6.2 Creating Classes

Classes and Objects in The Racket Guide introduces classes and objects.

The superclass-expr expression is evaluated when the class* expression is evaluated. The result must be a class value (possibly object%), otherwise the exn:fail:object exception is raised. The result of the superclass-expr expression is the new class’s superclass.

The interface-expr expressions are also evaluated when the class* expression is evaluated, after superclass-expr is evaluated. The result of each interface-expr must be an interface value, otherwise the exn:fail:object exception is raised. The interfaces returned by the interface-exprs are all implemented by the class. For each identifier in each interface, the class (or one of its ancestors) must declare a public method with the same name, otherwise the exn:fail:object exception is raised. The class’s superclass must satisfy the implementation requirement of each interface, otherwise the exn:fail:object exception is raised.

An inspect class-clause selects an inspector (see Structure Inspectors) for the class extension. The inspector-expr must evaluate to an inspector or #f when the class* form is evaluated. Just as for structure types, an inspector controls access to the class’s fields, including private fields, and also affects comparisons using equal?. If no inspect clause is provided, access to the class is controlled by the parent of the current inspector (see Structure Inspectors). A syntax error is reported if more than one inspect clause is specified.

The other class-clauses define initialization arguments, public and private fields, and public and private methods. For each id or maybe-renamed in a public, override, augment, pubment, overment, augride, public-final, override-final, augment-final, or private clause, there must be one method-definition. All other definition class-clauses create private fields. All remaining exprs are initialization expressions to be evaluated when the class is instantiated (see Creating Objects).

The result of a class* expression is a new class, derived from the specified superclass and implementing the specified interfaces. Instances of the class are created with the instantiate form or make-object procedure, as described in Creating Objects.

Each class-clause is (partially) macro-expanded to reveal its shapes. If a class-clause is a begin expression, its sub-expressions are lifted out of the begin and treated as class-clauses, in the same way that begin is flattened for top-level and embedded definitions.

Within a class* form for instances of the new class, this is bound to the object itself; this% is bound to the class of the object; super-instantiate, super-make-object, and super-new are bound to forms to initialize fields in the superclass (see Creating Objects); super is available for calling superclass methods (see Method Definitions); and inner is available for calling subclass augmentations of methods (see Method Definitions).

The original-datum is the original expression to use for reporting errors.

The name-id is used to name the resulting class; if it is #f, the class name is inferred.

The super-expr, interface-exprs, and class-clauses are as for class*.

If the deserialize-id-expr is not literally #f, then a serializable class is generated, and the result is two values instead of one: the class and a deserialize-info structure produced by make-deserialize-info. The deserialize-id-expr should produce a value suitable as the second argument to make-serialize-info, and it should refer to an export whose value is the deserialize-info structure.

Future optional forms may be added to the sequence that currently ends with deserialize-id-expr.

6.2.1 Initialization Variables

A class’s initialization variables, declared with init, init-field, and init-rest, are instantiated for each object of a class. Initialization variables can be used in the initial value expressions of fields, default value expressions for initialization arguments, and in initialization expressions. Only initialization variables declared with init-field can be accessed from methods; accessing any other initialization variable from a method is a syntax error.

The values bound to initialization variables are

• the arguments provided with instantiate or passed to make-object, if the object is created as a direct instance of the class; or,

• the arguments passed to the superclass initialization form or procedure, if the object is created as an instance of a derived class.

If an initialization argument is not provided for an initialization variable that has an associated default-value-expr, then the default-value-expr expression is evaluated to obtain a value for the variable. A default-value-expr is only evaluated when an argument is not provided for its variable. The environment of default-value-expr includes all of the initialization variables, all of the fields, and all of the methods of the class. If multiple default-value-exprs are evaluated, they are evaluated from left to right. Object creation and field initialization are described in detail in Creating Objects.

If an initialization variable has no default-value-expr, then the object creation or superclass initialization call must supply an argument for the variable, otherwise the exn:fail:object exception is raised.

Initialization arguments can be provided by name or by position. The external name of an initialization variable can be used with instantiate or with the superclass initialization form. Those forms also accept by-position arguments. The make-object procedure and the superclass initialization procedure accept only by-position arguments.

Arguments provided by position are converted into by-name arguments using the order of init and init-field clauses and the order of variables within each clause. When an instantiate form provides both by-position and by-name arguments, the converted arguments are placed before by-name arguments. (The order can be significant; see also Creating Objects.)

Unless a class contains an init-rest clause, when the number of by-position arguments exceeds the number of declared initialization variables, the order of variables in the superclass (and so on, up the superclass chain) determines the by-name conversion.

If a class expression contains an init-rest clause, there must be only one, and it must be last. If it declares a variable, then the variable receives extra by-position initialization arguments as a list (similar to a dotted “rest argument” in a procedure). An init-rest variable can receive by-position initialization arguments that are left over from a by-name conversion for a derived class. When a derived class’s superclass initialization provides even more by-position arguments, they are prefixed onto the by-position arguments accumulated so far.

If too few or too many by-position initialization arguments are provided to an object creation or superclass initialization, then the exn:fail:object exception is raised. Similarly, if extra by-position arguments are provided to a class with an init-rest clause, the exn:fail:object exception is raised.

Unused (by-name) arguments are to be propagated to the superclass, as described in Creating Objects. Multiple initialization arguments can use the same name if the class derivation contains multiple declarations (in different classes) of initialization variables with the name. See Creating Objects for further details.

See also Internal and External Names for information about internal and external names.

6.2.2 Fields

Each field, init-field, and non-method define-values clause in a class declares one or more new fields for the class. Fields declared with field or init-field are public. Public fields can be accessed and mutated by subclasses using inherit-field. Public fields are also accessible outside the class via class-field-accessor and mutable via class-field-mutator (see Field and Method Access). Fields declared with define-values are accessible only within the class.

A field declared with init-field is both a public field and an initialization variable. See Initialization Variables for information about initialization variables.

An inherit-field declaration makes a public field defined by a superclass directly accessible in the class expression. If the indicated field is not defined in the superclass, the exn:fail:object exception is raised when the class expression is evaluated. Every field in a superclass is present in a derived class, even if it is not declared with inherit-field in the derived class. The inherit-field clause does not control inheritance, but merely controls lexical scope within a class expression.

When an object is first created, all of its fields have the #<undefined> value (see Void). The fields of a class are initialized at the same time that the class’s initialization expressions are evaluated; see Creating Objects for more information.

See also Internal and External Names for information about internal and external names.

6.2.3 Methods

6.2.3.1 Method Definitions

Each public, override, augment, pubment, overment, augride, public-final, override-final, augment-final, and private clause in a class declares one or more method names. Each method name must have a corresponding method-definition. The order of public, etc., clauses and their corresponding definitions (among themselves, and with respect to other clauses in the class) does not matter.

As shown in the grammar for class*, a method definition is syntactically restricted to certain procedure forms, as defined by the grammar for method-procedure; in the last two forms of method-procedure, the body id must be one of the ids bound by let-values or letrec-values. A method-procedure expression is not evaluated directly. Instead, for each method, a class-specific method procedure is created; it takes an initial object argument, in addition to the arguments the procedure would accept if the method-procedure expression were evaluated directly. The body of the procedure is transformed to access methods and fields through the object argument.

A method declared with public, pubment, or public-final introduces a new method into a class. The method must not be present already in the superclass, otherwise the exn:fail:object exception is raised when the class expression is evaluated. A method declared with public can be overridden in a subclass that uses override, overment, or override-final. A method declared with pubment can be augmented in a subclass that uses augment, augride, or augment-final. A method declared with public-final cannot be overridden or augmented in a subclass.

A method declared with override, overment, or override-final overrides a definition already present in the superclass. If the method is not already present, the exn:fail:object exception is raised when the class expression is evaluated. A method declared with override can be overridden again in a subclass that uses override, overment, or override-final. A method declared with overment can be augmented in a subclass that uses augment, augride, or augment-final. A method declared with override-final cannot be overridden further or augmented in a subclass.

A method declared with augment, augride, or augment-final augments a definition already present in the superclass. If the method is not already present, the exn:fail:object exception is raised when the class expression is evaluated. A method declared with augment can be augmented further in a subclass that uses augment, augride, or augment-final. A method declared with augride can be overridden in a subclass that uses override, overment, or override-final. (Such an override merely replaces the augmentation, not the method that is augmented.) A method declared with augment-final cannot be overridden or augmented further in a subclass.

A method declared with private is not accessible outside the class expression, cannot be overridden, and never overrides a method in the superclass.

When a method is declared with override, overment, or override-final, then the superclass implementation of the method can be called using super form.

When a method is declared with pubment, augment, or overment, then a subclass augmenting method can be called using the inner form. The only difference between public-final and pubment without a corresponding inner is that public-final prevents the declaration of augmenting methods that would be ignored.

A method declared with abstract must be declared without an implementation. Subclasses may implement abstract methods via the override, overment, or override-final forms. Any class that contains or inherits any abstract methods is considered abstract and cannot be instantiated.

The second form is analogous to using apply with a procedure; the arg-list-expr must not be a parenthesized expression.

The second form is analogous to using apply with a procedure; the arg-list-expr must not be a parenthesized expression.

6.2.3.2 Inherited and Superclass Methods

Each inherit, inherit/super, inherit/inner, rename-super, and rename-inner clause declares one or more methods that are defined in the class, but must be present in the superclass. The rename-super and rename-inner declarations are rarely used, since inherit/super and inherit/inner provide the same access. Also, superclass and augmenting methods are typically accessed through super and inner in a class that also declares the methods, instead of through inherit/super, inherit/inner, rename-super, or rename-inner.

Method names declared with inherit, inherit/super, or inherit/inner access overriding declarations, if any, at run time. Method names declared with inherit/super can also be used with the super form to access the superclass implementation, and method names declared with inherit/inner can also be used with the inner form to access an augmenting method, if any.

Method names declared with rename-super always access the superclass’s implementation at run-time. Methods declared with rename-inner access a subclass’s augmenting method, if any, and must be called with the form

(id (lambda () default-expr) arg ...)

so that a default-expr is available to evaluate when no augmenting method is available. In such a form, lambda is a literal identifier to separate the default-expr from the arg. When an augmenting method is available, it receives the results of the arg expressions as arguments.

Methods that are present in the superclass but not declared with inherit, inherit/super, or inherit/inner or rename-super are not directly accessible in the class (though they can be called with send). Every public method in a superclass is present in a derived class, even if it is not declared with inherit in the derived class; the inherit clause does not control inheritance, but merely controls lexical scope within a class expression.

If a method declared with inherit, inherit/super, inherit/inner, rename-super, or rename-inner is not present in the superclass, the exn:fail:object exception is raised when the class expression is evaluated.

6.2.3.3 Internal and External Names

Each method declared with public, override, augment, pubment, overment, augride, public-final, override-final, augment-final, inherit, inherit/super, inherit/inner, rename-super, and rename-inner can have separate internal and external names when (internal-id external-id) is used for declaring the method. The internal name is used to access the method directly within the class expression (including within super or inner forms), while the external name is used with send and generic (see Field and Method Access). If a single id is provided for a method declaration, the identifier is used for both the internal and external names.

Method inheritance, overriding, and augmentation are based on external names only. Separate internal and external names are required for rename-super and rename-inner (for historical reasons, mainly).

Each init, init-field, field, or inherit-field variable similarly has an internal and an external name. The internal name is used within the class to access the variable, while the external name is used outside the class when providing initialization arguments (e.g., to instantiate), inheriting a field, or accessing a field externally (e.g., with class-field-accessor). As for methods, when inheriting a field with inherit-field, the external name is matched to an external field name in the superclass, while the internal name is bound in the class expression.

A single identifier can be used as an internal identifier and an external identifier, and it is possible to use the same identifier as internal and external identifiers for different bindings. Furthermore, within a single class, a single name can be used as an external method name, an external field name, and an external initialization argument name. Overall, each internal identifier must be distinct from all other internal identifiers, each external method name must be distinct from all other method names, each external field name must be distinct from all other field names, and each initialization argument name must be distinct from all other initialization argument names.

By default, external names have no lexical scope, which means, for example, that an external method name matches the same syntactic symbol in all uses of send. The define-local-member-name and define-member-name forms introduce scoped external names.

When a class expression is compiled, identifiers used in place of external names must be symbolically distinct (when the corresponding external names are required to be distinct), otherwise a syntax error is reported. When no external name is bound by define-member-name, then the actual external names are guaranteed to be distinct when class expression is evaluated. When any external name is bound by define-member-name, the exn:fail:object exception is raised by class if the actual external names are not distinct.

The binding introduced by define-local-member-name is a syntax binding that can be exported and imported with modules. Each evaluation of a define-local-member-name declaration generates a distinct hidden name (except as a top-level definition). The interface->method-names procedure does not expose hidden names.