Bigloo's object system is designed to be as simple as possible and
belongs to the
Clos [Bobrow et al. 88] object system family
in that it uses
classes,
generic functions and
methods. Its design has been strongly influenced by C. Queinnec's
Meroon [Queinnec93] It does not include any meta object protocol.
Classes are defined in a module declaration. A class declaration can
take place in a compiled or interpreted module. If a class declaration
takes place in a static module clause (see Section
Module Declaration) its scope is limited to the current module but if it
takes place in an export module clause, its scope is extended to all
modules that import the current module. The syntax of a class
declaration is:
| class ident field ... | bigloo module clause |
<class> ==> (class <ident> <constructor>? <field>+)
| (final-class <ident> <constructor>? <field>+)
| (wide-class <ident> <constructor>? <field>+)
| (abstract-class <ident> <constructor>? <field>+)
<constructor> ==> ( <expr> )
<field> ==> <ident>
| (<ident> <field-prop>)
<field-prop> ==> read-only
| (get <bigloo-exp>)
| (set <bigloo-exp>)
| (default <bigloo-exp>)
| (info <bigloo-exp>)
|
A class is a Bigloo type (see Section Atomic types) and the class
identifier is extracted from the <ident> of the class definition. If
<ident> is also a <typed-ident>, the type part of this identifier
denote the super-class of the class. If <ident> is a
<Ieee-ident>, the super-class of the class is the root of the
inheritance tree, the object class. This object class is
the only pre-existing class.
Final classes can only be sub-classed by wide classes.
Wide classes (only for compiled modules) can only inherit from final classes.
abstract classes can't be instantiated.
Wide-classes cannot be defined within the interpreter.
The optional constructor is an expression that must evaluate to a one
argument function. This function is automatically invoked each time
a new class instance is created. The constructor is passed the fresh
instance. If a class has not defined a constructor the super class'
constructors are searched. The first constructor found is invoked.
A constructor may be a generic function with a method specified
for one or more classes.
A class field may be a typed class field which is achieved
by using a <typed-ident> instead of a <Ieee-ident>
for the <ident> value.
Field marked with read-only declaration are immutables.
Default declarations allow default field values.
For the means of an example, the traditional points and colored points
can be defined as:
(module example
(static (abstract-class pt)
(class point::pt
x::double
y::double)
(class point-C::point
(color::string read-only))))
|
We illustrate final and wide classes by the example:
(module example
(export (final-class person
(name::string (default "Jones"))
(sex read-only)
children::pair-nil)
(wide-class married-person::person
mate::person)))
|
Fields may be virtual. A field is virtual as soon as its declaration
contain a get attribute. Virtual fields have no
physical implementation within the instance. When defining a virtual
field, the class declaration implements a getter and a setter
if that field is not a read only field. Access to the virtual field will
rely on invocation of the user getter and user setter. For
instance:
(module example
(static (class complex
mag::double
angle::double
(real::double (get (lambda (p)
(with-access::complex p (mag angle)
(* mag (cos angle)))))
read-only)
(imag::double (get (lambda (p)
(with-access::complex p (mag angle)
(* mag (sin angle)))))
read-only))))
(let ((p (instantiate::complex (mag 1.0) (angle 2.18))))
(with-access::complex p (real imag)
(print "real: " real)
(print "imag: " imag)))
|
Virtual fields cannot be associated default values. If a virtual field
is not provided with a setter it must be annotated as read only.
Info declarations allow arbitrary user information field values.
This value can be retrieved by introspection, by the means of the
class-field-info introspection function.
For the means of an example, with add to information to the slot of
the point class.
(module example
(static (class point
(x::double (info '(range 0.0 10.0)))
(y::double (info '(range -1.0 1.0)))))
|
|
10.2 Creating and accessing objects
|
Objects and classes are created and manipulated via library functions
and forms created automatically by Bigloo when a new class is defined.
| isa? obj class | bigloo procedure |
This function returns #t if obj is an instance
of class or an instance of a sub-class of class, otherwise,
it returns #f.
|
instantiate::class (ident value)... | bigloo syntax |
This forms allocates object of class class and fills the fields with
values found in the list of parameters (note that field are explicitly
named and that there is no ordering for field naming). Field values which
are not provided in the parameter list must have been declared with a
default value which is used to initialize the corresponding
field.
For instance:
(module example
(export
(class point (x (default 0)))
(class point2d::point y)))
(instantiate::point (x 0) (y 0))
(instantiate::point (y 0))
(instantiate::point (x 0))
=> Error because y has no default value
|
|
| class-nil class | bigloo procedure |
This function returns the NIL pre-existing class instance. This instance plays
the role of void * in C or null in Java. The value of each field
is unspecified but correct with respect to the Bigloo type system. Each
call to class-nileq?).
(module example
(export
(class point x)
(class point2d::point y)))
(eq? (class-nil point) (class-nil point))
=> #t
(eq? (class-nil point) (class-nil point2d))
=> #f
|
|
with-access::class obj (binding...) body | bigloo syntax |
A reference to any of the variables defined in as a binding is
replaced by the appropriate field access form. This is true for both
reference and assignment. A binding is either a symbol or a list
of two symbols. In the first place, it denotes a field. In the second
case, it denotes an aliases field.
For instance:
(with-access::point p (x (y1 y))
(with-access::point p2 (y)
(set! x (- x))
(set! y1 (- y1 y))))
|
|
-> var field | bigloo syntax |
Class instances can be accesses using the -> special form. The
the first argument must be the identifier of a local typed variable, otherwise
an error is raised. The form -> can be used to get or set value of
an instance field. For instance:
(define (example p1::point p2::point)
(set! (-> p1 x) (- (-> p1 x)))
(set! (-> p1 y) (- (-> p1 y) (-> p2 y))))
|
This is equivalent to:
(define (example p1::point p2::point)
(with-access::point p1 (x (y1 y))
(with-access::point p2 (y)
(set! x (- x))
(set! y1 (- y1 y)))))
|
|
co-instantiate ((var value) ...) body | bigloo syntax |
This form is only available from compiled modules. In other words, it
is not available from the interpreter. It permits the creation of
recursive instances. It is specially useful for creating instances for
which class declarations contain cyclic type references (for instance
a class c1 for a which a field is declared of class c2
and a class c2 for which a class is declared of type
c1). The syntax of a co-instantiate form is similar to a
let form. However the only legal values are
instantiate forms. The variables introduced in the binding of a
co-instantiate form are bound in body. In addition, they
are partially bound in the values expressions. In a
value position, a variable var can only be used to set the
value of a field of an instantiated class. It cannot be used in any
calculus. Example:
(module obj-example
(export (class c1 a b o2::c2)
(class c2 x y o1::c1)))
(co-instantiate ((o1 (instantiate::c1
(a 10)
(b 20)
(o2 o2)))
(o2 (instantiate::c2
(x 10)
(y 20)
(o1 o1))))
(with-access::c1 o1 (o2)
(with-access::c2 o2 (x y)
(+ x y))))
=> 30
|
|
duplicate::class obj (ident value)... | bigloo syntax |
This forms allocates an instance of class class. The field
values of the new object are picked up from the field values of
the old object unless they are explicitly given in the parameter
list.
For instance:
(with-access::point old (x)
(instantiate::point
(x x)
(y 10)))
|
is equivalent to:
(duplicate::point old (y 10))
|
The duplicated instance might be a subclass of duplicated instance. Example:
(module foo
(export (class point x y)
(class point3::point z)
(class point4::point3 t))
(main main))
(define (main x)
(let ((p (instantiate::point (x 1) (y 2))))
(print (duplicate::point4 p (z 6) (t 55)))))
|
|
A generic function is a bag of specific functions known as methods. When
invoked on a Bigloo object, a generic function determines the class of the
discriminating variable (corresponding to the first argument of the generic
function) and invokes the appropriate method. Generic functions implement
single inheritance and each is defined using the
define-generic
Bigloo syntax.
| define-generic (name arg...) default-body | bigloo syntax |
A generic function can be defined with a default body which will
be evaluated if no method can be found for the discriminating
variable. The default default-body signals an error.
|
As an example, here is a possible definition of the
object-display
generic function:
(define-generic (object-display obj::object . op)
(let ((port (if (pair? op)
(car op)
(current-output-port))))
(display "#\|" port)
(display (class-name (object-class obj)) port)
(display "\|" port)))
|
Methods can be defined to specialize a generic function and such
methods must have a
compatible variable list. That is, the first argument of the
method must be a sub-type (i.e. belong to a sub-class) of the
first argument of the generic function. Other formal parameters
must be of same types. Moreover, the result type of the method
must be a sub-type of the result of the generic function.
| define-method (name arg...) body | bigloo syntax |
| call-next-method | bigloo syntax |
If there is no appropriate method, an error is signaled.
Methods can use the form (call-next-method) to invoke the method
that would have been called if not present. The (call-next-method)
cannot be used out of method definition.
example:
(define-method (object-display p::person . op)
(let ((port (if (pair? op)
(car op)
(current-output-port))))
(fprint port "firstname : " (-> p fname))
(fprint port "name : " (-> p name))
(fprint port "sex : " (-> p sex))
p))
|
|
10.4 Widening and shrinking
|
Bigloo introduces a new kind of inheritance:
widening. This allows an
object to be temporarily
widened (that is transformed into an object
of another class, a
wide-class) and then
shrink-ed (that is
reshaped to its original class). This mechanism is very useful for
implementing short-term data storage. For instance, Bigloo compilation
passes are implemented using the
widening/shrinking mechanism. On
entry to a pass, objects are widened with the specific pass fields and, on
exit from a pass, objects are shrunk in order to forget the information
related to this pass.
Only instances of
final classes can be widened and objects can
only be widened in order to become instances of
wide classes.
Widening is performed by the
widen! syntax:
widen!::wide-class obj (id value) ... | bigloo syntax |
The object obj is widened to be instance of the wide class
wide-class. Fields values are either picked up from the
parameter list of the widen! form or
from the default values in the declaration of the wide class.
|
Objects are shrunk using the
shrink! syntax:
Here is a first example:
(module example
(static (final-class point
(x (default 0))
(y (default 0)))
(wide-class named-point::point name)))
(define *point* (instantiate::point))
|
Two classes have been declared and an instance
*point* of
point has been allocated. For now,
*point* is an instance
of
point but not an instance of
named-point and this can
be checked by:
(print (isa? *point* named)) ==> #t
(print (isa? *point* named-point)) ==> #f
|
Now, we
widen *point*...
(let ((n-point (widen!::named-point *point*
(name "orig"))))
|
And we check that now,
n-point is an instance of
named-point. Since
named-point is a subclass of
point,
n-point still is an instance of
point.
(print (isa? n-point named-point)) ==> #t
(print (isa? n-point named)) ==> #t
|
Widening affects the objects themselves. It does not operate any
copy operation. Hence,
*point* and
n-point are
eq?.
(print (eq? n-point *point*)) ==> #t
|
To end this example, we
shrink n-point and check
its class.
(shrink! n-point)
(print (isa? *point* named-point))) ==> #f
|
Here is a more complex example:
We illustrate widening and shrinking using our ``wedding simulator''.
First let us define three classes,
person (for man and woman),
married-woman and
married-man:
(module wedding
(static (final-class person
name::string
fname::string
(sex::symbol read-only))
(wide-class married-man::person
mate::person)
(wide-class married-woman::person
maiden-name::string
mate::person)))
|
As we can see people are allowed to change their name but not their sex.
The identity of a person can be printed as
(define-method (object-display p::person . op)
(with-access::person p (name fname sex)
(print "firstname : " fname)
(print "name : " name)
(print "sex : " sex)
p))
|
A married woman's identity is printed by (we suppose an equivalent method
definition for married-man)
(define-method (object-display p::married-woman . op)
(with-access::married-woman p (name fname sex mate)
(call-next-method)
(print "married to: " mate)
p))
|
We create a person with the
birth function:
(define (birth name::string fname::string sex)
[assert (sex) (memq sex '(male female))]
(instantiate::person
(name name)
(fname fname)
(sex sex)))
|
We celebrate a wedding using the
get-married! function:
(define (get-married! woman::person man::person)
(if (not (and (eq? (-> woman sex) 'female)
(eq? (-> man sex) 'male)))
(error "get-married"
"Illegal wedding"
(cons woman man))
(let* ((mname (-> woman name))
(wife (widen!::married-woman woman
(maiden-name mname)
(mate man))))
(person-name-set! wife (-> man name))
(widen!::married-man man
(mate woman)))))
|
We can check if two people are married by
(define (couple? woman::person man::person)
(and (isa? woman married-woman)
(isa? man married-man)
(eq? (with-access::married-woman woman (mate) mate) man)
(eq? (with-access::married-man man (mate) mate) woman)))
|
Now let us study the life a
Junior Jones and
Pamela Smith. Once upon a time...
(define *junior* (birth "Jones" "Junior" 'male))
(define *pamela* (birth "Smith" "Pamela" 'female))
|
Later on, they met each other and ... they got married:
(define *old-boy-junior* *junior*)
(define *old-girl-pamela* *pamela*)
(get-married! *pamela* *junior*)
|
This union can be checked:
(couple? *pamela* *junior*)
=> #t
|
We can look at the new identity of
*pamela*
(print *pamela*)
-| name : Jones
firstname : Pamela
sex : FEMALE
married to: Junior Jones
|
But
*pamela* and
*junior* still are the same persons:
(print (eq? *old-boy-junior* *junior*)) => #t
(print (eq? *old-girl-pamela* *pamela*)) => #t
|
Unfortunately all days are not happy days. After having been married
*pamela* and
*junior* have divorced:
(define (divorce! woman::person man::person)
(if (not (couple? woman man))
(error "divorce!"
"Illegal divorce"
(cons woman man))
(with-access::married-woman woman (maiden-name)
(begin
(shrink! woman)
(set! (-> woman name) maiden-name))
(shrink! man))))
(divorce! *pamela* *junior*)
|
We can look at the new identity of
*pamela*
(print *pamela*)
-| name : Smith
firstname : Pamela
sex : FEMALE
|
And
*pamela* and
*junior* still are the same persons:
(print (eq? *old-boy-junior* *junior*)) => #t
(print (eq? *old-girl-pamela* *pamela*)) => #t
|
No type denotes Bigloo's classes. These objects are handled
by the following library functions:
| find-class symbol | bigloo procedure |
Returns, if any, the class named symbol.
|
| class? obj | bigloo procedure |
Returns #t if and only if obj is a class.
|
| class-super class | bigloo procedure |
Returns the super-class of class.
|
| class-subclasses class | bigloo procedure |
Returns the subclasses of class.
|
| class-name class | bigloo procedure |
Returns the name (a symbol) of class.
|
| object-constructor class | bigloo procedure |
Returns class's constructor.
|
| object-class object | bigloo procedure |
Returns the class that object belongs to.
|
| wide-object? object | bigloo procedure |
Returns #t if object is a wide object otherwise it returns
#f.
|
| object-display object [port] | bigloo generic |
This generic function is invoked by display to display objects.
|
| object-write object [port] | bigloo generic |
This generic function is invoked by write to write objects.
|
| object->struct object | bigloo generic |
| struct->object struct | bigloo procedure |
These functions converts objects into Scheme structures and vice-versa.
|
| object-equal? object obj | bigloo generic |
This generic function is invoked by equal? when the first argument
is an instance of object.
|
| object-hashnumber object | bigloo generic |
This generic function returns an hash number of object.
|
| is-a? obj class | bigloo procedure |
Returns #t if obj belongs to class otherwise it
returns #f.
|
10.6 Object serialization
|
Objects can be
serialized and
un-serialized using
the regular
string->obj and
obj->string
functions. Objects can be stored on disk and restored from disk
by the use of the
output-obj and
input-obj
functions.
In addition to this standard serialization mechanism, custom object
serializers and un-serializers can be specified by the means of the
register-class-serialization! function (see Section
Serialization.
Two objects can be compared with the
equal? function. Two object
are equal if and only if they belong to a same class, all their field
values are equal and all their super class's field values are equal.
Bigloo provides the programmer with some object introspection facilities.
See section see
Object library for information on classes and
objects handling. Introspection facilities are, by default, available
for all classes. However, in order to shrink the code size generation,
it may be useful to disable class introspection. This decision can be
taken on a per class basis (i.e., one class may be provided with
introspection facilities while another one is not). The compiler
option
-fno-reflection
(see Chapter
Compiler Description) prevents the
compiler to generate the code required for introspecting the classes
defined in the compiled module.
| class-fields class | bigloo procedure |
Returns the a description of the fields of class. This description
is a list of field descriptions where each field description can be accessed by
the means of the following library functions. The fields are those
directly defined in class. That is class-fields does not
return fields defined in super classes of class.
|
| class-all-fields class | bigloo procedure |
Returns the a description of the fields of class. This description
is a list of field descriptions where each field description can be accessed by
the means of the following library functions. By contrast with
class-fields, this function returns fields that are also defined in
the super classes of class.
in th
|
| find-class-field class symbol | bigloo procedure |
Returns the field named symbol from class class. Returns
#f is such a field does not exist.
|
| class-field? obj | bigloo procedure |
Returns #t if obj is a class field descriptor. Otherwise returns #f.
|
| class-field-name field | bigloo procedure |
Returns the name of the field. The name is a symbol.
|
| class-field-accessor field | bigloo procedure |
Returns a procedure of one argument. Applying this function to an object
returns the value of the field described by field.
|
| class-field-mutable? field | bigloo procedure |
Returns #t if the described field is mutable and #f otherwise.
|
| class-field-mutator field | bigloo procedure |
Returns a procedure of two arguments. Applying this function to an object
changes the value of the field described by field. It is an
error to apply class-field-mutator to an immutable field.
|
| class-field-info field | bigloo procedure |
Returns the information associated to field (this the class declaration
info attribute).
|
For means of an example, here is a possible implementation of the
equal? test for objects:
(define (object-equal? obj1 obj2)
(define (class-field-equal? fd)
(let ((get-value (class-field-accessor fd)))
(equal? (get-value obj1) (get-value obj2))))
(let ((class1 (object-class obj1))
(class2 (object-class obj2)))
(cond
((not (eq? class1 class2))
#f)
(else
(let loop ((fields (class-fields class1))
(class class1))
(cond
((null? fields)
(let ((super (class-super class)))
(if (class? super)
(loop (class-fields super)
super)
#t)))
((class-field-equal? (car fields))
(loop (cdr fields) class))
(else
#f)))))))
|
| class-creator class | bigloo procedure |
Returns the creator for class. The creator is a function for which
the arity depends on the number of slots the class provides
(see Section see Creating and accessing objects).
When an instance is allocated by the means of the class-creator, as
for direct instantiation, the class constructor is
automatically invoked.
Example:
(module foo
(main main)
(static (class c1 (c1-constructor))))
(define c1-constructor
(let ((count 0))
(lambda (inst)
(set! count (+ 1 count))
(print "creating instance: " count)
inst)))
(define (main argv)
(let ((o1 (instantiate::c1))
(o2 (instantiate::c1))
(o3 ((class-creator c1))))
'done))
-| creating instance: 1
creating instance: 2
creating instance: 3
|
|
| class-predicate class | bigloo procedure |
Returns the predicate for class. This predicate returns #t
when applied to object of type class. It returns #f otherwise.
|