Built-in Decorators

TypeSpec

@continuationToken

Pagination property defining the token to get to the next page. It MUST be specified both on the request parameter and the response.

@continuationToken

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
  @continuationToken continuationToken: string;
}
@list op listPets(@continuationToken continuationToken: string): Page<Pet>;

@defaultVisibility

Declares the default visibility modifiers for a visibility class.

The default modifiers are used when a property does not have any visibility decorators applied to it.

The modifiers passed to this decorator MUST be members of the target Enum.

@defaultVisibility(...visibilities: valueof EnumMember[])

Target

Enum

Parameters

Name	Type	Description
visibilities	valueof EnumMember[]	the list of modifiers to use as the default visibility modifiers.

@deprecated

Caution

Deprecated: @deprecated decorator is deprecated. Use the #deprecated directive instead.

Mark this type as deprecated.

NOTE: This decorator should not be used, use the #deprecated directive instead.

@deprecated(message: valueof string)

Target

unknown

Parameters

Name	Type	Description
message	valueof string	Deprecation message.

Examples

Use the #deprecated directive instead:

#deprecated "Use ActionV2"
op Action<Result>(): Result;

@discriminator

Specify the property to be used to discriminate this type.

@discriminator(propertyName: valueof string)

Target

Model | Union

Parameters

Name	Type	Description
propertyName	valueof string	The property name to use for discrimination

Examples

@discriminator("kind")
union Pet{ cat: Cat, dog: Dog }

model Cat {kind: "cat", meow: boolean}
model Dog {kind: "dog", bark: boolean}

@discriminator("kind")
model Pet{ kind: string }

model Cat extends Pet {kind: "cat", meow: boolean}
model Dog extends Pet  {kind: "dog", bark: boolean}

@doc

Attach a documentation string.

@doc(doc: valueof string, formatArgs?: {})

Target

unknown

Parameters

Name	Type	Description
doc	valueof string	Documentation string
formatArgs	{}	Record with key value pair that can be interpolated in the doc.

Examples

@doc("Represent a Pet available in the PetStore")
model Pet {}

@encode

Specify how to encode the target type.

@encode(encodingOrEncodeAs: Scalar | valueof string | EnumMember, encodedAs?: Scalar)

Target

Scalar | ModelProperty

Parameters

Name	Type	Description
encodingOrEncodeAs	Scalar | valueof string | EnumMember	Known name of an encoding or a scalar type to encode as(Only for numeric types to encode as string).
encodedAs	Scalar	What target type is this being encoded as. Default to string.

Examples

offsetDateTime encoded with rfc7231

@encode("rfc7231")
scalar myDateTime extends offsetDateTime;

utcDateTime encoded with unixTimestamp

@encode("unixTimestamp", int32)
scalar myDateTime extends unixTimestamp;

encode numeric type to string

model Pet {
  @encode(string) id: int64;
}

@encodedName

Provide an alternative name for this type when serialized to the given mime type.

@encodedName(mimeType: valueof string, name: valueof string)

Target

unknown

Parameters

Name	Type	Description
mimeType	valueof string	Mime type this should apply to. The mime type should be a known mime type as described here https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types without any suffix (e.g. +json)
name	valueof string	Alternative name

Examples

model Certificate {
  @encodedName("application/json", "exp")
  @encodedName("application/xml", "expiry")
  expireAt: int32;
}

Invalid values

@encodedName("application/merge-patch+json", "exp")
             ^ error cannot use subtype

@error

Specify that this model is an error type. Operations return error types when the operation has failed.

@error

Target

Model

Parameters

None

Examples

@error
model PetStoreError {
  code: string;
  message: string;
}

@errorsDoc

Attach a documentation string to describe the error return types of an operation. If an operation returns a union of success and errors it only describes the errors. See @returnsDoc for success documentation.

@errorsDoc(doc: valueof string)

Target

Operation

Parameters

Name	Type	Description
doc	valueof string	Documentation string

Examples

@errorsDoc("Errors doc")
op get(): Pet | NotFound;

@example

Provide an example value for a data type.

@example(example: valueof unknown, options?: valueof ExampleOptions)

Target

Model | Enum | Scalar | Union | ModelProperty | UnionVariant

Parameters

Name	Type	Description
example	valueof unknown	Example value.
options	valueof ExampleOptions	Optional metadata for the example.

Examples

@example(#{name: "Fluffy", age: 2})
model Pet {
 name: string;
 age: int32;
}

@firstLink

Pagination property defining a link to the first page.

It is expected that navigating to the link will return the same set of responses as the operation that returned the current page.

@firstLink

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
  @nextLink next: url;
  @prevLink prev: url;
  @firstLink first: url;
  @lastLink last: url;
}
@list op listPets(): Page<Pet>;

@format

Specify a known data format hint for this string type. For example uuid, uri, etc. This differs from the @pattern decorator which is meant to specify a regular expression while @format accepts a known format name. The format names are open ended and are left to emitter to interpret.

@format(format: valueof string)

Target

string | bytes | ModelProperty

Parameters

Name	Type	Description
format	valueof string	format name.

Examples

@format("uuid")
scalar uuid extends string;

@friendlyName

Specifies how a templated type should name their instances.

@friendlyName(name: valueof string, formatArgs?: unknown)

Target

unknown

Parameters

Name	Type	Description
name	valueof string	name the template instance should take
formatArgs	unknown	Model with key value used to interpolate the name

Examples

@friendlyName("{name}List", T)
model List<Item> {
  value: Item[];
  nextLink: string;
}

@inspectType

A debugging decorator used to inspect a type.

@inspectType(text: valueof string)

Target

unknown

Parameters

Name	Type	Description
text	valueof string	Custom text to log

@inspectTypeName

A debugging decorator used to inspect a type name.

@inspectTypeName(text: valueof string)

Target

unknown

Parameters

Name	Type	Description
text	valueof string	Custom text to log

@invisible

Indicates that a property is not visible in the given visibility class.

This decorator removes all active visibility modifiers from the property within the given visibility class.

@invisible(visibilityClass: Enum)

Target

ModelProperty

Parameters

Name	Type	Description
visibilityClass	Enum	The visibility class to make the property invisible within.

Examples

model Example {
  @invisible(Lifecycle)
  hidden_property: string;
}

@key

Mark a model property as the key to identify instances of that type

@key(altName?: valueof string)

Target

ModelProperty

Parameters

Name	Type	Description
altName	valueof string	Name of the property. If not specified, the decorated property name is used.

Examples

model Pet {
  @key id: string;
}

@knownValues

Caution

Deprecated: This decorator has been deprecated. Use a named union of string literals with a string variant to achieve the same result without a decorator.

Provide a set of known values to a string type.

@knownValues(values: Enum)

Target

string | numeric | ModelProperty

Parameters

Name	Type	Description
values	Enum	Known values enum.

Examples

@knownValues(KnownErrorCode)
scalar ErrorCode extends string;

enum KnownErrorCode {
  NotFound,
  Invalid,
}

@lastLink

Pagination property defining a link to the last page.

It is expected that navigating to the link will return the same set of responses as the operation that returned the current page.

@lastLink

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
  @nextLink next: url;
  @prevLink prev: url;
  @firstLink first: url;
  @lastLink last: url;
}
@list op listPets(): Page<Pet>;

@list

Mark this operation as a list operation that returns a paginated list of items.

@list

Target

Operation

Parameters

None

@maxItems

Specify the maximum number of items this array should have.

@maxItems(value: valueof integer)

Target

unknown[] | ModelProperty

Parameters

Name	Type	Description
value	valueof integer	Maximum number

Examples

@maxItems(5)
model Endpoints is string[];

@maxLength

Specify the maximum length this string type should be.

@maxLength(value: valueof integer)

Target

string | ModelProperty

Parameters

Name	Type	Description
value	valueof integer	Maximum length

Examples

@maxLength(20)
scalar Username extends string;

@maxValue

Specify the maximum value this numeric type should be.

@maxValue(value: valueof numeric)

Target

numeric | ModelProperty

Parameters

Name	Type	Description
value	valueof numeric	Maximum value

Examples

@maxValue(200)
scalar Age is int32;

@maxValueExclusive

Specify the maximum value this numeric type should be, exclusive of the given value.

@maxValueExclusive(value: valueof numeric)

Target

numeric | ModelProperty

Parameters

Name	Type	Description
value	valueof numeric	Maximum value

Examples

@maxValueExclusive(50)
scalar distance is float64;

@minItems

Specify the minimum number of items this array should have.

@minItems(value: valueof integer)

Target

unknown[] | ModelProperty

Parameters

Name	Type	Description
value	valueof integer	Minimum number

Examples

@minItems(1)
model Endpoints is string[];

@minLength

Specify the minimum length this string type should be.

@minLength(value: valueof integer)

Target

string | ModelProperty

Parameters

Name	Type	Description
value	valueof integer	Minimum length

Examples

@minLength(2)
scalar Username extends string;

@minValue

Specify the minimum value this numeric type should be.

@minValue(value: valueof numeric)

Target

numeric | ModelProperty

Parameters

Name	Type	Description
value	valueof numeric	Minimum value

Examples

@minValue(18)
scalar Age is int32;

@minValueExclusive

Specify the minimum value this numeric type should be, exclusive of the given value.

@minValueExclusive(value: valueof numeric)

Target

numeric | ModelProperty

Parameters

Name	Type	Description
value	valueof numeric	Minimum value

Examples

@minValueExclusive(0)
scalar distance is float64;

@nextLink

Pagination property defining a link to the next page.

It is expected that navigating to the link will return the same set of responses as the operation that returned the current page.

@nextLink

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
  @nextLink next: url;
  @prevLink prev: url;
  @firstLink first: url;
  @lastLink last: url;
}
@list op listPets(): Page<Pet>;

@offset

Pagination property defining the number of items to skip.

@offset

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
}
@list op listPets(@offset skip: int32, @pageSize pageSize: int8): Page<Pet>;

@opExample

Provide example values for an operation’s parameters and corresponding return type.

@opExample(example: valueof OperationExample, options?: valueof ExampleOptions)

Target

Operation

Parameters

Name	Type	Description
example	valueof OperationExample	Example value.
options	valueof ExampleOptions	Optional metadata for the example.

Examples

@opExample(#{parameters: #{name: "Fluffy", age: 2}, returnType: #{name: "Fluffy", age: 2, id: "abc"})
op createPet(pet: Pet): Pet;

@overload

Specify this operation is an overload of the given operation.

@overload(overloadbase: Operation)

Target

Operation

Parameters

Name	Type	Description
overloadbase	Operation	Base operation that should be a union of all overloads

Examples

op upload(data: string | bytes, @header contentType: "text/plain" | "application/octet-stream"): void;
@overload(upload)
op uploadString(data: string, @header contentType: "text/plain" ): void;
@overload(upload)
op uploadBytes(data: bytes, @header contentType: "application/octet-stream"): void;

@pageIndex

Pagination property defining the page index.

@pageIndex

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
}
@list op listPets(@pageIndex page: int32, @pageSize pageSize: int8): Page<Pet>;

@pageItems

Specify the the property that contains the array of page items.

@pageItems

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
}
@list op listPets(@pageIndex page: int32, @pageSize pageSize: int8): Page<Pet>;

@pageSize

Specify the pagination parameter that controls the maximum number of items to include in a page.

@pageSize

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
}
@list op listPets(@pageIndex page: int32, @pageSize pageSize: int8): Page<Pet>;

@parameterVisibility

Sets which visibilities apply to parameters for the given operation.

@parameterVisibility(...visibilities: valueof string | EnumMember[])

Target

Operation

Parameters

Name	Type	Description
visibilities	valueof string | EnumMember[]	List of visibility strings which apply to this operation.

@pattern

Specify the the pattern this string should respect using simple regular expression syntax. The following syntax is allowed: alternations (|), quantifiers (?, *, +, and { }), wildcard (.), and grouping parentheses. Advanced features like look-around, capture groups, and references are not supported.

This decorator may optionally provide a custom validation message. Emitters may choose to use the message to provide context when pattern validation fails. For the sake of consistency, the message should be a phrase that describes in plain language what sort of content the pattern attempts to validate. For example, a complex regular expression that validates a GUID string might have a message like “Must be a valid GUID.”

@pattern(pattern: valueof string, validationMessage?: valueof string)

Target

string | bytes | ModelProperty

Parameters

Name	Type	Description
pattern	valueof string	Regular expression.
validationMessage	valueof string	Optional validation message that may provide context when validation fails.

Examples

@pattern("[a-z]+", "Must be a string consisting of only lower case letters and of at least one character.")
scalar LowerAlpha extends string;

@prevLink

Pagination property defining a link to the previous page.

It is expected that navigating to the link will return the same set of responses as the operation that returned the current page.

@prevLink

Target

ModelProperty

Parameters

None

Examples

model Page<T> {
  @pageItems items: T[];
  @nextLink next: url;
  @prevLink prev: url;
  @firstLink first: url;
  @lastLink last: url;
}
@list op listPets(): Page<Pet>;

@projectedName

Caution

Deprecated: Use @encodedName instead for changing the name over the wire.

DEPRECATED: Use @encodedName instead.

Provide an alternative name for this type.

@projectedName(targetName: valueof string, projectedName: valueof string)

Target

unknown

Parameters

Name	Type	Description
targetName	valueof string	Projection target
projectedName	valueof string	Alternative name

Examples

model Certificate {
  @projectedName("json", "exp")
  expireAt: int32;
}

@removeVisibility

Removes visibility modifiers from a property.

If the visibility modifiers for a visibility class have not been initialized, this decorator will use the default visibility modifiers for the visibility class as the default modifier set.

@removeVisibility(...visibilities: valueof EnumMember[])

Target

The property to remove visibility from. ModelProperty

Parameters

Name	Type	Description
visibilities	valueof EnumMember[]	The visibility modifiers to remove from the target property.

Examples

model Example {
  // This property will have the Create and Update visibilities, but not the
  // Read visibility, since it is removed.
  @removeVisibility(Lifecycle.Read)
  secret_property: string;
}

@returnsDoc

Attach a documentation string to describe the successful return types of an operation. If an operation returns a union of success and errors it only describes the success. See @errorsDoc for error documentation.

@returnsDoc(doc: valueof string)

Target

Operation

Parameters

Name	Type	Description
doc	valueof string	Documentation string

Examples

@returnsDoc("Returns doc")
op get(): Pet | NotFound;

@returnTypeVisibility

Sets which visibilities apply to the return type for the given operation.

@returnTypeVisibility(...visibilities: valueof string | EnumMember[])

Target

Operation

Parameters

Name	Type	Description
visibilities	valueof string | EnumMember[]	List of visibility strings which apply to this operation.

@secret

Mark this string as a secret value that should be treated carefully to avoid exposure

@secret

Target

string | ModelProperty

Parameters

None

Examples

@secret
scalar Password is string;

@service

Mark this namespace as describing a service and configure service properties.

@service(options?: ServiceOptions)

Target

Namespace

Parameters

Name	Type	Description
options	ServiceOptions	Optional configuration for the service.

Examples

@service
namespace PetStore;

Setting service title

@service({title: "Pet store"})
namespace PetStore;

Setting service version

@service({version: "1.0"})
namespace PetStore;

@summary

Typically a short, single-line description.

@summary(summary: valueof string)

Target

unknown

Parameters

Name	Type	Description
summary	valueof string	Summary string.

Examples

@summary("This is a pet")
model Pet {}

@tag

Attaches a tag to an operation, interface, or namespace. Multiple @tag decorators can be specified to attach multiple tags to a TypeSpec element.

@tag(tag: valueof string)

Target

Namespace | Interface | Operation

Parameters

Name	Type	Description
tag	valueof string	Tag value

@visibility

Indicates that a property is only considered to be present or applicable (“visible”) with the in the given named contexts (“visibilities”). When a property has no visibilities applied to it, it is implicitly visible always.

As far as the TypeSpec core library is concerned, visibilities are open-ended and can be arbitrary strings, but the following visibilities are well-known to standard libraries and should be used with standard emitters that interpret them as follows:

• “read”: output of any operation.
• “create”: input to operations that create an entity..
• “query”: input to operations that read data.
• “update”: input to operations that update data.
• “delete”: input to operations that delete data.

See also: Automatic visibility

@visibility(...visibilities: valueof string | EnumMember[])

Target

ModelProperty

Parameters

Name	Type	Description
visibilities	valueof string | EnumMember[]	List of visibilities which apply to this property.

Examples

model Dog {
  // the service will generate an ID, so you don't need to send it.
  @visibility(Lifecycle.Read) id: int32;
  // the service will store this secret name, but won't ever return it
  @visibility(Lifecycle.Create, Lifecycle.Update) secretName: string;
  // the regular name is always present
  name: string;
}

@withDefaultKeyVisibility

Set the visibility of key properties in a model if not already set.

This will set the visibility modifiers of all key properties in the model if the visibility is not already explicitly set, but will not change the visibility of any properties that have visibility set explicitly, even if the visibility is the same as the default visibility.

Visibility may be explicitly set using any of the following decorators:

• @visibility
• @removeVisibility
• @invisible

@withDefaultKeyVisibility(visibility: valueof string | EnumMember)

Target

Model

Parameters

Name	Type	Description
visibility	valueof string | EnumMember	The desired default visibility value. If a key property already has visibility set, it will not be changed.

@withLifecycleUpdate

Transforms the target model to include only properties that are visible during the “Update” lifecycle phase.

Any nested models of optional properties will be transformed into the “CreateOrUpdate” lifecycle phase instead of the “Update” lifecycle phase, so that nested models may be fully updated.

@withLifecycleUpdate

Target

The model to apply the transformation to. Model

Parameters

None

Examples

model Dog {
  @visibility(Lifecycle.Read)
  id: int32;

  @visibility(Lifecycle.Create, Lifecycle.Update)
  secretName: string;

  name: string;
}

@withLifecycleUpdate
model DogUpdate {
  ...Dog
}

@withOptionalProperties

Returns the model with required properties removed.

@withOptionalProperties

Target

Model

Parameters

None

@withoutDefaultValues

Returns the model with any default values removed.

@withoutDefaultValues

Target

Model

Parameters

None

@withoutOmittedProperties

Returns the model with the given properties omitted.

@withoutOmittedProperties(omit: string | Union)

Target

Model

Parameters

Name	Type	Description
omit	string | Union	List of properties to omit

@withPickedProperties

Returns the model with only the given properties included.

@withPickedProperties(pick: string | Union)

Target

Model

Parameters

Name	Type	Description
pick	string | Union	List of properties to include

@withUpdateableProperties

Returns the model with non-updateable properties removed.

@withUpdateableProperties

Target

Model

Parameters

None

@withVisibility

Removes properties that are not considered to be present or applicable (“visible”) in the given named contexts (“visibilities”). Can be used together with spread to effectively spread only visible properties into a new model.

See also: Automatic visibility

When using an emitter that applies visibility automatically, it is generally not necessary to use this decorator.

@withVisibility(...visibilities: valueof string | EnumMember[])

Target

Model

Parameters

Name	Type	Description
visibilities	valueof string | EnumMember[]	List of visibilities which apply to this property.

Examples

model Dog {
  @visibility("read") id: int32;
  @visibility("create", "update") secretName: string;
  name: string;
}

// The spread operator will copy all the properties of Dog into DogRead,
// and @withVisibility will then remove those that are not visible with
// create or update visibility.
//
// In this case, the id property is removed, and the name and secretName
// properties are kept.
@withVisibility("create", "update")
model DogCreateOrUpdate {
  ...Dog;
}

// In this case the id and name properties are kept and the secretName property
// is removed.
@withVisibility("read")
model DogRead {
  ...Dog;
}

@withVisibilityFilter

Applies the given visibility filter to the properties of the target model.

This transformation is recursive, so it will also apply the filter to any nested or referenced models that are the types of any properties in the target.

@withVisibilityFilter(filter: valueof VisibilityFilter)

Target

The model to apply the visibility filter to. Model

Parameters

Name	Type	Description
filter	valueof VisibilityFilter	The visibility filter to apply to the properties of the target model.

Examples

model Dog {
  @visibility(Lifecycle.Read)
  id: int32;

  name: string;
}

@withVisibilityFilter(#{ all: #[Lifecycle.Read] })
model DogRead {
 ...Dog
}