►The Racket Reference
▼5 Classes and Objects
►5.4 Field and Method Access
On this page:
5.4.1 Methods
send
send/ apply
send*
with-method
5.4.2 Fields
get-field
set-field!
field-bound?
class-field-accessor
class-field-mutator
5.4.3 Generics
generic
send-generic
make-generic
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5.4 Field and Method Access

In expressions within a class definition, the initialization variables, fields, and methods of the class are all part of the environment. Within a method body, only the fields and other methods of the class can be referenced; a reference to any other class-introduced identifier is a syntax error. Elsewhere within the class, all class-introduced identifiers are available, and fields and initialization variables can be mutated with set!.

5.4.1 Methods

Method names within a class can only be used in the procedure position of an application expression; any other use is a syntax error.

To allow methods to be applied to lists of arguments, a method application can have the following form:

(method-id arg ... . arg-list-expr)

This form calls the method in a way analogous to (apply method-id arg ... arg-list-expr). The arg-list-expr must not be a parenthesized expression.

Methods are called from outside a class with the send and send/apply forms.

(send obj-expr method-id arg ...)
(send obj-expr method-id arg ... . arg-list-expr)
Evaluates obj-expr to obtain an object, and calls the method with (external) name method-id on the object, providing the arg results as arguments. Each arg is as for #%app: either arg-expr or keyword arg-expr. In the second form, arg-list-expr cannot be a parenthesized expression.

If obj-expr does not produce an object, the exn:fail:contract exception is raised. If the object has no public method named method-id, the exn:fail:object exception is raised.

(send/apply obj-expr method-id arg ... arg-list-expr)
Like the dotted form of send, but arg-list-expr can be any expression.

(send* obj-expr msg ...)
 
msg = (method-id arg ...)
  | (method-id arg ... . arg-list-expr)
Calls multiple methods (in order) of the same object. Each msg corresponds to a use of send.

For example,

(send* edit (begin-edit-sequence)
            (insert "Hello")
            (insert #\newline)
            (end-edit-sequence))

is the same as

(let ([o edit])
  (send o begin-edit-sequence)
  (send o insert "Hello")
  (send o insert #\newline)
  (send o end-edit-sequence))

(with-method ((id (obj-expr method-id)) ...)
  body ...+)
Extracts methods from an object and binds a local name that can be applied directly (in the same way as declared methods within a class) for each method. Each obj-expr must produce an object, which must have a public method named by the corresponding method-id. The corresponding id is bound so that it can be applied directly (see Methods).

Example:

(let ([s (new stack%)])
  (with-method ([push (s push!)]
                [pop (s pop!)])
    (push 10)
    (push 9)
    (pop)))

is the same as

(let ([s (new stack%)])
  (send s push! 10)
  (send s push! 9)
  (send s pop!))
5.4.2 Fields

(get-field id obj-expr)
Extracts the field with (external) name id from the value of obj-expr.

If obj-expr does not produce an object, the exn:fail:contract exception is raised. If the object has no id method, the exn:fail:object exception is raised.

(set-field! id obj-expr expr)
Sets the field with (external) name id from the value of obj-expr to the value of expr.

If obj-expr does not produce an object, the exn:fail:contract exception is raised. If the object has no id method, the exn:fail:object exception is raised.

(field-bound? id obj-expr)
Produces #t if the object result of obj-expr has a field with (external) name id, #f otherwise.

If obj-expr does not produce an object, the exn:fail:contract exception is raised.

(class-field-accessor class-expr field-id)
Returns an accessor procedure that takes an instance of the class produced by class-expr and returns the value of the object’s field with (external) name field-id.

If class-expr does not produce a class, the exn:fail:contract exception is raised. If the class has no field-id field, the exn:fail:object exception is raised.

(class-field-mutator class-expr field-id)
Returns a mutator procedure that takes an instance of the class produced by class-expr and a value, and sets the value of the object’s field with (external) name field-id to the given value. The result is #<void>.

If class-expr does not produce a class, the exn:fail:contract exception is raised. If the class has no field-id field, the exn:fail:object exception is raised.

5.4.3 Generics

A generic can be used instead of a method name to avoid the cost of relocating a method by name within a class.

(generic class-or-interface-expr id)
Produces a generic that works on instances of the class or interface produced by class-or-interface-expr (or an instance of a class/interface derived from class-or-interface) to call the method with (external) name id.

If class-or-interface-expr does not produce a class or interface, the exn:fail:contract exception is raised. If the resulting class or interface does not contain a method named id, the exn:fail:object exception is raised.

(send-generic obj-expr generic-expr arg ...)
(send-generic obj-expr generic-expr arg ... . arg-list-expr)
Calls a method of the object produced by obj-expr as indicated by the generic produced by generic-expr. Each arg is as for #%app: either arg-expr or keyword arg-expr. The second form is analogous to calling a procedure with apply, where arg-list-expr is not a parenthesized expression.

If obj-expr does not produce a object, or if generic-expr does not produce a generic, the exn:fail:contract exception is raised. If the result of obj-expr is not an instance of the class or interface encapsulated by the result of generic-expr, the exn:fail:object exception is raised.

(make-generic type method-name) → generic?
  type : (or/c class? interface?)
  method-name : symbol?
Like the generic form, but as a procedure that accepts a symbolic method name.

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