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no_site_title | title |
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true | Preserves Schema |
Tony Garnock-Jones tonyg@leastfixedpoint.com
June 2021. Version 0.1.2.
This document proposes a Schema language for the Preserves data model.
Introduction
A Preserves schema connects Preserves Value
s to host-language data
structures. Each definition within a schema can be processed by a
compiler to produce
-
a host-language type definition;
-
a partial parsing function from
Value
s to instances of the produced type; and -
a total serialization function from instances of the type to
Value
s.
Every parsed Value
retains enough information to always be able to
be serialized again, and every instance of a host-language data
structure contains, by construction, enough information to be
successfully serialized.
Example. Sending the schema
version 1 .
Date = <date @year int @month int @day int>.
Person = <person @name string @birthday Date>.
to the TypeScript schema compiler produces types,
type Date = {"year": number, "month": number, "day": number};
type Person = {"name": string, "birthday": Date};
constructors,
function Date({year, month, day}: {year: number, month: number, day: number}): Date;
function Person({name, birthday}: {name: string, birthday: Date}): Person;
partial parsing functions which throw on parse failure,
function asDate(v: _val): Date;
function asPerson(v: _val): Person;
total parsing functions which yield undefined
on parse failure,
function toDate(v: _val): undefined | Date;
function toPerson(v: _val): undefined | Person;
and total serialization functions,
function fromDate(_v: Date): _val;
function fromPerson(_v: Person): _val;
Concepts
Bundle. A collection of schemas, each named by a module path.
Definition. A named pattern within a schema. When compiled, a definition will usually produce a type (plus associated constructors and predicates), a parser function, and a serializer function.
Metaschema. The Preserves metaschema is a schema describing the abstract syntax of all schema instances (including itself).
Module path. A sequence of symbols, denoting a leaf in a tree with symbol-labelled edges.
Pattern. A pattern describes a collection of Value
s as well as
providing names for the portions of matching Value
s that should be
captured in a host-language data type.
Schema abstract syntax tree (AST). Schema-manipulating tools will
usually work with schema AST; that is, with Value
s conforming to the
metaschema or instances of the corresponding host-language
datastructures.
Schema domain-specific language (DSL). While human beings can work directly with Preserves documents matching the metaschema, the schema DSL provides an easier-to-read and -write language for working with schemas that can be translated into instances
Schema. A collection of definitions, plus an optional schema-wide reference to a schema describing embedded values.
Concrete (DSL) Syntax
In this section, we use an ABNF-like notation to define a textual syntax that is easy for people to read and write. Most of the examples in this document are written using this syntax. In the following section, we will define the abstract syntax that this surface syntax translates into.
Schema files and bundles.
Each schema should be placed in a single file. Schema files usually
end with extension .prs
, and consist of a sequence of Preserves
Value
s1 separated into clauses by the Preserves
Symbol
".
".
A bundle of schema files is a directory tree containing .prs
files.
Clauses.
Clause = (Version / EmbeddedTypeName / Include / Definition) "."
Version = "version" "1"
EmbeddedTypeName = "embeddedType" ("#f" / Ref)
Include = "include" string
Definition = id "=" (OrPattern / AndPattern / Pattern)
Version specification. Mandatory. Names the version of the schema
language used in the file. This version of the specification is
referred to in schema files as version 1
.
Embedded type name. Optional. If given as #f
(the default), it
declares that values parsed by the schema do not contain embedded
Value
s of any particular type. If given as a Ref
, a reference to a
definition in this or a neighbouring schema, it declares that embedded
Value
s must themselves conform to the named definition.
Include. Experimental. Includes the contents of a neighbouring file as if it were textually inserted in place of this clause. The file path may be relative to the current file, or absolute.
Definition. Each definition clause implicitly connects a pattern with a type name and a set of associated functions.
Union definitions.
OrPattern = AltPattern "/" AltPattern *("/" AltPattern)
The right-hand-side of a definition may supply two or more alternatives. When parsing, the alternatives are tried in order; the result of the first successful alternative is the result of the entire parse.
The type corresponding to an OrPattern
is a union type, a variant
type, or an algebraic sum type, depending on the host language.
Each alternative with an OrPattern
must have a definition-unique
name. The name can either be given explicitly as @name
(see
discussion of NamedPattern
below) or inferred. It can only be
inferred from the label of a record pattern, from the name of a
reference to another definition, or from the text of a "sufficiently
identifierlike" literal pattern - one that matches a string, symbol,
number or boolean:
AltPattern = "@" id SimplePattern
/ "<" id PatternSequence ">"
/ Ref
/ LiteralPattern -- with a side condition
Intersection definitions.
AndPattern = NamedPattern "&" NamedPattern *("&" NamedPattern)
The right-hand-side of a definition may supply two or more patterns, the intersection of whose denotations is the denotation of the overall definition. When parsing, every pattern is tried; if all succeed, the resulting information is combined into a single record type.
When serializing, the terms resulting from serializing at each pattern are merged together.
Experimental.
Intersections are an experimental feature. They can be used to express optional dictionary entries:2
MyDict = {a: int, b: string} & @c MaybeC .
MaybeC = @present {c: symbol} / @invalid {c: any} / @absent {} .
It is not yet clear whether they pull their weight. In particular, the semantics of serializing a value defined by intersection are not completely clear.
Patterns.
Pattern = SimplePattern / CompoundPattern
Patterns come in two kinds:
-
the parsers for simple patterns yield a single host-language value; for example, a string, an array, a pointer, and so on.
-
the parsers for compound patterns yield zero or more fields which combine into an overall record type associated with a definition.
Simple patterns
SimplePattern = AnyPattern
/ AtomKindPattern
/ EmbeddedPattern
/ LiteralPattern
/ SequenceOfPattern
/ SetOfPattern
/ DictOfPattern
/ Ref
The any
pattern matches any input Value
:
AnyPattern = "any"
Specifying the name of a kind of Atom
matches that kind of atom:
AtomKindPattern = "bool" / "float" / "double" / "int" / "string" / "bytes" / "symbol"
Embedded input Value
s are matched with embedded patterns. The
portion under the #!
prefix is the interface schema for the
embedded value.3 The result of a match is an
instance of the schema-wide embeddedType
, if one is supplied.
EmbeddedPattern = "#!" SimplePattern
A literal pattern may be expressed in any of three ways: non-symbol
atoms stand for themselves directly; symbols, prefixed with an equal
sign, are matched literally; and any Value
at all may be quoted by
placing it in a <<lit> ... >
record:
LiteralPattern = "="symbol / "<<lit>" value ">" / non-symbol-atom
Brackets containing an item pattern and a literal ellipsis match a sequence of items, each matching the nested item pattern. Sets and uniform dictionaries are similar.
SequenceOfPattern = "[" SimplePattern "..." "]"
SetOfPattern = "#{" SimplePattern "}"
DictOfPattern = "{" SimplePattern ":" SimplePattern "...:..." "}"
Finally, a reference to some other definition, in this schema or a neighbouring schema within this bundle, is made by mentioning the possibly-qualified name of the definition as a bare symbol:
Ref = symbol
Periods ".
" in such symbols are special:
Name
refers to the definition namedName
in the current schema.Mod.Submod.Name
refers to definitionName
inMod.Submod
, some other schema in the bundle.
Each period-separated portion of a reference name must be an id
, an
identifier.
Compound patterns
CompoundPattern = RecordPattern
/ TuplePattern
/ VariableTuplePattern
/ DictionaryPattern
A record pattern matches an input record. It may be specified as a
record with a literal in the label position, or as a quoted <<rec> ... >
record with a pattern for each of the label and field-sequence
positions:4
RecordPattern = "<<rec>" NamedPattern NamedPattern ">"
/ "<" value PatternSequence ">"
PatternSequence = *(NamedPattern) [NamedSimplePattern "..."]
A tuple pattern matches a fixed-length sequence with specific patterns in each position. A variable tuple pattern is the same, but with an additional pattern for matching additional elements following the fixed-position patterns.
TuplePattern = "[" *(NamedPattern) "]"
VariableTuplePattern = "[" *(NamedPattern) NamedSimplePattern "..." "]"
A dictionary pattern matches specific literal keys in an input
dictionary. If no explicit name is given for a particular
NamedSimplePattern
, but the key for the pattern is a symbol, then
that symbol is used as the name for that dictionary entry.
DictionaryPattern = "{" *(value ":" NamedSimplePattern) "}"
Identifiers and Bindings: NamedPattern and NamedSimplePattern
Compound patterns specifications contain NamedPattern
s or
NamedSimplePattern
s rather than ordinary Pattern
s:
NamedPattern = "@" id SimplePattern / Pattern
NamedSimplePattern = "@" id SimplePattern / SimplePattern
Use of an @name
prefix generally results in creation of a field with
the given name in the overall record type for a definition. The type
of value contained in the field will correspond to the Pattern
or
SimplePattern
given.
An id
is a symbol that matches the regular expression
^[a-zA-Z][a-zA-Z_0-9]*$
. This is a lowest-common-denominator
constraint that allows for a reasonable mapping to the identifiers of
many programming languages.
Appendix: Metaschema
The metaschema defines the structure of the abstract syntax (AST) of schemas, using the concrete DSL syntax described above.
The text below is taken from
schema/schema.prs
in the source code repository.
A Bundle
collects a number of Schema
s, each named by a
ModulePath
:5
Bundle = <bundle @modules Modules>.
Modules = { ModulePath: Schema ...:... }.
ModulePath = [symbol ...].
Schema = <schema {
version: Version
embeddedType: EmbeddedTypeName
definitions: Definitions
}>.
A Version
names the version of the schema language in use. At
present, it must be 1
.
; version 1 .
Version = 1 .
An EmbeddedTypeName
specifies the type of embedded values within
values parsed by a given schema:
EmbeddedTypeName = Ref / #f.
Ref = <ref @module ModulePath @name symbol>.
The Definitions
are a named collection of definitions within a
schema. Note the special mention of pattern0
and pattern1
: these
ensure that each or
or and
record has at least two members.
Definitions = { symbol: Definition ...:... }.
Definition =
; Pattern / Pattern / ...
/ <or [@pattern0 NamedAlternative
@pattern1 NamedAlternative
@patternN NamedAlternative ...]>
; Pattern & Pattern & ...
/ <and [@pattern0 NamedPattern
@pattern1 NamedPattern
@patternN NamedPattern ...]>
; Pattern
/ Pattern
.
NamedAlternative = [@variantLabel string @pattern Pattern].
Each Pattern
is either a simple or compound pattern:
Pattern = SimplePattern / CompoundPattern .
Simple patterns are as described above:
SimplePattern =
; any
/ =any
; special builtins: bool, float, double, int, string, bytes, symbol
/ <atom @atomKind AtomKind>
; matches an embedded value in the input: #!p
/ <embedded @interface SimplePattern>
; =symbol, <<lit> any>, or plain non-symbol atom
/ <lit @value any>
; [p ...] ----> <seqof <ref p>>; see also tuplePrefix below.
/ <seqof @pattern SimplePattern>
; #{p} ----> <setof <ref p>>
/ <setof @pattern SimplePattern>
; {k: v, ...:...} ----> <dictof <ref k> <ref v>>
/ <dictof @key SimplePattern @value SimplePattern>
; symbol, symbol.symbol, symbol.symbol.symbol, ...
/ Ref
.
AtomKind = =Boolean
/ =Float
/ =Double
/ =SignedInteger
/ =String
/ =ByteString
/ =Symbol .
Compound patterns involve optionally-named subpatterns:
CompoundPattern =
; <label a b c> ----> <rec <lit label> <tuple [<ref a> <ref b> <ref c>]>>
; except for record labels
; <<rec> x y> ---> <rec <ref x> <ref y>>
/ <rec @label NamedPattern @fields NamedPattern>
; [a b c] ----> <tuple [<ref a> <ref b> <ref c>]>
/ <tuple @patterns [NamedPattern ...]>
; [a b c ...] ----> <tuplePrefix [<ref a> <ref b>] <seqof <ref c>>>
/ <tuplePrefix @fixed [NamedPattern ...] @variable NamedSimplePattern>
; {a: b, c: d} ----> <dict {a: <ref b>, c: <ref d>}>
/ <dict @entries DictionaryEntries>
.
DictionaryEntries = { any: NamedSimplePattern ...:... }.
Explicitly-named subpatterns are always SimplePattern
s; but,
depending on context, if a name is omitted, the pattern may be a
Pattern
or may be restricted to SimplePattern
as well:
NamedSimplePattern = @named Binding / @anonymous SimplePattern .
NamedPattern = @named Binding / @anonymous Pattern .
Binding = <named @name symbol @pattern SimplePattern>.
Appendix: Metaschema instance
The following is a (lightly-reformatted) Preserves document which is the output of DSL-to-AST compilation of the DSL source text of the metaschema.
<schema {
version: 1,
embeddedType: #f,
definitions: {
Pattern: <or [
["SimplePattern", <ref [] SimplePattern>],
["CompoundPattern", <ref [] CompoundPattern>]
]>,
CompoundPattern: <or [
["rec", <rec <lit rec> <tuple [
<named label <ref [] NamedPattern>>,
<named fields <ref [] NamedPattern>>
]>>],
["tuple", <rec <lit tuple> <tuple [<named patterns <seqof <ref [] NamedPattern>>>]>>],
["tuplePrefix", <rec <lit tuplePrefix> <tuple [
<named fixed <seqof <ref [] NamedPattern>>>,
<named variable <ref [] NamedSimplePattern>>
]>>],
["dict", <rec <lit dict> <tuple [<named entries <ref [] DictionaryEntries>>]>>]
]>,
Modules: <dictof <ref [] ModulePath> <ref [] Schema>>,
Ref: <rec <lit ref> <tuple [
<named module <ref [] ModulePath>>,
<named name <atom Symbol>>
]>>,
Bundle: <rec <lit bundle> <tuple [<named modules <ref [] Modules>>]>>,
Binding: <rec <lit named> <tuple [
<named name <atom Symbol>>,
<named pattern <ref [] SimplePattern>>
]>>,
Definition: <or [
["or", <rec <lit or> <tuple [<tuplePrefix [
<named pattern0 <ref [] NamedAlternative>>,
<named pattern1 <ref [] NamedAlternative>>
] <named patternN <seqof <ref [] NamedAlternative>>>>]>>],
["and", <rec <lit and> <tuple [<tuplePrefix [
<named pattern0 <ref [] NamedPattern>>,
<named pattern1 <ref [] NamedPattern>>
] <named patternN <seqof <ref [] NamedPattern>>>>]>>],
["Pattern", <ref [] Pattern>]
]>,
NamedSimplePattern: <or [
["named", <ref [] Binding>],
["anonymous", <ref [] SimplePattern>]
]>,
EmbeddedTypeName: <or [
["Ref", <ref [] Ref>],
["false", <lit #f>]
]>,
ModulePath: <seqof <atom Symbol>>,
AtomKind: <or [
["Boolean", <lit Boolean>],
["Float", <lit Float>],
["Double", <lit Double>],
["SignedInteger", <lit SignedInteger>],
["String", <lit String>],
["ByteString", <lit ByteString>],
["Symbol", <lit Symbol>]
]>,
DictionaryEntries: <dictof any <ref [] NamedSimplePattern>>,
Version: <lit 1>,
NamedPattern: <or [
["named", <ref [] Binding>],
["anonymous", <ref [] Pattern>]
]>,
SimplePattern: <or [
["any", <lit any>],
["atom", <rec <lit atom> <tuple [<named atomKind <ref [] AtomKind>>]>>],
["embedded", <rec <lit embedded> <tuple [<named interface <ref [] SimplePattern>>]>>],
["lit", <rec <lit lit> <tuple [<named value any>]>>],
["seqof", <rec <lit seqof> <tuple [<named pattern <ref [] SimplePattern>>]>>],
["setof", <rec <lit setof> <tuple [<named pattern <ref [] SimplePattern>>]>>],
["dictof", <rec <lit dictof> <tuple [
<named key <ref [] SimplePattern>>,
<named value <ref [] SimplePattern>>
]>>],
["Ref", <ref [] Ref>]
]>,
NamedAlternative: <tuple [
<named variantLabel <atom String>>,
<named pattern <ref [] Pattern>>
]>,
Definitions: <dictof <atom Symbol> <ref [] Definition>>,
Schema: <rec <lit schema> <tuple [<dict {
version: <named version <ref [] Version>>,
embeddedType: <named embeddedType <ref [] EmbeddedTypeName>>,
definitions: <named definitions <ref [] Definitions>>
}>]>>
}
}>
Appendix: Example generated types
The following are the (abridged) TypeScript and Racket generated type definitions for the metaschema.
TypeScript.
import * as _ from "@preserves/core";
// ...
export type _embedded = any;
export type _val = _.Value<_embedded>;
// ...
export type Bundle = {"modules": Modules};
export type Modules = _.KeyedDictionary<ModulePath, Schema, _embedded>;
export type Schema = {
"version": Version,
"embeddedType": EmbeddedTypeName,
"definitions": Definitions
};
export type Version = null;
export type EmbeddedTypeName = ({"_variant": "Ref", "value": Ref} | {"_variant": "false"});
export type Definitions = _.KeyedDictionary<symbol, Definition, _embedded>;
export type Definition = (
{
"_variant": "or",
"pattern0": NamedAlternative,
"pattern1": NamedAlternative,
"patternN": Array<NamedAlternative>
} |
{
"_variant": "and",
"pattern0": NamedPattern,
"pattern1": NamedPattern,
"patternN": Array<NamedPattern>
} |
{"_variant": "Pattern", "value": Pattern}
);
export type Pattern = (
{"_variant": "SimplePattern", "value": SimplePattern} |
{"_variant": "CompoundPattern", "value": CompoundPattern}
);
export type SimplePattern = (
{"_variant": "any"} |
{"_variant": "atom", "atomKind": AtomKind} |
{"_variant": "embedded", "interface": SimplePattern} |
{"_variant": "lit", "value": _val} |
{"_variant": "seqof", "pattern": SimplePattern} |
{"_variant": "setof", "pattern": SimplePattern} |
{"_variant": "dictof", "key": SimplePattern, "value": SimplePattern} |
{"_variant": "Ref", "value": Ref}
);
export type CompoundPattern = (
{"_variant": "rec", "label": NamedPattern, "fields": NamedPattern} |
{"_variant": "tuple", "patterns": Array<NamedPattern>} |
{
"_variant": "tuplePrefix",
"fixed": Array<NamedPattern>,
"variable": NamedSimplePattern
} |
{"_variant": "dict", "entries": DictionaryEntries}
);
export type DictionaryEntries = _.KeyedDictionary<_val, NamedSimplePattern, _embedded>;
export type AtomKind = (
{"_variant": "Boolean"} |
{"_variant": "Float"} |
{"_variant": "Double"} |
{"_variant": "SignedInteger"} |
{"_variant": "String"} |
{"_variant": "ByteString"} |
{"_variant": "Symbol"}
);
export type NamedAlternative = {"variantLabel": string, "pattern": Pattern};
export type NamedSimplePattern = (
{"_variant": "named", "value": Binding} |
{"_variant": "anonymous", "value": SimplePattern}
);
export type NamedPattern = (
{"_variant": "named", "value": Binding} |
{"_variant": "anonymous", "value": Pattern}
);
export type Binding = {"name": symbol, "pattern": SimplePattern};
export type Ref = {"module": ModulePath, "name": symbol};
export type ModulePath = Array<symbol>;
Racket.
(struct AtomKind-Symbol () #:prefab)
(struct AtomKind-ByteString () #:prefab)
(struct AtomKind-String () #:prefab)
(struct AtomKind-SignedInteger () #:prefab)
(struct AtomKind-Double () #:prefab)
(struct AtomKind-Float () #:prefab)
(struct AtomKind-Boolean () #:prefab)
(struct Bundle (modules) #:prefab)
(struct CompoundPattern-dict (entries) #:prefab)
(struct CompoundPattern-tuplePrefix (fixed variable) #:prefab)
(struct CompoundPattern-tuple (patterns) #:prefab)
(struct CompoundPattern-rec (label fields) #:prefab)
(struct Definition-Pattern (value) #:prefab)
(struct Definition-and (pattern0 pattern1 patternN) #:prefab)
(struct Definition-or (pattern0 pattern1 patternN) #:prefab)
(struct EmbeddedTypeName-false () #:prefab)
(struct EmbeddedTypeName-Ref (value) #:prefab)
(struct NamedAlternative (variantLabel pattern) #:prefab)
(struct NamedPattern-anonymous (value) #:prefab)
(struct NamedPattern-named (value) #:prefab)
(struct NamedSimplePattern-anonymous (value) #:prefab)
(struct NamedSimplePattern-named (value) #:prefab)
(struct Binding (name pattern) #:prefab)
(struct Pattern-CompoundPattern (value) #:prefab)
(struct Pattern-SimplePattern (value) #:prefab)
(struct Ref (module name) #:prefab)
(struct Schema (definitions embeddedType version) #:prefab)
(struct SimplePattern-Ref (value) #:prefab)
(struct SimplePattern-dictof (key value) #:prefab)
(struct SimplePattern-setof (pattern) #:prefab)
(struct SimplePattern-seqof (pattern) #:prefab)
(struct SimplePattern-lit (value) #:prefab)
(struct SimplePattern-embedded (interface) #:prefab)
(struct SimplePattern-atom (atomKind) #:prefab)
(struct SimplePattern-any () #:prefab)
Appendix: Future work
-
There are side conditions on AST instances. It would be nice to eventually be able to express these within the metaschema.
-
It'd be interesting to, Ometa-like, be able to specify the DSL-to-AST translation process as a schema. One challenge in doing so is the way schemas are required to be reversible at present.
-
Should
include
accept URLs, to be able to retrieve schema from the web? -
It'd be nice to firm up the interpretation of embedded interface schemas. I have in mind something like the higher-order contracts of Dimoulas. Essentially, a schema is a contract, and embedded pointers-to-behaviour are like closures/channels/objects/etc, which demand higher-order contracts. Future work could pin this down further; also, consideration of dependent schemas (analogous to dependent contracts) could be of interest.
Example. In the following fragment,
#!Session
is the handle a connected user uses to interact with a chatroom. In the implementation,Says
messages are dropped if theirwho
doesn't match theuid
supplied in theJoin
assertion. It'd be nice to capture that using a dependent schema, passing in the specificuid
value to theSession
constructor, something like#!(Session uid)
.Join = <joinedUser @uid UserId @handle #!Session>. Session = @observeSpeech <Observe =says @observer #!Says> / Says . Says = <says @who UserId @what string>.
Notes
-
That is, schema files use Preserves as a kind of S-expression! ↩︎
-
This encoding is not ideal. It passes responsibility for checking for invalid inputs up to the user, rather than handling it completely at the Schema layer. ↩︎
-
Embedded patterns are experimental. One interpretation is that an embedded value denotes a reference to some stateful actor in a potentially-distributed system, and that the interface schema associated with an embedded value describes the messages that may be sent to that actor.
Examples.
#!any
may denote a reference to an Actor able to receive any value as a message;#!#t
, a reference to an Actor expecting only the "true" message;#!Session
, a reference to an Actor expecting any message matching a schema defined asSession
in this file. ↩︎ -
Note that
<label
ps>
can be thought of as roughly equivalent to<<rec> <<lit> label> [
ps]>
. The following two definitions are equivalent:
↩︎D1 = <foo @a string @b string @extra any ... >. D2 = <<rec> <<lit> foo> [@a string @b string @extra any ...]>.
-
The semantics of module path references remain to be specified! ↩︎