## -----
## This is the schema describing the schema declarations for IPLD Schemas.
## Yes, it's self-describing! :)
## -----

## Schema is a the root element of an IPLD Schema document.
##
## A complete (if quite short) Schema might look like this:
##
## ```
## {
##   "types": {
##     "MyFooType": {
##       "type": "string"
##     }
##   }
## }
## ```
##
## The main bulk of a schema is the types map,
## which is TypeName mapped to TypeDefn.
##
## Some additional top level fields are optional,
## such as some maps which may store data about where ADLs
## should be expected to be used within the data described by the schema.
## However, not all schemas use these features.
##
type Schema struct {
	types {TypeName:TypeDefn}
	advanced optional AdvancedDataLayoutMap
}

## Type names are a simple alias of string.
##
## There are some additional rules that should be applied. Type names:
##   - *Must* only contain alphanumeric ASCII characters and underscores
##   - *Must* begin with a capital letter
##   - *Should* avoid more than one connected underscore character,
##     multiple-underscores may be used for codegen
##
## Type names are strings meant for human consumption at a local scope.
## When making a Schema, note that the TypeName is the key of the map:
## a TypeName must be unique within the Schema.
##
type TypeName string

## AdvancedDataLayoutName defines the name of an ADL as a string.
##
## The same constraints and conventions apply as for TypeName.
##
## This identifier is used for keys in the AdvancedDataLayoutMap and also as
## references to ADLs where the "advanced" representation strategy is used for
## the types that support it.
##
type AdvancedDataLayoutName string

## AdvancedDataLayoutMap defines the set of ADLs found within the schema. It
## maps the name (AdvancedDataLayoutName) to the AdvancedDataLayout, which is
## currently an empty map.
##
type AdvancedDataLayoutMap {AdvancedDataLayoutName:AdvancedDataLayout}

## TypeDefn is a union type, where each of the possible members describes one kind of type.
## For example, TypeDefnBool is a member of the TypeDefn union, as is TypeDefnMap.
##
## The TypeDefn union is serialized using "keyed" union representation,
## which means in the serial form, we will always see a map with one key,
## and that key will indicate which member type is coming up as the value.
##
## Some of the kinds of type are so simple the union discriminant is the only
## content at all, e.g. strings:
##
## ```
## {
##   "string": {}
## }
## ```
##
## Other types have more content.  Consider this example of a map type:
##
## ```
## {
##   "map": {
##     "keyType": "String",
##     "valueType": "Int"
##   }
## }
## ```
##
type TypeDefn union {
	| TypeDefnBool "bool"
	| TypeDefnString "string"
	| TypeDefnBytes "bytes"
	| TypeDefnInt "int"
	| TypeDefnFloat "float"
	| TypeDefnMap "map"
	| TypeDefnList "list"
	| TypeDefnLink "link"
	| TypeDefnUnion "union"
	| TypeDefnStruct "struct"
	| TypeDefnEnum "enum"
	| TypeDefnUnit "unit"
	| TypeDefnAny "any"
	| TypeDefnCopy "copy"
} representation keyed

## TypeKind enumerates all the major kinds of type.
## Notice this enum's members are the same as the set of strings used as
## discriminants in the TypeDefn union.
## (Almost!  TypeDefn also contains TypeDefnCopy, which is a slight outlier.
## (TypeDefnCopy can be used to define a type, but isn't a typekind itself.))
##
## This enum is not actually used elsewhere in the schema-schema,
## but does correspond to the discriminant values used in the TypeDefn union.
type TypeKind enum {
	| Bool ("bool")
	| String ("string")
	| Bytes ("bytes")
	| Int ("int")
	| Float ("float")
	| Map ("map")
	| List ("list")
	| Link ("link")
	| Union ("union")
	| Struct ("struct")
	| Enum ("enum")
	| Unit ("unit")
	| Any ("any")
}

## RepresentationKind is similar to TypeKind, but includes only those concepts
## which exist at the IPLD *Data Model* level.
##
## In other words, structs, unions, and enumerations are not present:
## those concepts are introduced in the IPLD Schema system, and when serialized,
## all of them must be transformable to one of these representation kinds
## (e.g. a "struct" TypeKind will usually be transformed to a "map"
## RepresentationKind; "enum" TypeKind are typically a "string" RepresentationKind;
## and so on; exactly what RepresentationKind a type defintion will have
## is determined by its representation strategy).
##
## RepresentationKind strings are sometimes used to to indicate part of the
## definition in the details of TypeDefn; for example, they're used describing
## some of the detailed behaviors of a "kinded"-style union type.
type RepresentationKind enum {
	| Bool ("bool")
	| String ("string")
	| Bytes ("bytes")
	| Int ("int")
	| Float ("float")
	| Map ("map")
	| List ("list")
	| Link ("link")
}

## AnyScalar defines a union of the basic non-complex kinds.
##
## Useful defining usage of IPLD nodes that do compose from other nodes.
##
type AnyScalar union {
	| Bool bool
	| String string
	| Bytes bytes
	| Int int
	| Float float
} representation kinded

## AdvancedDataLayout defines `advanced` definitions which are stored in the
## top-level "advanced" map (AdvancedDataLayoutMap)
##
## Used as `advanced Foo` rather than `type Foo` to indicate an advanced data
## layout (ADL) with that name which can be used as a representation for type
## definitions whose kind the ADL is able to support.
##
## The AdvancedDataLayoutName is currently the only identifier that can be used
## to make a connection with the algorithm/logic behind this ADL. Future
## iterations may formalize this connection by some other means.
##
type AdvancedDataLayout struct {}

## TypeDefnBool describes a simple boolean type.
## It has no details.
##
type TypeDefnBool struct {}

## TypeDefnString describes a simple string type.
## It has no details.
##
type TypeDefnString struct {}

## TypeDefnBytes describes a simple byte array type.
##
type TypeDefnBytes struct {
	representation BytesRepresentation
}

## BytesRepresentation specifies how a TypeDefnBytes is to be serialized. By
## default it will be stored as bytes in the data model but it may be replaced
## with an ADL.
##
type BytesRepresentation union {
	| BytesRepresentation_Bytes "bytes"
	| AdvancedDataLayoutName "advanced"
} representation keyed

## BytesRepresentation_Bytes is the default representation for TypeDefnBytes and
## will be used implicitly if no representation is specified.
##
type BytesRepresentation_Bytes struct {}

## TypeDefnInt describes a simple integer numeric type.
## It has no details.
##
type TypeDefnInt struct {}

## TypeDefnFloat describes a simple floating point numeric type.
## It has no details.
##
type TypeDefnFloat struct {}

## TypeDefnMap describes a key-value map.
## The keys and values of the map have some specific type of their own.
##
## A constraint on keyType is that the referenced type must have a string
## representation kind. The IPLD Data Model only allows for string keys on maps,
## so this constraint is imposed here.
##
type TypeDefnMap struct {
	keyType TypeName
	valueType TypeNameOrInlineDefn
	valueNullable Bool (implicit false)
	representation optional MapRepresentation
}

## MapRepresentation describes how a map type should be mapped onto
## its IPLD Data Model representation.
##
## By default, a map type is also represented as a map in the Data Model,
## but other representation strategies can be configured
##
## Note that the `TypeDefnMap.representation` field is optional --
## the default behavior is demarcated by the lack of any of these values.
##
type MapRepresentation union {
	| MapRepresentation_StringPairs "stringpairs"
	| MapRepresentation_ListPairs "listpairs"
	| AdvancedDataLayoutName "advanced"
} representation keyed

## MapRepresentation_StringPairs describes that a map should be encoded as a
## string of delimited "k/v" entries, e.g. "k1=v1,k2=v2".
## The separating delimiter may be specified with "entryDelim", and the k/v
## delimiter may be specified with "innerDelim". So a "k=v" naive
## comma-separated form would use an "innerDelim" of "=" and an "entryDelim"
## of ",".
##
## This serial representation is limited: the domain of keys must
## exclude the "innerDelim" and values and keys must exclude ",".
## There is no facility for escaping, such as in escaped CSV.
## This also leads to a further restriction that this representation is only
## valid for maps whose keys and values may all be encoded to string form
## without conflicts in delimiter character. It is recommended, therefore,
## that its use be limited to maps containing values with the basic data
## model kinds that exclude multiple values (i.e. no maps, lists, and therefore
## structs or unions).
##
type MapRepresentation_StringPairs struct {
	innerDelim String
	entryDelim String
}

## MapRepresentation_ListPairs describes that a map should be encoded as a
## list in the IPLD Data Model. This list comprises a sub-list for each entry,
## in the form: [[k1,v1],[k2,v2]].
##
## This representation type is similar to StructRepresentation_Tuple except
## it includes the keys. This is critical for maps since the keys are not
## defined in the schema (hence "tuple" representation isn't available for
## maps).
##
type MapRepresentation_ListPairs struct {}

## TypeDefnList describes a list.
## The values of the list have some specific type of their own.
##
type TypeDefnList struct {
	valueType TypeNameOrInlineDefn
	valueNullable Bool (implicit false)
	representation optional ListRepresentation
}

## ListRepresentation describes how a map type should be mapped onto
## its IPLD Data Model representation.  By default a list is a list in the
## data model but it may be replaced with an ADL.
##
## Note that the `TypeDefnList.representation` field is optional --
## the default behavior is demarcated by the lack of any of these values.
##
type ListRepresentation union {
	| AdvancedDataLayoutName "advanced"
} representation keyed

## TypeDefnLink describes an IPLD link, which is a content-addressable pointer to more data.
## (Links in IPLD are implemented by CIDs: they're a hash that identifies another another block of data,
## plus a codec hint for how to parse it into IPLD Data Model).
##
## A typed link can optionally state an "expectedType".
## This provides a mechanism for suggesting what we should expect find
## if we were to follow the link.
## (Note that this cannot be strictly enforced by a node or block-level schema validation!
## But may be enforced elsewhere in an application using a schema, and
## is generally enforced as soon as possible when traversing typed links.)
##
## In the Schema DSL, links are specified with the `&FooBar` syntax --
## The ampersand character denotes a link, and the rest of the declaration
## is the name of the expected type for the linked content.
##
## If you want the equivalent of untyped links, in the DSL, you can say `&Any`.
## This is also available in the Prelude, and that type is simply name "Link"
## (in other words, the Prelude contains `type Link &Any`).
##
type TypeDefnLink struct {
	expectedType TypeName (implicit "Any")
}

## TypeDefnUnion describes a union (sometimes called a "sum type", or
## more verbosely, a "discriminated union", or in yet other literature, a "variant" type).
## A union is a type that can have a value of several different types, but
## unlike maps or structs, in a union, only one of those values may be present
## at a time.
##
## Unions can be represented in several significantly different ways:
## see the documentation of the UnionRepresentation type for details.
## Also note that there is no default representation for union types --
## you must _always_ explicitly specify a representation strategy when defining unions!
##
type TypeDefnUnion struct {
	members [UnionMember]
	representation UnionRepresentation
}

## UnionMember is a type for identifying the members of a union.
## Most commonly, this is simply a TypeName string;
## however, it can also be a UnionMemberInlineDefn,
## which is used to allow inline link definitions within a kinded union as a shorthand
## (rather than requiring all links be declared as a named type before being usable within a union).
type UnionMember union {
	| TypeName string
	| UnionMemberInlineDefn map
} representation kinded

## UnionMemberInlineDefn is a very similar purpose to InlineDefn,
## but found specifically within UnionMember.
## It only allows describing a link type (and not maps nor lists, as InlineDefn does),
## which is a constraint applied to union membership largely to make sure
## if there are errors in processing unions, we can make legible messages about it!
type UnionMemberInlineDefn union {
	| TypeDefnLink "link"
} representation keyed

## UnionRepresentation is a union of all the distinct ways a TypeDefnUnion's values
## can be mapped onto a serialized format for the IPLD Data Model.
##
## There are six strategies that can be used to encode a union:
## "keyed", "envelope", "inline", "stringprefix", "bytesprefix", and "kinded".
## The "keyed", "envelope", and "inline" strategies are all ways to produce
## representations in a map format, using map keys as type discriminators
## (some literature may describe this as a "tagged" style of union).
## The "stringprefix" strategy, only available for unions in which all member
## types themselves represent as strings in the data model, uses a prefix
## string as the type discrimination hint (and like the map-oriented strategies,
## may also be seen as a form of "tagged" style unions).
## The "bytesprefix" strategy, only available for unions in which all member
## types themselves represent as bytes in the data model, similar to
## "stringprefix" but with bytes.
## The "kinded" strategy can describe a union in which member types have
## several different representation kinds, and uses the representation kind
## itself as the type discrimination hint to do so.
##
## Note: Unions can be used to produce a "nominative" style of type declarations
## -- yes, even given that IPLD Schema systems are natively "structural" typing!
##
type UnionRepresentation union {
	| UnionRepresentation_Kinded "kinded"
	| UnionRepresentation_Keyed "keyed"
	| UnionRepresentation_Envelope "envelope"
	| UnionRepresentation_Inline "inline"
	| UnionRepresentation_StringPrefix "stringprefix"
	| UnionRepresentation_BytesPrefix "bytesprefix"
} representation keyed

## "Kinded" union representations describe a bidirectional mapping between
## a RepresentationKind and the type which should be the
## union member decoded when one sees this RepresentationKind.
##
## The referenced type must of course produce the RepresentationKind it's
## matched with!
type UnionRepresentation_Kinded {RepresentationKind:UnionMember}

## "Keyed" union representations will encode as a map, where the map has
## exactly one entry, the key string of which will be used to look up the name
## of the Type; and the value should be the content, and be of that Type.
##
## Note: when writing a new protocol, it may be wise to prefer keyed unions
## over the other styles wherever possible; keyed unions tend to have good
## performance characteristics, as they have most "mechanical sympathy" with
## parsing and deserialization implementation order.
type UnionRepresentation_Keyed {String:UnionMember}

## "Envelope" union representations will encode as a map, where the map has
## exactly two entries: the two keys should be of the exact strings specified
## for this envelope representation.  The value for the discriminant key
## should be one of the strings in the discriminant table.  The value for
## the content key should be the content, and be of the Type matching the
## lookup in the discriminant table.
type UnionRepresentation_Envelope struct {
	discriminantKey String
	contentKey String
	discriminantTable {String:UnionMember}
}

## "Inline" union representations require that all of their members encode
## as a map, and encode their type info into the same map as the member data.
## Thus, the map for an inline union may have any number of entries: it is
## however many fields the member value has, plus one (for the discriminant).
##
## All members of an inline union must be struct types and must encode to
## the map RepresentationKind.  Other types which encode to map (such as map
## types themselves!) cannot be used: the potential for content values with
## with keys overlapping with the discriminantKey would result in undefined
## behavior!  Similarly, the member struct types may not have fields which
## have names that collide with the discriminantKey.
##
## When designing a new protocol, use inline unions sparingly; despite
## appearing simple, they have the most edge cases of any kind of union
## representation, and their implementation is generally the most complex and
## is difficult to optimize deserialization to support.
type UnionRepresentation_Inline struct {
	discriminantKey String
	discriminantTable {String:TypeName}
}

## UnionRepresentation_StringPrefix describes a union representation for unions
## whose member types are all strings. Strings used for this representation
## strategy use the first characters as the discriminator, and the subsequent
## characters as the discriminated type's value.
##
## There is currently no limitation on prefix length, other than needing to be
## at least one character. Nor is there a requirement that they all be of the
## same length, although they must all represent unique prefixes.
##
## stringprefix is an invalid representation for any union that contains a type
## that does not have a string representation.
##
type UnionRepresentation_StringPrefix struct {
	prefixes {String:TypeName}
}

## UnionRepresentation_BytesPrefix describes a union representation for unions
## whose member types are all bytes. It is encoded to a byte array whose
## first bytes are the discriminator and subsequent bytes form the discriminated
## type.
##
## Discriminators are represented as hexadecimal strings. There is currently
## no limitation on their length, other than needing to be at least one byte.
## Nor is there a requirement that they all be of the same length, although
## they must all represent unique prefixes.
##
## Only valid, upper-case, hexadecimal strings representing at least one byte
## are allowed.
##
## bytesprefix is an invalid representation for any union that contains a type
## that does not have a bytes representation.
##
type UnionRepresentation_BytesPrefix struct {
	prefixes {HexString:TypeName}
}

## HexString is an alias for string, to denote and clarify that it's not a regular freetext string.
## It's seen used in the UnionRepresentation_BytesPrefix type.
##
## (We use hexadecimal strings in the schema-schema in some places,
## even though we could've used bytes types, because the schema DSL also uses hex strings,
## and consistency (and, the ability to keep the schema-schema in plain JSON!) is valuable.)
type HexString string

## TypeDefnStruct describes a type which has a group of fields of varying Type.
## Each field has a name, which is used to access its value, similarly to
## accessing values in a map.
##
## The most typical representation of a struct is as a map, in which case field
## names also serve as the the map keys (though this is a default, and details
## of this representation may be configured; and other representation strategies
## also exist).
##
type TypeDefnStruct struct {
	fields {FieldName:StructField}
	representation StructRepresentation
}

## FieldName is an alias of string.
##
## There are some additional rules that should be applied:
##   - Field names should by convention begin with a lower-case letter;
##   - Field names must be all printable characters (no whitespace);
##   - Field names must not contain punctuation other than underscores
##     (dashes, dots, etc.).
##
## Field names are strings meant for human consumption at a local scope.
## When making a Schema, note that the FieldName is the key of the map:
## a FieldName must be unique within the Schema.
##
type FieldName string

## StructField describes the properties of each field declared by a TypeDefnStruct.
##
## StructField contains properties similar to TypeDefnMap -- namely, it describes
## a content type (as a TypeNameOrInlineDefn -- it supports inline definitions) -- and
## has a boolean property for whether or not the value is permitted to be null.
##
## In addition, StructField also has a property called "optional".
## An "optional" field is one which is permitted to be absent entirely.
## This is distinct from "nullable": a field can be optional=false and
## nullable=true, in which case it's an error if the key is missing entirely,
## but null is of course valid.  Conversely, if a field is optional=true and
## nullable=false, it's an error if the field is present and assigned null, but
## fine for a map to be missing a key of the field's name entirely and still be
## recognized as this struct.
## (The specific behavior of optionals may vary per StructRepresentation.)
##
## Note that the 'optional' and 'nullable' properties are not themselves
## optional... however, in the IPLD serial representation of schemas, you'll
## often see them absent from the map encoding a StructField.  This is because
## these fields are specified to be implicitly false.
## Implicits in a map representation of a struct mean that those entries may
## be missing from the map encoding... but unlike with "optional" fields, there
## is no "undefined" value; absence is simply interpreted as the value specified
## as the implicit.
## (With implicit fields, an explicitly encoded implicit value is actually an
## error instead!)  "Optional" fields give rise to N+1 cardinality logic,
## just like "nullable" fields; "implicit" fields *do not*.
##
type StructField struct {
	type TypeNameOrInlineDefn
	optional Bool (implicit false)
	nullable Bool (implicit false)
}

## TypeNameOrInlineDefn is a union of either TypeName or an InlineDefn.
## It's used for the value type in the recursive types (maps, lists, and the fields of structs),
## which allows the use of InlineDefn in any of those positions.
##
## TypeNameOrInlineDefn is simply a TypeName if the kind of data is a string;
## this is simple and common case.
## If the data is a map, then it requires further recognition as an InlineDefn.
##
## Note that TypeNameOrInlineDefn isn't used to describe *keys* in the recursive types that
## have them (maps, structs) -- recursive types in keys would not lend itself
## well to serialization!
## TypeNameOrInlineDefn also isn't used to describe members in Unions -- this is a choice
## aimed to limit syntactical complexity (both at type definition authoring
## time, as well as for the sake of error messaging during typechecking).
##
type TypeNameOrInlineDefn union {
	| TypeName string
	| InlineDefn map
} representation kinded

## InlineDefn represents a declaration of an anonymous type of one of the simple
## recursive kinds (e.g. map or list) which is found "inline" in another type's definition.
## InlineDefn also allows description of anonymous but typed links for similar reasons.
## InlineDefn is the more complex option of the TypeNameOrInlineDefn union.
##
## Note that the representation of this union -- the use of a keyed representation,
## as well as the keywords for its members -- align exactly with those
## in the TypeNameOrInlineDefn union.  Technically, this isn't a necessary property (in that
## nothing would break if that sameness was violated) but it's awfully nice for
## sanity; what we're saying here is that the representation of the types in an
## InlineDefn should look *exactly the same* as the top-level type declarations...
## it's just that within an InlineDefn, we're restricted to a subset of the members.
##
type InlineDefn union {
	| TypeDefnMap "map"
	| TypeDefnList "list"
	| TypeDefnLink "link"
} representation keyed

## StructRepresentation describes how a struct type should be mapped onto
## its IPLD Data Model representation.
##
## The default representation strategy for struct types is a map,
## with the struct's field names as keys.
## However, that can be configured.
## For example, the map representation can be configured with
## directives to use different keys in the representation form;
## or, configured to consider some fields as having values which should be seen as the implicit value for that field,
## meaning the entire field shouldn't get an entry in the representation map if that value is found in that field.
## Or, wholly different representation strategies can be used
## (such as the tuple strategy, which results in a data model list kind,
## or stringjoin, which results a data model string kind!).
##
type StructRepresentation union {
	| StructRepresentation_Map "map"
	| StructRepresentation_Tuple "tuple"
	| StructRepresentation_StringPairs "stringpairs"
	| StructRepresentation_StringJoin "stringjoin"
	| StructRepresentation_ListPairs "listpairs"
} representation keyed

## StructRepresentation_Map describes a way to map a struct type onto a map
## representation. Field serialization options may optionally be configured to
## enable mapping serialized keys using the 'rename' option, or implicit values
## specified where the field is omitted from the serialized form using the
## 'implicit' option.
##
## See StructRepresentation_Map_FieldDetails for details on the 'rename' and
## 'implicit' options.
##
type StructRepresentation_Map struct {
	fields optional {FieldName:StructRepresentation_Map_FieldDetails}
}

## StructRepresentation_Map_FieldDetails describes additional properties of a
## struct field when represented as a map.  For example, fields may be renamed,
## or implicit values associated.
##
## If an implicit value is defined, then during marshalling, if the actual value
## is the implicit value, that field will be omitted from the map; and during
## unmarshalling, correspondingly, the absence of that field will be interpreted
## as being the implicit value.
##
## Note that fields with implicits are distinct from fields which are optional!
## The cardinality of membership of an optional field is is incremented:
## e.g., the cardinality of "fieldname Bool" is 2; "fieldname optional Bool" is
## membership cardinality *3*, because it may also be undefined.
## By contrast, the cardinality of membership of a field with an implicit value
## remains unchanged; there is serial state which can map to an undefined value.
##
## Note that 'rename' supports exactly one string, and not a list: this is
## intentional.  The rename feature is meant to allow serial representations
## to use a different key string than the schema type definition field name;
## it is not intended to be used for migration purposes.
##
type StructRepresentation_Map_FieldDetails struct {
	rename optional String
	implicit optional AnyScalar
}

## StructRepresentation_Tuple describes a way to map a struct type into a list
## representation.
##
## Tuple representations are less flexible than map representations:
## field order can be specified in order to override the order defined
## in the type, but optionals and implicits are not (currently) supported.
## A `fieldOrder` list must include quoted strings (FieldName is a string
## type) which are coerced to the names of the struct fields. e.g.:
##   fieldOrder ["Foo", "Bar", "Baz"]
##
type StructRepresentation_Tuple struct {
	fieldOrder optional [FieldName]
}

## StructRepresentation_StringPairs describes that a struct should be encoded
## as a string of delimited "k/v" entries, e.g. "k1=v1,k2=v2".
## The separating delimiter may be specified with "entryDelim", and the k/v
## delimiter may be specified with "innerDelim". So a "k=v" naive
## comma-separated form would use an "innerDelim" of "=" and an "entryDelim"
## of ",".
##
## Serialization a struct with stringpairs works the same way as serializing
## a map with stringpairs and the same character limitations exist. See
## MapRepresentation_StringPairs for more details on these limitations.
##
type StructRepresentation_StringPairs struct {
	innerDelim String
	entryDelim String
}

## StructRepresentation_StringJoin describes a way to encode a struct to
## a string in the IPLD Data Model. Similar to tuple representation, the
## keys are dropped as they may be inferred from the struct definition.
## values are concatenated, in order, and separated by a "join" delimiter.
## For example, specifying ":" as the "join": "v1,v2,v3".
##
## stringjoin is necessarily restrictive and therefore only valid for structs
## whose values may all be encoded to string form without conflicts in "join"
## character. It is recommended, therefore, that its use be limited to structs
## containing values with the basic data model kinds that exclude multiple
## values (i.e. no maps, lists, and therefore structs or unions).
##
type StructRepresentation_StringJoin struct {
	join String
	fieldOrder optional [FieldName]
}

## StructRepresentation_ListPairs describes that a struct, should be encoded as
## a list in the IPLD Data Model. This list comprises a sub-list for each
## entry, in the form: [[k1,v1],[k2,v2]].
##
## This representation type encodes in the same way as
## MapStructRepresentation_Tuple. It is also similar to
## StructRepresentation_Tuple except it includes the keys in nested lists.
## A tuple representation for a struct will encode more compact than listpairs.
##
type StructRepresentation_ListPairs struct {}

## TypeDefnEnum describes a type which has a known, pre-defined set of possible values.
## Each of the member values is named by a string (of EnumMember type).
##
## Enums can have either string or int-based representations.
## Integer and string values (for int and string representations respectively)
## are provided in parens in the DSL. Where the string used in serialization is
## the same as the EnumMember, it may be omitted. For int representation enums,
## all int values are required.
##
type TypeDefnEnum struct {
	members [EnumMember]
	representation EnumRepresentation
}

## EnumMember is a string that that names a member of an Enum type.
##
## The range of valid values for an EnumMember are the same as for TypeName
## but without the convention of an uppercase first character.
## Capitalization is left up to the discretion of the schema writer.
##
type EnumMember string

## EnumRepresentation describes how an enum type should be mapped onto
## its IPLD Data Model representation. By default an enum is represented as a
## string kind but it may also be represented as an int kind.
##
type EnumRepresentation union {
	| EnumRepresentation_String "string"
	| EnumRepresentation_Int "int"
} representation keyed

## EnumRepresentation_String describes the way an enum is represented as a
## string in the data model. By default, the strings used as EnumMember will be
## used at the serialization. A custom string may be provided (with `Foo ("x")`
## in the DSL) which will be stored here in the representation block. Missing
## entries in this map will use the default.
##
type EnumRepresentation_String {EnumMember:String}

## EnumRepresentation_Int describes the way an enum is represented as an int
## in the data model. A mapping of names to ints is required to perform the
## conversion from int to enum value. In the DSL, int values _must_ be provided
## for each EnumMember (with `Foo ("100")`, those are stored here.
##
type EnumRepresentation_Int {EnumMember:Int}

## TypeDefnUnit describes a type which contains no data at all (other than that fact of its existence).
## (If this seems strange, consider that the cardinality of a bool type is 2;
## the cardinality of a unit type is simply 1.)
##
type TypeDefnUnit struct {
	representation UnitRepresentation
}

## UnitRepresentation is an enum for describing how a TypeDefnUnit should be represented in the data model.
##
## Unit types are commonly seen represented in several ways.
## A null token is a common one.
## A true token is sometimes seen (especially, when people encode "sets" in json:
## often this will be seen as a map where the values are keys and the map values are 'true').
## Also, an empty map can be a useful unit value;
## an empty map accurately communicates a lack of data.
## (The emptymap strategy can be a particularly interesting choice if you want to
## have a schema that is evolvable in the future to start using a struct or map
## in the same place as the unit type currently stands, while having older documents
## continue to be parsable by the evolved schema.)
##
## Unlike many of the other representation information types seen in the schema-schema,
## this one is just an enum, rather than being a union.
## That's because there's no possibility of every needing to annotate more customization
## onto values in the unit type... because there are no possible values in the unit type.
##
## Note that there is no discernible logical difference between
## `type Foo struct {}` and `type Foo unit representation emptymap`;
## only that the latter can be said to be a more explicit description of intent.
## Both will result in identical representations, and both have identical cardinality (which is 1).
##
type UnitRepresentation enum {
	| Null ("null")
	| True ("true")
	| False ("false")
	| Emptymap ("emptymap")
}

## TypeDefnAny describes a type which can contain data of any kind.
## It's essentially an escape valve; it says "we don't really know what lies beyond here".
## The type-level data model kind of an "any" type can be anything;
## it depends on what the inhabitant value is.
## The representation-level kind can similarly be anything;
## it will match whatever the type-level kind is.
##
## It is not possible to regain useful types on deeper values after using an 'any' type;
## from then on, the rest of the data is locked in on having exactly that 'any' type,
## and having no further ability to have separate type-level and representation-level behaviors.
##
## 'any' was introduced as a typekind after discovering it is not possible
## to emulate its behavior by constructing a union; see
## https://github.com/ipld/specs/issues/318 for some discussion of this.
type TypeDefnAny struct {}

## TypeDefnCopy describes a special "copy" unit that indicates that a type name
## should copy the type descriptor of another type. TypeDefnCopy does not redirect a
## name to another type. Instead, it copies the entire type definition and
## assigns it to another type.
##
## The DSL defines a TypeDefnCopy as `type NewThing = CopiedThing`, where
## "CopiedThing" refers to a `type` defined elsewhere in a schema and is not
## one of TypeKind or an inline type descriptor (`{}`, `[]`, `&`).
##
type TypeDefnCopy struct {
	fromType TypeName
}