17.14.6 Class, Interface, and Veneer Clause Macros🔗ℹ

The compile-time body block returns the expansion result. The result must be a sequence of groups to be spliced in place of the macro use, where each group can be either a another class clause, an expression, a definition, or an export.

In addition to ~op_stx and ~all_stx options like defn.macro, the optional ~info body form binds an id to a class_meta.Info value. Use the class_meta.Info.lookup function to access information about the class declaration that appears before the use of the class-clause macro.

> class_clause.macro 'lazy_method $id(): $(body :: Block)':     'private field result: #false      method $id():        result || (block:                     def v $body                     result := v                     v)'

> class Person(name):     lazy_method greeting():       "Hello, " +& name

> def ming = Person("Ming")

> ming.greeting()

"Hello, Ming"

> ming.greeting() === ming.greeting()

#true

Returns an object containing information about name, which must be bound as a class name or internal class name for class_meta.describe, as an interface name or internal interface name for interface_meta.describe, or as a veneer name for veneer_meta.describe. Use class_meta.Info.lookup, interface_meta.Info.lookup, or veneer_meta.Info.lookup to extract details from the information object.

A value satisfying class_meta.Info can be obtained from class_meta.describe or received by a class-clause macro through an ~info declaration. Similarly, a value satisfying interface_meta.Info or veneer_meta.Info can be obtained by interface_meta.describe or veneer_meta.describe, or received by an interface- or veneer-clause macro through an ~info declaration. In a macro declared with class_and_interface_clause.macro, a value received by ~info is an instance of either class_meta.Info or interface_meta.Info, which indicates the context where the macro is used.

The class_meta.Info.lookup, interface_meta.Info.lookup, and veneer_meta.Info.lookup functions access information via a key symbol. The currently recognized keys are described below, but more may be added in the future.

When received within a class-, interface-, or veneer-clause macro, the information reported for a key covers only class, interface, or veneer clauses before the macro use, so a use of the same key in the same class but in a later clause may report different information. Furthermore, the information has not been fully checked; for example, the name of a class, interface, or veneer being extended is reported as it appears, and has not yet been checked to be a valid class, interface, or veneer name, and multiple names may be reported if a class contains multiple extends clauses. Similarly, field- or method-name lists may include duplicates.

When obtained via class_meta.describe, interface_meta.describe, or veneer_meta.describe, the information covers the whole body of the referenced class, interface, or veneer form. When a class or interface name is used, then information about private fields and methods is omitted from queries; using an internal name for a class or interface includes information about private fields and methods. A class- or interface-clause macro, which directly receives only partial information, can expand to a use of an expression macro that uses class_meta.describe or interface_meta.describe to get more information based on a retained or newly declared internal name.

Recognized keys for classes:

• #'name: An identifier for the defined class, including any dotted-name prefix in the class declaration.

• #'extends: A list of indentifiers for classes declared as superclasses. Normally, the list will be empty or have one element, but a multi-identifier list can be reported for a clause macro if a class form has multiple extends clauses (even though an error will be reported later).

• #'implements: A list of indentifiers for interfaces declared to be implemented by the class. The list does not include interfaces that are implied by superclasses and implemented interfaces. For a clause macro or starting from class_meta.describe with a class internal name, the list includes privately implemented interfaces.

• #'implements_visibilities: A list of symbols in parallel to a #'implements list that categorizes each implementation’s visibility: #'public or #'private.

• #'internal_names: A list of identifiers declared as internal names for this class. This list will be non-empty only in the information provided to a clause macro.

• #'field_names: A list of identifiers for fields declared for the class in the order of the field declarations. For a clause macro, the list does not include fields that will be inherited from a superclass, while inherited public fields are included in information obtained via class_meta.describe.

• #'field_mutabilities: A list of symbols in parallel to a #'field_names list that categorizes each field’s mutability: #'immutable or #'immutable.

• #'field_visibilities: A list of symbols in parallel to a #'field_names list that categorizes each field’s visibility: #'public or #'private.

• #'field_constructives: A list of symbols in parallel to a #'field_names list that categorizes each field’s use in the default constructor: #'required, #'optional, or #'absent. All private fields are classified as #'absent.

• #'field_keywords: A list of keywords and #falses in parallel to a #'field_names, where a keyword indicates that the field uses that keyword as an argument for the default constructor.

• #'method_names: A list of identifiers for methods declared for the class in the order of the method declarations. The list does not include methods that will be inherited from a superclass or implemented interface, while inherited public methods are included in information obtained via class_meta.describe.

• #'method_visibilities: A list of symbols in parallel to a #'methods_names list that categorizes each methods’s visibility: #'public or #'private.

• #'method_arities: A list of arity representations in parallel to a #'methods_names list. An arity representation can be an integer, in which case each bit set in the integer represents a number of arguments accepted by the method, and no keyword argument are accepted. An arity representation can be a list, in which case the first element is an integer (like an integer by itself), the second element is a list of required keywords for arguments, and the last element is either a list of allowed keywords or #false if all keywords are accepted. An arity representation can be #false if no information about accepted arguments is available. Keep in mind that when a subclass overrides a method, it is not limited to the existing method’s arity.

• #'property_names: A list of identifiers for properties declared for the class in the order of the property declarations. The list does not include properties that will be inherited from a superclass or implemented interface, while inherited public properties are included in information obtained via class_meta.describe.

• #'property_visibilities: A list of symbols in parallel to a #'property_names list that categorizes each property’s visibility: #'public or #'private.

• #'property_arities: A list of arity representations in parallel to a #'property_names list. An arity representation is 1 if the property does not support assignment, 3 if it does support assignment, or #false if the property’s support for assignments is unknown.

• #'uses_default_constructor: A boolean indicating whether the class name in an expression position refers to a default constructor function.

• #'uses_default_binding: A boolean indicating whether the class name in a binding position refers to a default binding form.

• #'uses_default_annotation: A boolean indicating whether the class name in an annotation position refers to a default annotation form.

Recognized keys for interfaces:

• #'name: An identifier for the interface being defined, including any dotted-name prefix in the interface declaration.

• #'extends: A list of indentifiers for interfaces declared as superinterfaces. The list does not include interfaces that are implied by superinterfaces.

• #'internal_names: The same as for classes.

• #'method_names: The same as for classes.

• #'method_arities: The same as for classes.

• #'method_visibilities: The same as for classes.

• #'property_names: The same as for classes.

• #'property_arities: The same as for classes.

• #'property_visibilities: The same as for classes.

• #'uses_default_annotation: A boolean indicating whether the interface name in an annotation position refers to a default annotation form.

Recognized keys for veneers:

• #'name: An identifier for the veneer being defined, including any dotted-name prefix in the veneer declaration.

• #'extends: A list of indentifiers for veneers declared as superveneers. Normally, the list will be empty or have one element, but a multi-identifier list can be reported for a clause macro if a veneer form has multiple extends clauses (even though an error will be reported later).

• #'internal_names: The same as for classes.

• #'method_names: The same as for classes.

• #'method_arities: The same as for classes.

• #'method_visibilities: The same as for classes.

• #'property_names: The same as for classes.

• #'property_arities: The same as for classes.

• #'property_visibilities: The same as for classes.