---
no_site_title: true
title: "Preserves Schema"
---

Tony Garnock-Jones <tonyg@leastfixedpoint.com>  
June 2021. Version 0.1.2.

  [abnf]: https://tools.ietf.org/html/rfc7405

This document proposes a Schema language for the
[Preserves data model](./preserves.html).

## 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][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`s[^like-sexps] separated into *clauses* by the Preserves
`Symbol` "`.`".

[^like-sexps]: That is, schema files use Preserves as a kind of
    S-expression!

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 *(NamedPattern) ">"
               / 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*:[^not-ideal-optional-encoding]

    MyDict = {a: int, b: string} & @c MaybeC .
    MaybeC = @present {c: symbol} / @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.

[^not-ideal-optional-encoding]: This encoding is not ideal. Given a
    term `{a: 1, b: "", c: sym}`, it will yield a `present` result,
    and given `{a: 1, b: ""}`, will yield an `absent` result; but
    `absent` will also result from `{a: 1, b: "", c: "notasymbol"}`.

### 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.[^interface-schema] 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 named `Name` in the current schema.
 - `Mod.Submod.Name` refers to definition `Name` in `Mod.Submod`, some other schema in the bundle.

Each period-separated portion of a reference name must be an `id`, an
identifier.

[^interface-schema]: 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 as
    `Session` in this file.

#### 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:

    RecordPattern = "<<rec>" NamedPattern *(NamedPattern) ">"
                  / "<" value *(NamedPattern) ">"

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 = "[" 1*(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`](https://gitlab.com/preserves/preserves/-/blob/main/schema/schema.prs)
in the source code repository.

A `Bundle` collects a number of `Schema`s, each named by a
`ModulePath`:[^todo-semantics-of-bundles]

    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>.

[^todo-semantics-of-bundles]: The semantics of module path references
    remain to be specified!

## 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](https://en.wikipedia.org/wiki/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](https://www2.ccs.neu.edu/racket/pubs/dissertation-dimoulas.pdf).
   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 their `who` doesn't
   match the `uid` supplied in the `Join` assertion. It'd be nice to
   capture that using a dependent schema, passing in the specific
   `uid` value to the `Session` constructor, something like
   `#!(Session uid)`.

       Join = <joinedUser @uid UserId @handle #!Session>.
       Session = @observeSpeech <Observe =says @observer #!Says> / Says .
       Says = <says @who UserId @what string>.


<!-- Heading to visually offset the footnotes from the main document: -->
## Notes