Generated: Wednesday, July 11, 2012, 10:53:44 A SchemeDoc Manual

Abstraction Step Language - ASL 2.1

Kurt NÝrmark Aalborg University, Denmark

Source file: y:/Kurt/Files/projects/p2025/abstraction-step-language/ASL2-1/man/../asl.scm

This is SchemeDoc documentation of the constituents of the ASL2 experimental programming language, version 2.1. The ASL2 programming language is embedded in R5RS Scheme. ASL2 is created to explore an extreme variant of object-oriented programming, which starts with concrete objects (created to solve a particular problem) and ends with a traditional class hierarchy. Throughout the development process the programmer carries out a sequence of Abstraction Steps. Version 2 of ASL - ASL2 - accompanies a paper titled 'Object-oriented Programming with Gradual Abstraction'. In version 2.1 a few renamings have been done to allow for more smooth execution in some R5RS Scheme systems. Specifically, new is renamed to new-object, this is renamed to this-object, Object is renamed to root-object, and Class is renamed to class-object.

Table of Contents:
1. ASL object and class Primitives. 4. Function objects. 7. Generation of class forms.
2. ASL Predicates. 5. Expressions.
3. Object inspection. 6. General ASL2 Standard Objects.

Alphabetic index:
add-member! (add-member! object field-name field-value) Add a new member (data or function) to an object by mutation.
apply-function (apply-function function-object actual-parameter-list) Apply the function object on the given actual parmeters.
as-independent-object! (as-independent-object! object) Disconnect an object with class from its class, and transfer the methods from the class to the object.
as-instance-of-class! (as-instance-of-class! object class) Mutate object be to an instance of class.
asl-object? (asl-object? x) Is x an object.
class-object class-object The implicit superclass of all classes.
class-of (class-of obj) Return the class object of object, or #f if obj is not an instance of a class object.
class-source (class-source class-object class-name superclass-name file-path [role]) Generate a string with the class source form on a file.
clone-object (clone-object obj . initializer-fields) Make a shallow copy of obj, and re-assign the field values of the copy as prescribed by initializer-fields
delete-member! (delete-member! object field-name) Delete an existing member (data or method) from an object, by mutation of object.
environment-of-function (environment-of-function function) The environment of a function.
eval-expr (eval-exr expr env) Evaluate the expression expr in the environment env.
expression-of-function (expression-of-function function) The expression of a function.
formal-parameters-of-function (formal-parameters-of-function function) The formal parameters of a function.
function (function formal-parameters body-form ...) Make and return a function object with the given formal parameter list and a given body.
function? (function? x) Is x a function?
generalize (generalize class-list member-names) Return a new class which generalizes all classes in class-list.
get-field (get-field object field-name) Get the value of a named field in an object.
inspect (inspect object [variation]) Prints useful information about object.
instance-of? (instance-of? object class) Is object an instance of class.
kappa (kappa . objects) Infer the class of the objects, and return the object that represents the class.
kappa? (kappa? x) Is x a class object?
make-class (make-class [superclass] [field-name field-value]...) Make a class-object with a given superclass and given fields.
make-object (make-object [object-role-name] [field-name field-value]...) A operation that makes an object with given fields, either as an independent object or as an instance of a class (in case the role asked for is mapped to a class).
new-object (new class-object [field-name field-value]...) Make a new object as an instance of a class, as represented by class-object.
object-role (object-role object) Return the role of object.
object-with-class? (object-with-class? x) Is x an object, which is an instance of some class.
object-without-class? (object-without-class? x) Is x an object, which is not an instance of some class.
root-object root-object The root class in the system.
send (send object selector . actual-parameters) Apply a method in object, as selected by the symbol selector, on the given actual parameters.
send-super (send-super object selector . actual-parameters) Like send, but locates the method 'in a superclass' relative to the receiver.
set-field! (set-field! object field-name field-value) In object, set the value of an existing field named field-name to field-value.
specialize (specialize class . fields) Make and return a specialization of class.
superclass-of (superclass-of class-obj) Provided that class-obj is a class object, return the superclass of class-obj.


1 ASL object and class Primitives.
This section contains the most important object and class related primitives in ASL2. Among these, ASL operations that create objects and classes.

new-object
Form (new class-object [field-name field-value]...)
Description Make a new object as an instance of a class, as represented by class-object. The initial values of the fields are taken from the default field values of the class. The default initial field values can be overwritten (by mutation) done on the basis of the remaining parameters. This primitive represents is concretization step.
Parameters class-object An object of kind class.
field-name The name of an existing field in the class.
field-value The new value of field-name in the new object.
See also Scheme source file new-object
Note In version 2 this primitive was called new.

make-object
Form (make-object [object-role-name] [field-name field-value]...)
Description A operation that makes an object with given fields, either as an independent object or as an instance of a class (in case the role asked for is mapped to a class). The object role name is an optional string, which indicates the user's intention. If the object role name is provided (as string), we organize the objects by their role in an association list keyed by the object role string.
Parameters fields The fields given as a property list.
object-role-name. An informal object role name, such as the perceived type of the object. All objects with the same role name are collected in ... A string.
field-name. A symbol.
field-valued. Of arbitrary type.
Returns An object.
See also Scheme source file make-object
Note field-names prefixed with a dollar character, such as $kind and $superclass, are used for internal book-keeping purposes.

make-class
Form (make-class [superclass] [field-name field-value]...)
Description Make a class-object with a given superclass and given fields. This primtive represents a concretization step rather than an abstraction step, because it makes a more special class from a general class. The value of given fields of in class serve as default field values when the class is instantiated. As an alternative to use of make-class, it is possible to derive a class from an object with use of kappa.
Parameters superclass An existing superclass object. Defaults to root-object.
fields The fields given as a property list. These fields become default values of forthcoming instances. Defaults to the empty property list.
Returns An class object.
See also Scheme source file make-class
Alternative kappa
Sugared variant specialize
Note Internally, a class which does not have a $superclass field is considered a subclass of class root-object. Class root-object is the root class in the system.

kappa
Form (kappa . objects)
Description Infer the class of the objects, and return the object that represents the class. It is possible to pass one or more objects as input to kappa. The field values of the first object become default field values of the inferred class. The first object passed to kappa prescribes the fields which must be present in all other objects. As a side effect, all objects passed to kappa become instances of the class (kappa calls as-instance-of! on all objects). As an additional side effect, if all objects possess a unique object role, this object role is mapped to the new class. (This affects the working of make-object).
Precondition The parameter objects is non-empty. None of the objects represents a class.
Returns The object which represents the inferred class.
See also Scheme source file kappa

add-member!
Form (add-member! object field-name field-value)
Description Add a new member (data or function) to an object by mutation. Can used on both independent objects, instances of classes and class objects. If field-name already exists in object the old field is removed before the new field is added. add-member! has no return value.
Parameters object An object to which we will add a new member
field-name The name of the new member. A symbol.
field-value The value of the new member.
See also Scheme source file add-member!
Note This is a mutating procedure. object is mutated to contain a new field.

delete-member!
Form (delete-member! object field-name)
Description Delete an existing member (data or method) from an object, by mutation of object. Can be used on any kind of object.
Parameters object An object from which to delete a member.
field-name The name of the member to delete. A symbol.
See also Scheme source file delete-member!
non mutating counterpart delete-member
Note This is a mutating procedure which modifies the state of object.

get-field
Form (get-field object field-name)
Description Get the value of a named field in an object. You should only this get-field as a back door accessor. Accessible fields should be accessed via the automatically generated access methods, or lexically (if accessed from inside a method). The function get-field can be used on both independent objects, instances of classes, and class objects. As a special case on instances of classes, take the value from the default value of the class, if it does not exist in the object (first case in the body of this function). If get-field is applied in a function, which is added as a method to an object/class, the special variable this can be passed as first parameter to get-field, in order to read the values of instance variables.
Parameters object An object.
field-name The name of a field (a Scheme symbol).
See also Scheme source file get-field

set-field!
Form (set-field! object field-name field-value)
Description In object, set the value of an existing field named field-name to field-value. You should only use set-field! as a low-level back door facility. Values of accessible fields should be set by the automatically generated setter methods, or internally by using set! on the name of an instance varibale. set-field! has no return value.
Parameters object An object
field-name A name of a field. (A Scheme symbol).
field-value The new value of the field named field-name.
See also Scheme source file set-field!
Note Mutating. Changes the state of an existing object.

send
Form (send object selector . actual-parameters)
Description Apply a method in object, as selected by the symbol selector, on the given actual parameters. A method is a function object, as created by make-function, which has been added as a member to an object or to a class. Take the method from the object if it exists, or from the class of which the object is an instance. Free variables of the function behind the method are bound in the static environment of the function (as provided as part of the function). Access to instance variables (get/set) are provided via the primitives get-field and set-field! The special variable this is fluently bound to object during the act of sending the message.
Parameters object The receiving object
selector A name of a function. A symbol.
actual-parameters A list of actual parameters of the method.
See also Scheme source file send
getter/setter get-field set-field!
similar function send-super

send-super
Form (send-super object selector . actual-parameters)
Description Like send, but locates the method 'in a superclass' relative to the receiver.
Parameters object The receving object
selector A name of a function, selected in the superclass of the class of object.
actual-parameters A list of actual parameters of the method.
See also Scheme source file send-super
similar function send

clone-object
Form (clone-object obj . initializer-fields)
Description Make a shallow copy of obj, and re-assign the field values of the copy as prescribed by initializer-fields
Parameters obj An object.
initializer-fields Initializers represented as a property list.
See also Scheme source file clone-object

as-instance-of-class!
Form (as-instance-of-class! object class)
Description Mutate object be to an instance of class. It is required that all members of the object are present in the class. Methods in the object are removed from the object. It is an error if object does not possess all the fields of class. If object already is registered as an instance of class, then object is just returned.
Parameters object An object
class A class object
See also Scheme source file as-instance-of-class!

as-independent-object!
Form (as-independent-object! object)
Description Disconnect an object with class from its class, and transfer the methods from the class to the object. If objects is without a class, just return object.
See also Scheme source file as-independent-object!

generalize
Form (generalize class-list member-names)
Description Return a new class which generalizes all classes in class-list. This is an abstraction step. Mutate the existing classes to have the new class as their superclass. Take the default values of non-function fields from the first class in the list. Transfer all methods from all classes in the list. It is not necessary that all members enumerated in member-name are members of all classes in class-list. With this semantics, multiple generalization may lead to definition of new members in some of the subclasses. Methods that belong to more than one subclass are assumed to be equal to each other. (This is not - and cannot - be tested).
Parameters class-list A list of class objects.
member-names A list of members (a list of symbols).
See also Scheme source file generalize

specialize
Form (specialize class . fields)
Description Make and return a specialization of class. The class is extended with fields.
Parameters class A class object
fields Fields which extends the set of fields in class. Should be provided as a property list.
Returns The specialized class object.
See also Scheme source file specialize
similar primitive make-class
Internal remark The specialize primitive is really just syntactic suggar of make-class. Notice also that there is no need of a mutating specialize! variant (because the general class does not refer to its specializations).

class-of
Form (class-of obj)
Description Return the class object of object, or #f if obj is not an instance of a class object.
Parameters obj An object.
See also Scheme source file class-of

superclass-of
Form (superclass-of class-obj)
Description Provided that class-obj is a class object, return the superclass of class-obj. If the class does not have an explicit superclass, return Class.
Parameters class-obj A class object
See also Scheme source file superclass-of
root-object root-object

object-role
Form (object-role object)
Description Return the role of object. A string. Some objects may have an empty string as role name. Relies on the particular ASL object representation.
See also Scheme source file object-role


2 ASL Predicates.
A number of predicates which identifies ASL objects and classes relative to other Scheme objects.

asl-object?
Form (asl-object? x)
Description Is x an object.
See also Scheme source file asl-object?

object-without-class?
Form (object-without-class? x)
Description Is x an object, which is not an instance of some class. An non-classified or a weakly classified object.
See also Scheme source file object-without-class?

object-with-class?
Form (object-with-class? x)
Description Is x an object, which is an instance of some class. A strongly classified object.
See also Scheme source file object-with-class?

kappa?
Form (kappa? x)
Description Is x a class object?
See also Scheme source file kappa?

instance-of?
Form (instance-of? object class)
Description Is object an instance of class. True if object has all the fields prescribed by class
Parameters object An object.
class An object of kind class.
See also Scheme source file instance-of?


3 Object inspection.
Functions that can be used to inspect (pretty print) an ASL object.

inspect
Form (inspect object [variation])
Description Prints useful information about object.
Parameters object An object
variation Either full or short (a symbol).
See also Scheme source file inspect


4 Function objects.
Function objects form the basis for methods. When a function object is added to an object or a class, it becomes a method in the object.

function
Form (function formal-parameters body-form ...)
Description Make and return a function object with the given formal parameter list and a given body.
Parameters body An expression or a sequence of statements, represented as S-expressions.
formal-parameters A list of formal parameter names, either a proper list, an improper list, or a symbol. An symbol, or the tail of an improper list is used as a rest parameter (like in Scheme).
See also Scheme source file function
Note This is a syntactic abstraction which calls an underlying constructor function, make-function. Therefore formal-parameters and body are not to be quoted.

function?
Form (function? x)
Description Is x a function?
See also Scheme source file function?

expression-of-function
Form (expression-of-function function)
Description The expression of a function.
See also Scheme source file expression-of-function

formal-parameters-of-function
Form (formal-parameters-of-function function)
Description The formal parameters of a function.
See also Scheme source file formal-parameters-of-function

environment-of-function
Form (environment-of-function function)
Description The environment of a function.
See also Scheme source file environment-of-function

apply-function
Form (apply-function function-object actual-parameter-list)
Description Apply the function object on the given actual parmeters.
See also Scheme source file apply-function


5 Expressions.
Expressions are encapsluated in functions, which in turn form the basis of methods. The method eval-expr is the main function of a simple, meta circular Scheme interpreter. It has been adapted from the book 'The Scheme Programming Language' by R. Kent Dybvig.

eval-expr
Form (eval-exr expr env)
Description Evaluate the expression expr in the environment env. A set of primitive Scheme functions are implicitly found at the rear end of any environment. In case a name is not found in either env, or in the implicit primitive-environment, the name is evaluated in the implementation's interaction environment.
Parameters expr The expression to evaluate. An Scheme S-expression.
env The environment, represented as an association list of symbol/value pairs.
See also Scheme source file eval-expr


6 General ASL2 Standard Objects.

root-object
Form root-object
Description The root class in the system. The only class without a superclass.
See also Scheme source file root-object
Note In version 2 this ASL2 object was called Object.

class-object
Form class-object
Description The implicit superclass of all classes. The superclass of Class is root-object.
See also Scheme source file class-object
Superclass root-object
Note In version 2 this ASL2 object was called Class.


7 Generation of class forms.
The primitive that turns an ASL2 class object into a class description in a source file.

class-source
Form (class-source class-object class-name superclass-name file-path [role])
Description Generate a string with the class source form on a file. If role is provided, generate a mapping for it to the class.
Parameters class-object The class object from which to generate the source form
class-name The name of the class (a string)
superclass-name The name of the superclass (a string), or #f if there is no superclass. Do not attempt to pass Class.
file-path The path to the file where to write the class source form
role The role of the objects, which will be mapped to the class (represented by class-object).
See also Scheme source file class-source

Generated: Wednesday, July 11, 2012, 10:53:44
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