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,
send/apply, and send/keyword-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/keyword-apply obj-expr method-id | | keyword-list-expr value-list-expr | | arg ... arg-list-expr) |
|
Calls the method on obj whose name matches
method-name, passing along all given vs and
kw-args.
| (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
5.4.2 Fields
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.
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.
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.
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.
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.
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.
Like the
generic form, but as a procedure that accepts a
symbolic method name.