►The Racket Reference
▼5 Classes and Objects
►5.4 Field and Method Access
On this page:
5.4.1 Methods
send
send/ apply
send/ keyword-apply
dynamic-send
send*
send+
with-method
5.4.2 Fields
get-field
dynamic-get-field
set-field!
dynamic-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 used 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, send/apply, and send/keyword-apply forms.

syntax

(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.

syntax

(send/apply obj-expr method-id arg ... arg-list-expr)

Like the dotted form of send, but arg-list-expr can be any expression.

syntax

(send/keyword-apply obj-expr method-id
                    keyword-list-expr value-list-expr
                    arg ... arg-list-expr)
Like send/apply, but with expressions for keyword and argument lists like keyword-apply.

procedure

(dynamic-send obj    
  method-name    
  v ...    
  #:<kw> kw-arg ...) → any
  obj : object?
  method-name : symbol?
  v : any/c
  kw-arg : any/c
Calls the method on obj whose name matches method-name, passing along all given vs and kw-args.

syntax

(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))

syntax

(send+ obj-expr msg ...)

 
msg = (method-id arg ...)
  | (method-id arg ... . arg-list-expr)
Calls methods (in order) starting with the object produced by obj-expr. Each method call will be invoked on the result of the last method call, which is expected to be an object. Each msg corresponds to a use of send.

This is the functional analogue of send*.

Examples:

(define point%
  (class object%
    (super-new)
    (init-field [x 0] [y 0])
    (define/public (move-x dx)
      (new this% [x (+ x dx)]))
    (define/public (move-y dy)
      (new this% [y (+ y dy)]))))
> (send+ (new point%)
         (move-x 5)
         (move-y 7)
         (move-x 12))

(object:point% ...)

syntax

(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

syntax

(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 field, the exn:fail:object exception is raised.

procedure

(dynamic-get-field field-name obj) → any/c

  field-name : symbol?
  obj : object?
Extracts the field from obj with the (external) name that matches field-name. If the object has no field matching field-name, the exn:fail:object exception is raised.

syntax

(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 field, the exn:fail:object exception is raised.

procedure

(dynamic-set-field! field-name obj v) → void?

  field-name : symbol?
  obj : object?
  v : any/c
Sets the field from obj with the (external) name that matches field-name to v. If the object has no field matching field-name, the exn:fail:object exception is raised.

syntax

(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.

syntax

(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.

syntax

(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.

syntax

(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.

syntax

(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.

procedure

(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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