`leadbyte(t,n,m)`, where `t`,`n`∈{0,1,2,3} and 0≤`m`<16:
|
||
|
||
leadbyte(t,n,m) = [t*64 + n*16 + m]
|
||
|
||
The arguments `t` and `n` describe the rest of the
|
||
representation:[^some-encodings-unused]
|
||
|
||
[^some-encodings-unused]: Some encodings are unused. All such
|
||
encodings are reserved for future versions of this specification.
|
||
|
||
- `t`=0, `n`=0 (format A) represents an `Atom` with fixed-length binary representation.
|
||
- `t`=0, `n`=1 (format A) represents certain small `SignedInteger`s.
|
||
- `t`=0, `n`=2 (format C) is a Stream Start byte.
|
||
- `t`=0, `n`=3 (format C) is a Stream End byte.
|
||
- `t`=1 (format B) represents an `Atom` with variable-length binary representation.
|
||
- `t`=2 (format B) represents a `Record`.
|
||
- `t`=3 (format B) represents a `Sequence`, `Set` or `Dictionary`.
|
||
|
||
#### Encoding data of fixed length (format A)
|
||
|
||
Each specific type of data defines its own rules for this format.
|
||
|
||
#### Encoding data of known length (format B)
|
||
|
||
A `Repr` where the length of the `Value` to be encoded is variable but
|
||
known uses the value of `m` in `leadbyte` to encode its length. The
|
||
length counts *bytes* for atomic `Value`s, but counts *contained
|
||
values* for compound `Value`s.
|
||
|
||
- A length `l` between 0 and 14 is represented using `leadbyte` with
|
||
`m=l`.
|
||
- A length of 15 or greater is represented by `m=15` and additional
|
||
bytes describing the length following the lead byte.
|
||
|
||
The function `header(t,n,m)` yields an appropriate sequence of bytes
|
||
describing a `Repr`'s type and length when `t`, `n` and `m` are
|
||
appropriate non-negative integers:
|
||
|
||
header(t,n,m) = leadbyte(t,n,m) when m < 15
|
||
or leadbyte(t,n,15) ++ varint(m) otherwise
|
||
|
||
The additional length bytes are formatted as
|
||
[base 128 varints][varint]. We write `varint(m)` for the
|
||
varint-encoding of `m`. Quoting the [Google Protocol Buffers][varint]
|
||
definition,
|
||
|
||
> 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.
|
||
|
||
**Examples.**
|
||
|
||
- The varint representation of 15 is just the byte 15.
|
||
- 300 (binary, grouped into 7-bit chunks, `10 0101100`) varint-encodes to the two bytes 172 and 2.
|
||
- 1000000000 (binary `11 1011100 1101011 0010100 0000000`) varint-encodes to bytes 128, 148, 235, 220, and 3.
|
||
|
||
#### Streaming data of unknown length (format C)
|
||
|
||
A `Repr` where the length of the `Value` to be encoded is variable and
|
||
not known at the time serialization of the `Value` starts is encoded
|
||
by a single Stream Start (“open”) byte, followed by zero or more
|
||
*chunks*, followed by a matching Stream End (“close”) byte:
|
||
|
||
open(t,n) = leadbyte(0,2, t*4 + n)
|
||
close(t,n) = leadbyte(0,3, t*4 + n)
|
||
|
||
For a `Repr` of a `Value` containing binary data, each chunk is to be
|
||
a format B `Repr` of a `ByteString`, no matter the type of the overall
|
||
`Repr`.
|
||
|
||
For a `Repr` of a `Value` containing other `Value`s, each chunk is to
|
||
be a single `Repr`.
|
||
|
||
### Records
|
||
|
||
Format B (known length):
|
||
|
||
[[ L(F_1...F_m) ]] = header(2,3,m+1) ++ [[L]] ++ [[F_1]] ++...++ [[F_m]]
|
||
|
||
For `m` fields, `m+1` is supplied to `header`, to account for the
|
||
encoding of the record label.
|
||
|
||
Format C (streaming):
|
||
|
||
[[ L(F_1...F_m) ]] = open(2,3) ++ [[L]] ++ [[F_1]] ++...++ [[F_m]] ++ close(2,3)
|
||
|
||
Applications *SHOULD* prefer the known-length format for encoding
|
||
`Record`s.
|
||
|
||
#### Application-specific short form for labels
|
||
|
||
Any given protocol using Preserves may additionally define an
|
||
interpretation for `n`∈{0,1,2}, mapping each *short form label
|
||
number* `n` to a specific record label. When encoding `m` fields with
|
||
short form label number `n`, format B becomes
|
||
|
||
header(2,n,m) ++ [[F_1]] ++...++ [[F_m]]
|
||
|
||
and format C becomes
|
||
|
||
open(2,n) ++ [[F_1]] ++...++ [[F_m]] ++ close(2,n)
|
||
|
||
**Examples.** For example, a protocol may choose to map records
|
||
labelled `void` to `n=0`, making
|
||
|
||
[[void()]] = header(2,0,0) = [0x80]
|
||
|
||
or it may map records labelled `person` to short form label number 1,
|
||
making
|
||
|
||
[[person("Dr", "Elizabeth", "Blackwell")]]
|
||
= header(2,1,3) ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]]
|
||
= [0x93] ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]]
|
||
|
||
for format B, or
|
||
|
||
= open(2,1) ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]] ++ close(2,1)
|
||
= [0x29] ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]] ++ [0x39]
|
||
|
||
for format C.
|
||
|
||
### Sequences, Sets and Dictionaries
|
||
|
||
Format B (known length):
|
||
|
||
[[ [X_1...X_m] ]] = header(3,0,m) ++ [[X_1]] ++...++ [[X_m]]
|
||
[[ #set{X_1...X_m} ]] = header(3,1,m) ++ [[X_1]] ++...++ [[X_m]]
|
||
[[ {K_1:V_1...K_m:V_m} ]] = header(3,2,m*2) ++ [[K_1]] ++ [[V_1]] ++...
|
||
++ [[K_m]] ++ [[V_m]]
|
||
|
||
Note that `m*2` is given to `header` for a `Dictionary`, since there
|
||
are two `Value`s in each key-value pair.
|
||
|
||
Format C (streaming):
|
||
|
||
[[ [X_1...X_m] ]] = open(3,0) ++ [[X_1]] ++...++ [[X_m]] ++ close(3,0)
|
||
[[ #set{X_1...X_m} ]] = open(3,1) ++ [[X_1]] ++...++ [[X_m]] ++ close(3,1)
|
||
[[ {K_1:V_1...K_m:V_m} ]] = open(3,2) ++ [[K_1]] ++ [[V_1]] ++...
|
||
++ [[K_m]] ++ [[V_m]] ++ close(3,2)
|
||
|
||
Applications may use whichever format suits their needs on a
|
||
case-by-case basis.
|
||
|
||
There is *no* ordering requirement on the `X_i` elements or
|
||
`K_i`/`V_i` pairs.[^no-sorting-rationale] They may appear in any
|
||
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, because (a)
|
||
where canonicalization is used for cryptographic signatures, it is
|
||
more reliable to simply retain the exact binary form of the signed
|
||
document than to depend on canonical de- and re-serialization, and
|
||
(b) sorting keys or elements makes no sense in streaming
|
||
serialization formats.
|
||
|
||
However, a quality implementation may wish to offer the programmer
|
||
the option of serializing with set elements and dictionary keys in
|
||
sorted order.
|
||
|
||
Note that `header(3,3,m)` and `open(3,3)`/`close(3,3)` are unused and reserved.
|
||
|
||
### SignedIntegers
|
||
|
||
Format B/A (known length/fixed-size):
|
||
|
||
[[ x ]] when x ∈ SignedInteger = header(1,0,m) ++ intbytes(x) if x<-3 ∨ 13≤x
|
||
header(0,1,x+16) if -3≤x<0
|
||
header(0,1,x) if 0≤x<13
|
||
|
||
Integers in the range [-3,12] are compactly represented using format A
|
||
because they are so frequently used. Other integers are represented
|
||
using format B.
|
||
|
||
Format C *MUST NOT* be used for `SignedInteger`s.
|
||
|
||
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)` <!-- for `x`≠0 --> is the sign bit.[^zero-intbytes]
|
||
|
||
[^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.
|
||
|
||
For example,
|
||
|
||
[[ -257 ]] = 42 FE FF [[ -3 ]] = 1D [[ 128 ]] = 42 00 80
|
||
[[ -256 ]] = 42 FF 00 [[ -2 ]] = 1E [[ 255 ]] = 42 00 FF
|
||
[[ -255 ]] = 42 FF 01 [[ -1 ]] = 1F [[ 256 ]] = 42 01 00
|
||
[[ -254 ]] = 42 FF 02 [[ 0 ]] = 10 [[ 32767 ]] = 42 7F FF
|
||
[[ -129 ]] = 42 FF 7F [[ 1 ]] = 11 [[ 32768 ]] = 43 00 80 00
|
||
[[ -128 ]] = 41 80 [[ 12 ]] = 1C [[ 65535 ]] = 43 00 FF FF
|
||
[[ -127 ]] = 41 81 [[ 13 ]] = 41 0D [[ 65536 ]] = 43 01 00 00
|
||
[[ -4 ]] = 41 FC [[ 127 ]] = 41 7F [[ 131072 ]] = 43 02 00 00
|
||
|
||
### Strings, ByteStrings and Symbols
|
||
|
||
Syntax for these three types varies only in the value of `n` supplied
|
||
to `header`, `open`, and `close`. In each case, the payload following
|
||
the header is a binary sequence; for `String` and `Symbol`, it is a
|
||
UTF-8 encoding of the `Value`'s code points, while for `ByteString` it
|
||
is the raw data contained within the `Value` unmodified.
|
||
|
||
Format B (known length):
|
||
|
||
[[ S ]] = header(1,n,m) ++ encode(S)
|
||
where m = |encode(S)|
|
||
and (n,encode(S)) = (1,utf8(S)) if S ∈ String
|
||
(2,S) if S ∈ ByteString
|
||
(3,utf8(S)) if S ∈ Symbol
|
||
|
||
To stream a `String`, `ByteString` or `Symbol`, emit `open(1,n)` and
|
||
then a sequence of zero or more format B chunks, followed by
|
||
`close(1,n)`. Every chunk must be a `ByteString`.
|
||
|
||
While the overall content of a streamed `String` or `Symbol` must be
|
||
valid UTF-8, individual chunks do not have to conform to UTF-8.
|
||
|
||
### Fixed-length Atoms
|
||
|
||
Fixed-length atoms all use format A, and do not have a length
|
||
representation. They repurpose the bits that format B `Repr`s use to
|
||
specify lengths. Applications *MUST NOT* use format C with
|
||
`open(0,n)` or `close(0,n)` for any `n`.
|
||
|
||
#### Booleans
|
||
|
||
[[ #false ]] = header(0,0,0) = [0x00]
|
||
[[ #true ]] = header(0,0,1) = [0x01]
|
||
|
||
#### Floats and Doubles
|
||
|
||
[[ F ]] when F ∈ Float = header(0,0,2) ++ binary32(F)
|
||
[[ D ]] when D ∈ Double = header(0,0,3) ++ 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.
|
||
|
||
## Examples
|
||
|
||
### Simple examples
|
||
|
||
<!-- TODO: Give some examples of large and small Preserves, perhaps -->
|
||
<!-- translated from various JSON blobs floating around the internet. -->
|
||
|
||
For the following examples, imagine an application that maps `Record`
|
||
short form label number 0 to label `discard`, 1 to `capture`, and 2 to
|
||
`observe`.
|
||
|
||
| Value | Encoded hexadecimal byte sequence |
|
||
|---------------------------------------------------|----------------------------------------------------------------------|
|
||
| `capture(discard())` | 91 80 |
|
||
| `observe(speak(discard(), capture(discard())))` | A1 B3 75 73 70 65 61 6B 80 91 80 |
|
||
| `[1 2 3 4]` (format B) | C4 11 12 13 14 |
|
||
| `[1 2 3 4]` (format C) | 2C 11 12 13 14 3C |
|
||
| `[-2 -1 0 1]` | C4 1E 1F 10 11 |
|
||
| `"hello"` (format B) | 55 68 65 6C 6C 6F |
|
||
| `"hello"` (format C, 2 chunks) | 25 62 68 65 63 6C 6C 6F 35 |
|
||
| `"hello"` (format C, 5 chunks) | 25 62 68 65 62 6C 6C 60 60 61 6F 35 |
|
||
| `["hello" there #"world" [] #set{} #true #false]` | C7 55 68 65 6C 6C 6F 75 74 68 65 72 65 65 77 6F 72 6C 64 C0 D0 01 00 |
|
||
| `-257` | 42 FE FF |
|
||
| `-1` | 1F |
|
||
| `0` | 10 |
|
||
| `1` | 11 |
|
||
| `255` | 42 00 FF |
|
||
| `1.0f` | 02 3F 80 00 00 |
|
||
| `1.0` | 03 3F F0 00 00 00 00 00 00 |
|
||
| `-1.202e300` | 03 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", date(1821 2 3), "Dr")
|
||
|
||
encodes to
|
||
|
||
B5 ;; Record, generic, 4+1
|
||
C5 ;; Sequence, 5
|
||
76 74 69 74 6C 65 64 ;; Symbol, "titled"
|
||
76 70 65 72 73 6F 6E ;; Symbol, "person"
|
||
12 ;; SignedInteger, "2"
|
||
75 74 68 69 6E 67 ;; Symbol, "thing"
|
||
11 ;; SignedInteger, "1"
|
||
41 65 ;; SignedInteger, "101"
|
||
59 42 6C 61 63 6B 77 65 6C 6C ;; String, "Blackwell"
|
||
B4 ;; Record, generic, 3+1
|
||
74 64 61 74 65 ;; Symbol, "date"
|
||
42 07 1D ;; SignedInteger, "1821"
|
||
12 ;; SignedInteger, "2"
|
||
13 ;; SignedInteger, "3"
|
||
52 44 72 ;; String, "Dr"
|
||
|
||
[^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:
|
||
|
||
E2
|
||
55 "Image"
|
||
EC
|
||
55 "Width" 42 03 20
|
||
55 "Title" 5F 14 "View from 15th Floor"
|
||
58 "Animated" 75 "false"
|
||
56 "Height" 42 02 58
|
||
59 "Thumbnail"
|
||
E6
|
||
55 "Width" 41 64
|
||
53 "Url" 5F 26 "http://www.example.com/image/481989943"
|
||
56 "Height" 41 7D
|
||
53 "IDs" C4
|
||
41 74
|
||
42 03 AF
|
||
42 00 EA
|
||
43 00 97 89
|
||
|
||
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:
|
||
|
||
C2
|
||
EF 10
|
||
59 "precision" 53 "zip"
|
||
58 "Latitude" 03 40 42 E2 26 80 9D 49 52
|
||
59 "Longitude" 03 C0 5E 99 56 6C F4 1F 21
|
||
57 "Address" 50
|
||
54 "City" 5D "SAN FRANCISCO"
|
||
55 "State" 52 "CA"
|
||
53 "Zip" 55 "94107"
|
||
57 "Country" 52 "US"
|
||
EF 10
|
||
59 "precision" 53 "zip"
|
||
58 "Latitude" 03 40 42 AF 9D 66 AD B4 03
|
||
59 "Longitude" 03 C0 5E 81 AA 4F CA 42 AF
|
||
57 "Address" 50
|
||
54 "City" 59 "SUNNYVALE"
|
||
55 "State" 52 "CA"
|
||
53 "Zip" 55 "94085"
|
||
57 "Country" 52 "US"
|
||
|
||
## Conventions for Common Data Types
|
||
|
||
The `Value` data type is essentially an S-Expression, able to
|
||
represent semi-structured data over `ByteString`, `String`,
|
||
`SignedInteger` atoms and so on.[^why-not-spki-sexps]
|
||
|
||
[^why-not-spki-sexps]: Rivest's S-Expressions are in many ways
|
||
similar to Preserves. However, while they include binary data and
|
||
sequences, and an obvious equivalence for them exists, they lack
|
||
numbers *per se* as well as any kind of unordered structure such
|
||
as sets or maps. In addition, while "display hints" allow
|
||
labelling of binary data with an intended interpretation, they
|
||
cannot be attached to any other kind of structure, and the "hint"
|
||
itself can only be a binary blob.
|
||
|
||
However, users need a wide variety of data types for representing
|
||
domain-specific values such as various kinds of encoded and normalized
|
||
text, calendrical values, machine words, and so on.
|
||
|
||
Appropriately-labelled `Record`s denote these domain-specific data
|
||
types.[^why-dictionaries]
|
||
|
||
[^why-dictionaries]: Given `Record`'s existence, it may seem odd
|
||
that `Dictionary`, `Set`, `Float`, etc. are given special
|
||
treatment. Preserves aims to offer a useful basic equivalence
|
||
predicate to programmers, and so if a data type demands a special
|
||
equivalence predicate, as `Dictionary`, `Set` and `Float` all do,
|
||
then the type should be included in the base language. Otherwise,
|
||
it can be represented as a `Record` and treated separately. Both
|
||
`Boolean` and `String` are seeming exceptions: they merit
|
||
inclusion because of their cultural importance.
|
||
|
||
All of these conventions are optional. They form a layer atop the core
|
||
`Value` structure. Non-domain-specific tools do not in general need to
|
||
treat them specially.
|
||
|
||
**Validity.** Many of the labels we will describe in this section come
|
||
with side-conditions on the contents of labelled `Record`s. It is
|
||
possible to construct an instance of `Value` that violates these
|
||
side-conditions without ceasing to be a `Value` or becoming
|
||
unrepresentable. However, we say that such a `Value` is *invalid*
|
||
because it fails to honour the necessary side-conditions.
|
||
Implementations *SHOULD* allow two modes of working: one which
|
||
treats all `Value`s identically, without regard for side-conditions,
|
||
and one which enforces validity (i.e. side-conditions) when reading,
|
||
writing, or constructing `Value`s.
|
||
|
||
### MIME-type tagged binary data
|
||
|
||
Many internet protocols use
|
||
[media types](https://tools.ietf.org/html/rfc6838) (a.k.a MIME types)
|
||
to indicate the format of some associated binary data. For this
|
||
purpose, we define `MIMEData` to be a record labelled `mime` with two
|
||
fields, the first being a `Symbol`, the media type, and the second
|
||
being a `ByteString`, the binary data.
|
||
|
||
While each media type may define its own rules for comparing
|
||
documents, we define ordering among `MIMEData` *representations* of
|
||
such media types following the general rules for ordering of
|
||
`Record`s.
|
||
|
||
**Examples.**
|
||
|
||
| Value | Encoded hexadecimal byte sequence |
|
||
|--------------------------------------------|-------------------------------------------------------------------------------------------------------------------|
|
||
| `mime(application/octet-stream #"abcde")` | B3 74 6D 69 6D 65 7F 18 61 70 70 6C 69 63 61 74 69 6F 6E 2F 6F 63 74 65 74 2D 73 74 72 65 61 6D 65 61 62 63 64 65 |
|
||
| `mime(text/plain #"ABC")` | B3 74 6D 69 6D 65 7A 74 65 78 74 2F 70 6C 61 69 6E 63 41 42 43 |
|
||
| `mime(application/xml #"<xhtml/>")` | B3 74 6D 69 6D 65 7F 0F 61 70 70 6C 69 63 61 74 69 6F 6E 2F 78 6D 6C 68 3C 78 68 74 6D 6C 2F 3E |
|
||
| `mime(text/csv #"123,234,345")` | B3 74 6D 69 6D 65 78 74 65 78 74 2F 63 73 76 6B 31 32 33 2C 32 33 34 2C 33 34 35 |
|
||
|
||
Applications making heavy use of `mime` records may choose to use a
|
||
short form label number for the record type. For example, if short
|
||
form label number 1 were chosen, the second example above,
|
||
`mime(text/plain "ABC")`, would be encoded with "92" in place of "B3
|
||
74 6D 69 6D 65".
|
||
|
||
### Unicode normalization forms
|
||
|
||
Unicode defines multiple
|
||
[normalization forms](http://unicode.org/reports/tr15/) for text.
|
||
While no particular normalization form is required for `String`s,
|
||
users may need to unambiguously signal or require a particular
|
||
normalization form. A `NormalizedString` is a `Record` labelled with
|
||
`unicode-normalization` and having two fields, the first of which is a
|
||
`Symbol` specifying the normalization form used (e.g. `nfc`, `nfd`,
|
||
`nfkc`, `nfkd`), and the second of which is a `String` whose
|
||
underlying code point representation *MUST* be normalized according to
|
||
the named normalization form.
|
||
|
||
### IRIs (URIs, URLs, URNs, etc.)
|
||
|
||
An `IRI` is a `Record` labelled with `iri` and having one field, a
|
||
`String` which is the IRI itself and which *MUST* be a valid absolute
|
||
or relative IRI.
|
||
|
||
### Machine words
|
||
|
||
The definition of `SignedInteger` captures all integers. However, in
|
||
certain circumstances it can be valuable to assert that a number
|
||
inhabits a particular range, such as a fixed-width machine word.
|
||
|
||
A family of labels `i`*n* and `u`*n* for *n* ∈ {8,16,32,64} denote
|
||
*n*-bit-wide signed and unsigned range restrictions, respectively.
|
||
Records with these labels *MUST* have one field, a `SignedInteger`,
|
||
which *MUST* fall within the appropriate range. That is, to be valid,
|
||
- in `i8(`*x*`)`, -128 <= *x* <= 127.
|
||
- in `u8(`*x*`)`, 0 <= *x* <= 255.
|
||
- in `i16(`*x*`)`, -32768 <= *x* <= 32767.
|
||
- etc.
|
||
|
||
### Anonymous Tuples and Unit
|
||
|
||
A `Tuple` is a `Record` with label `tuple` and zero or more fields,
|
||
denoting an anonymous tuple of values.
|
||
|
||
The 0-ary tuple, `tuple()`, denotes the empty tuple, sometimes called
|
||
"unit" or "void" (but *not* e.g. JavaScript's "undefined" value).
|
||
|
||
### Null and Undefined
|
||
|
||
Tony Hoare's
|
||
"[billion-dollar mistake](https://en.wikipedia.org/wiki/Tony_Hoare#Apologies_and_retractions)"
|
||
can be represented with the 0-ary `Record` `null()`. An "undefined"
|
||
value can be represented as `undefined()`.
|
||
|
||
### Dates and Times
|
||
|
||
Dates, times, moments, and timestamps can be represented with a
|
||
`Record` with label `rfc3339` having a single field, a `String`, which
|
||
*MUST* conform to one of the `full-date`, `partial-time`, `full-time`,
|
||
or `date-time` productions of
|
||
[section 5.6 of RFC 3339](https://tools.ietf.org/html/rfc3339#section-5.6).
|
||
|
||
## Security Considerations
|
||
|
||
**Empty chunks.** Streamed (format C) `String`s, `ByteString`s and
|
||
`Symbol`s may include chunks of zero length. This opens up a
|
||
possibility for denial-of-service: an attacker may begin streaming a
|
||
string, sending an endless sequence of zero length chunks, appearing
|
||
to make progress but not actually doing so. Implementations may place
|
||
optional reasonable restrictions on the number of consecutive empty
|
||
chunks that may appear in a stream, and may even supply an optional
|
||
mode that rejects empty chunks entirely.
|
||
|
||
**Whitespace.** Similarly, the textual format for `Value`s allows
|
||
arbitrary whitespace in many positions. In streaming transfer
|
||
situations, consider optional restrictions on the amount of
|
||
consecutive whitespace and comments that may appear in a serialized
|
||
`Value`.
|
||
|
||
**Canonical form for cryptographic hashing and signing.** As
|
||
specified, neither the textual nor the compact binary encoding rules
|
||
for `Value`s force canonical serializations. Two serializations of the
|
||
same `Value` may yield different binary `Repr`s.
|
||
|
||
## Appendix. Table of lead byte values
|
||
|
||
00 - False
|
||
01 - True
|
||
02 - Float
|
||
03 - Double
|
||
(0x) RESERVED 04-0F
|
||
1x - Small integers 0..12,-3..-1
|
||
2x - Start Stream
|
||
3x - End Stream
|
||
|
||
4x - SignedInteger
|
||
5x - String
|
||
6x - ByteString
|
||
7x - Symbol
|
||
|
||
8x - short form Record label index 0
|
||
9x - short form Record label index 1
|
||
Ax - short form Record label index 2
|
||
Bx - Record
|
||
|
||
Cx - Sequence
|
||
Dx - Set
|
||
Ex - Dictionary
|
||
(Fx) RESERVED F0-FF
|
||
|
||
## Appendix. Bit fields within lead byte values
|
||
|
||
tt nn mmmm contents
|
||
---------- ---------
|
||
|
||
00 00 0000 False
|
||
00 00 0001 True
|
||
00 00 0010 Float, 32 bits big-endian binary
|
||
00 00 0011 Double, 64 bits big-endian binary
|
||
|
||
00 01 xxxx Small integers 0..12,-3..-1
|
||
|
||
00 10 ttnn Start Stream <tt,nn>
|
||
When tt = 00 --> error
|
||
01 --> each chunk is a ByteString
|
||
1x --> each chunk is a single encoded Value
|
||
00 11 ttnn End Stream <tt,nn> (must match preceding Start Stream)
|
||
|
||
01 00 mmmm SignedInteger, big-endian binary
|
||
01 01 mmmm String, UTF-8 binary
|
||
01 10 mmmm ByteString
|
||
01 11 mmmm Symbol, UTF-8 binary
|
||
|
||
10 00 mmmm application-specific Record
|
||
10 01 mmmm application-specific Record
|
||
10 10 mmmm application-specific Record
|
||
10 11 mmmm Record
|
||
|
||
11 00 mmmm Sequence
|
||
11 01 mmmm Set
|
||
11 10 mmmm Dictionary
|
||
|
||
If mmmm = 1111, a varint(m) follows, giving the length, before
|
||
the body; otherwise, m is the length of the body to follow.
|
||
|
||
## Appendix. Representing Values in Programming Languages
|
||
|
||
We have given a definition of `Value` and its semantics, and proposed
|
||
a concrete syntax for communicating and storing `Value`s. We now turn
|
||
to **suggested** representations of `Value`s as *programming-language
|
||
values* for various programming languages.
|
||
|
||
When designing a language mapping, an important consideration is
|
||
roundtripping: serialization after deserialization, and vice versa,
|
||
should both be identities.
|
||
|
||
### JavaScript
|
||
|
||
- `Boolean` ↔ `Boolean`
|
||
- `Float` and `Double` ↔ numbers
|
||
- `SignedInteger` ↔ numbers or `BigInt` (see [here](https://developers.google.com/web/updates/2018/05/bigint) and [here](https://github.com/tc39/proposal-bigint))
|
||
- `String` ↔ strings
|
||
- `ByteString` ↔ `Uint8Array`
|
||
- `Symbol` ↔ `Symbol.for(...)`
|
||
- `Record` ↔ `{ "_label": theLabel, "_fields": [field0, ..., fieldN] }`, plus convenience accessors
|
||
- `(undefined)` ↔ the undefined value
|
||
- `(rfc3339 F)` ↔ `Date`, if `F` matches the `date-time` RFC 3339 production
|
||
- `Sequence` ↔ `Array`
|
||
- `Set` ↔ `{ "_set": M }` where `M` is a `Map` from the elements of the set to `true`
|
||
- `Dictionary` ↔ a `Map`
|
||
|
||
### Scheme/Racket
|
||
|
||
- `Boolean` ↔ booleans
|
||
- `Float` and `Double` ↔ inexact numbers (Racket: single- and double-precision floats)
|
||
- `SignedInteger` ↔ exact numbers
|
||
- `String` ↔ strings
|
||
- `ByteString` ↔ byte vector (Racket: "Bytes")
|
||
- `Symbol` ↔ symbols
|
||
- `Record` ↔ structures (Racket: prefab struct)
|
||
- `Sequence` ↔ lists
|
||
- `Set` ↔ Racket: sets
|
||
- `Dictionary` ↔ Racket: hash-table
|
||
|
||
### Java
|
||
|
||
- `Boolean` ↔ `Boolean`
|
||
- `Float` and `Double` ↔ `Float` and `Double`
|
||
- `SignedInteger` ↔ `Integer`, `Long`, `BigInteger`
|
||
- `String` ↔ `String`
|
||
- `ByteString` ↔ `byte[]`
|
||
- `Symbol` ↔ a simple data class wrapping a `String`
|
||
- `Record` ↔ in a simple implementation, a generic `Record` class; else perhaps a bean mapping?
|
||
- `(mime T B)` ↔ an implementation of `javax.activation.DataSource`?
|
||
- `Sequence` ↔ an implementation of `java.util.List`
|
||
- `Set` ↔ an implementation of `java.util.Set`
|
||
- `Dictionary` ↔ an implementation of `java.util.Map`
|
||
|
||
### Erlang
|
||
|
||
- `Boolean` ↔ `true` and `false`
|
||
- `Float` and `Double` ↔ floats (unsure how Erlang deals with single-precision)
|
||
- `SignedInteger` ↔ integers
|
||
- `String` ↔ pair of `utf8` and a binary
|
||
- `ByteString` ↔ a binary
|
||
- `Symbol` ↔ pair of `atom` and a binary
|
||
- `Record` ↔ triple of `obj`, label, and field list
|
||
- `Sequence` ↔ a list
|
||
- `Set` ↔ a `sets` set
|
||
- `Dictionary` ↔ a [map][erlang-map] (new in Erlang/OTP R17)
|
||
|
||
This is a somewhat unsatisfactory mapping because: (a) Erlang doesn't
|
||
garbage-collect its atoms, meaning that (a.1) representing `Symbol`s
|
||
as atoms could lead to denial-of-service and (a.2) representing
|
||
`Symbol`-labelled `Record`s as Erlang records must be rejected for the
|
||
same reason; (b) even if it did, Erlang's boolean values are atoms,
|
||
which would then clash with the `Symbol`s `true` and `false`; and (c)
|
||
Erlang has no distinct string type, making for a trilemma where
|
||
`String`s are in danger of clashing with `ByteString`s, `Sequence`s,
|
||
or `Record`s.
|
||
|
||
### Python
|
||
|
||
- `Boolean` ↔ `True` and `False`
|
||
- `Float` ↔ a `Float` wrapper-class for a double-precision value
|
||
- `Double` ↔ float
|
||
- `SignedInteger` ↔ int and long
|
||
- `String` ↔ `unicode`
|
||
- `ByteString` ↔ `bytes`
|
||
- `Symbol` ↔ a simple data class wrapping a `unicode`
|
||
- `Record` ↔ something like `namedtuple`, but that doesn't care about class identity?
|
||
- `Sequence` ↔ `tuple` (but accept `list` during encoding)
|
||
- `Set` ↔ `frozenset` (but accept `set` during encoding)
|
||
- `Dictionary` ↔ a hashable (immutable) dictionary-like thing (but accept `dict` during encoding)
|
||
|
||
### Squeak Smalltalk
|
||
|
||
- `Boolean` ↔ `true` and `false`
|
||
- `Float` ↔ perhaps a subclass of `Float`?
|
||
- `Double` ↔ `Float`
|
||
- `SignedInteger` ↔ `Integer`
|
||
- `String` ↔ `WideString`
|
||
- `ByteString` ↔ `ByteArray`
|
||
- `Symbol` ↔ `WideSymbol`
|
||
- `Record` ↔ a simple data class
|
||
- `Sequence` ↔ `ArrayedCollection` (usually `OrderedCollection`)
|
||
- `Set` ↔ `Set`
|
||
- `Dictionary` ↔ `Dictionary`
|
||
|
||
## Appendix. Why not Just Use JSON?
|
||
|
||
<!-- JSON lacks semantics: JSON syntax doesn't denote anything -->
|
||
|
||
JSON offers *syntax* for numbers, strings, booleans, null, arrays and
|
||
string-keyed maps. However, it suffers from two major problems. First,
|
||
it offers no *semantics* for the syntax: it is left to each
|
||
implementation to determine how to treat each JSON term. This causes
|
||
[interoperability](http://seriot.ch/parsing_json.php) and even
|
||
[security](http://web.archive.org/web/20180906202559/http://docs.couchdb.org/en/stable/cve/2017-12635.html)
|
||
issues. Second, JSON's lack of support for type tags leads to awkward
|
||
and incompatible *encodings* of type information in terms of the fixed
|
||
suite of constructors on offer.
|
||
|
||
There are other minor problems with JSON having to do with its syntax.
|
||
Examples include its relative verbosity and its lack of support for
|
||
binary data.
|
||
|
||
### JSON syntax doesn't *mean* anything
|
||
|
||
When are two JSON values the same? When are they different?
|
||
<!-- When is one JSON value "less than" another? -->
|
||
|
||
The specifications are largely silent on these questions. Different
|
||
JSON implementations give different answers.
|
||
|
||
Specifically, JSON does not:
|
||
|
||
- assign any meaning to numbers,[^meaning-ieee-double]
|
||
- determine how strings are to be compared,[^string-key-comparison]
|
||
- determine whether object key ordering is significant,[^json-member-ordering] or
|
||
- determine whether duplicate object keys are permitted, what it
|
||
would mean if they were, or how to determine a duplicate in the
|
||
first place.[^json-key-uniqueness]
|
||
|
||
In short, JSON syntax doesn't *denote* anything.[^xml-infoset] [^other-formats]
|
||
|
||
[^meaning-ieee-double]:
|
||
[Section 6 of RFC 8259](https://tools.ietf.org/html/rfc8259#section-6)
|
||
does go so far as to indicate “good interoperability can be
|
||
achieved” by imagining that parsers are able reliably to
|
||
understand the syntax of numbers as denoting an IEEE 754
|
||
double-precision floating-point value.
|
||
|
||
[^string-key-comparison]:
|
||
[Section 8.3 of RFC 8259](https://tools.ietf.org/html/rfc8259#section-8.3)
|
||
suggests that *if* an implementation compares strings used as
|
||
object keys “code unit by code unit”, then it will interoperate
|
||
with *other such implementations*, but neither requires this
|
||
behaviour nor discusses comparisons of strings used in other
|
||
contexts.
|
||
|
||
[^json-member-ordering]:
|
||
[Section 4 of RFC 8259](https://tools.ietf.org/html/rfc8259#section-4)
|
||
remarks that “[implementations] differ as to whether or not they
|
||
make the ordering of object members visible to calling software.”
|
||
|
||
[^json-key-uniqueness]:
|
||
[Section 4 of RFC 8259](https://tools.ietf.org/html/rfc8259#section-4)
|
||
is the only place in the specification that mentions the issue. It
|
||
explicitly sanctions implementations supporting duplicate keys,
|
||
noting only that “when the names within an object are not unique,
|
||
the behavior of software that receives such an object is
|
||
unpredictable.” Implementations are free to choose any behaviour
|
||
at all in this situation, including signalling an error, or
|
||
discarding all but one of a set of duplicates.
|
||
|
||
[^xml-infoset]: The XML world has the concept of
|
||
[XML infoset](https://www.w3.org/TR/xml-infoset/). Loosely
|
||
speaking, XML infoset is the *denotation* of an XML document; the
|
||
*meaning* of the document.
|
||
|
||
[^other-formats]: Most other recent data languages are like JSON in
|
||
specifying only a syntax with no associated semantics. While some
|
||
do make a sketch of a semantics, the result is often
|
||
underspecified (e.g. in terms of how strings are to be compared),
|
||
overly machine-oriented (e.g. treating 32-bit integers as
|
||
fundamentally distinct from 64-bit integers and from
|
||
floating-point numbers), overly fine (e.g. giving visibility to
|
||
the order in which map entries are written), or all three.
|
||
|
||
Some examples:
|
||
|
||
- are the JSON values `1`, `1.0`, and `1e0` the same or different?
|
||
- are the JSON values `1.0` and `1.0000000000000001` the same or different?
|
||
- are the JSON strings `"päron"` (UTF-8 `70c3a4726f6e`) and `"päron"`
|
||
(UTF-8 `7061cc88726f6e`) the same or different?
|
||
- are the JSON objects `{"a":1, "b":2}` and `{"b":2, "a":1}` the same
|
||
or different?
|
||
- which, if any, of `{"a":1, "a":2}`, `{"a":1}` and `{"a":2}` are the
|
||
same? Are all three legal?
|
||
- are `{"päron":1}` and `{"päron":1}` the same or different?
|
||
|
||
### JSON can multiply nicely, but it can't add very well
|
||
|
||
JSON includes a fixed set of types: numbers, strings, booleans, null,
|
||
arrays and string-keyed maps. Domain-specific data must be *encoded*
|
||
into these types. For example, dates and email addresses are often
|
||
represented as strings with an implicit internal structure.
|
||
|
||
There is no convention for *labelling* a value as belonging to a
|
||
particular category. This makes it difficult to extract, say, all
|
||
email addresses, or all URLs, from an arbitrary JSON document.
|
||
|
||
Instead, JSON-encoded data are often labelled in an ad-hoc way.
|
||
Multiple incompatible approaches exist. For example, a "money"
|
||
structure containing a `currency` field and an `amount` may be
|
||
represented in any number of ways:
|
||
|
||
{ "_type": "money", "currency": "EUR", "amount": 10 }
|
||
{ "type": "money", "value": { "currency": "EUR", "amount": 10 } }
|
||
[ "money", { "currency": "EUR", "amount": 10 } ]
|
||
{ "@money": { "currency": "EUR", "amount": 10 } }
|
||
|
||
This causes particular problems when JSON is used to represent *sum*
|
||
or *union* types, such as "either a value or an error, but not both".
|
||
Again, multiple incompatible approaches exist.
|
||
|
||
For example, imagine an API for depositing money in an account. The
|
||
response might be either a "success" response indicating the new
|
||
balance, or one of a set of possible errors.
|
||
|
||
Sometimes, a *pair* of values is used, with `null` marking the option
|
||
not taken.[^interesting-failure-mode]
|
||
|
||
{ "ok": { "balance": 210 }, "error": null }
|
||
{ "ok": null, "error": "Unauthorized" }
|
||
|
||
[^interesting-failure-mode]: What is the meaning of a document where
|
||
both `ok` and `error` are non-null? What might happen when a
|
||
program is presented with such a document?
|
||
|
||
The branch not chosen is sometimes present, sometimes omitted as if it
|
||
were an optional field:
|
||
|
||
{ "ok": { "balance": 210 } }
|
||
{ "error": "Unauthorized" }
|
||
|
||
Sometimes, an array of a label and a value is used:
|
||
|
||
[ "ok", { "balance": 210 } ]
|
||
[ "error", "Unauthorized" ]
|
||
|
||
Sometimes, the shape of the data is sufficient to distinguish among
|
||
the alternatives, and the label is left implicit:
|
||
|
||
{ "balance": 210 }
|
||
"Unauthorized"
|
||
|
||
JSON itself does not offer any guidance for which of these options to
|
||
choose. In many real cases on the web, poor choices have led to
|
||
encodings that are irrecoverably ambiguous.
|
||
|
||
# Open questions
|
||
|
||
Q. Should "symbols" instead be URIs? Relative, usually; relative to
|
||
what? Some domain-specific base URI?
|
||
|
||
Q. Literal small integers: are they pulling their weight? They're not
|
||
absolutely necessary. They mess up the connection between
|
||
value-ordering and repr-ordering!
|
||
|
||
Q. Should we go for trying to make the data ordering line up with the
|
||
encoding ordering? We'd have to only use streaming forms, and avoid
|
||
the small integer encoding, and not store record arities, and sort
|
||
sets and dictionaries, and mask floats and doubles (perhaps
|
||
[like this](https://stackoverflow.com/questions/43299299/sorting-floating-point-values-using-their-byte-representation)),
|
||
and pick a specific `NaN`, and I don't know what to do about
|
||
SignedIntegers. Perhaps make them more like float formats, with the
|
||
byte count acting as a kind of exponent underneath the sign bit.
|
||
|
||
- Perhaps define separate additional canonicalization restrictions?
|
||
Doesn't help the ordering, but does help the equivalence.
|
||
|
||
- Canonicalization and early-bailout-equivalence-checking are in
|
||
tension with support for streaming values.
|
||
|
||
Q. The postfix fields in the textual syntax come unannounced: "oh, and
|
||
another thing, what you just read is a label, and here are some
|
||
fields." This is a problem for interactive reading of textual syntax,
|
||
because after a complete term, it needs to see the next character to
|
||
tell whether it is an open-parenthesis or not! For this reason, I've
|
||
disallowed whitespace between a label `Value` and the open-parenthesis
|
||
of the fields. Is this reasonable??
|
||
|
||
Q. To remain compatible with JSON, portions of the text syntax have to
|
||
remain case-insensitive (`%i"..."`). However, non-JSON extensions do
|
||
not. There's only one (?) at the moment, the `%i"f"` in `Float`;
|
||
should it be changed to case-sensitive?
|
||
|
||
## Notes
|