Multi-class inheritance

Author: Tung Thanh Le
Contact: ttungl at gmail dot com

Introduction

Used Scheme (Dr. Racket) to modify the interpreter for creating new functions of a language. In this work, multi-class inheritance is created. A new instance generated is inherited to all the methods from the joined classes. Used Scheme language for implementation.

In-Detail

This work extends a new capability of a Classes language in Scheme. It allows a class that can be created from two existing classes and owns its inherited properties (fields & methods) from two existing classes. It also allows to set/get a value of a class’s field.

Grammar

• Program::= {ClassDecl}* {ClassExtendMerge} * expression

• ClassDecl::= class Identifier {field Identifier}* {MethodDecl}*

• MethodDecl::= method Identifier ({Identifier}*(,)) expression

• ClassExtendMerge::= classmerge Identifier Identifier Identifier

• Expression::= setfield identifier identifier identifier

• Expression::= getfield identifier identifier

  Description: classmerge allows merging two existing classes and adding to a third class. This could be added up to n classes and all the methods and fields in n classes are inherited to the class (n+1)^th.

Abstract syntax/ADT

• setfield-exp: Identifier x Identifier x Expression -> Unspecified
• getfield-exp: Identifier x Identifier -> Expval
• Expval = Int + Bool + Proc + listof(expval) + object
• Denval = Ref(expval)

Main modifications:

1. Data-types:
   • Object: add a field-names.
   • Method: eliminate a super-name.
   • Class: replace super-name by prototype, and add the fields value. a-class datatype has the prototype, the field-names and fields value, and method-env.
2. proto-merge function:
   • This allows two existing classes can be merged and be added to a new class by appending fields, field-names, and merging method-env of both classes, and update this to the global class environment the-class-env by using add-to-class-env! with the new class merged.
3. Find-method:
   • Find a method based on the method name in the class’s prototype.
4. Construct-field-names and construct-fields:
   • Construct the fields and field-names into the class’s prototypes.
5. add-to-class-env!:
   • Update a new class to the list of the global class environment the-class-env. It also checks the class is whether or not in the existing class of the list, if so, it updates the latest one.
6. Update-class-env:
   • Updates a new class to the list of the global class environment.

How to use

class c1
	field f1
	field f2
	field f4
	method initialize() set f1 = 1
	method getf4() f4
class c2
	field f3
	field f4
	method initialize() set f2 = 2
	method setf1(n)set f1 = n
	method setf2(n)set f2 = n
	method setf3(n)set f3 = n
	method setf4(n)set f4 = n
	method getf1() f1
	method getf2() f2
	method getf3() f3

classmerge c4 = c1 & c2

let o = new c4()
t1=0
t2=0
t3=0
t4=0
in begin
	setfield c1 f1 = 5;
	send o setf1(20);
	set t2 = getfield c1 f1;
	set t1 = send o getf1();
	send o setf4(30);
	setfield c2 f4 =15;
	set t3 = getfield c2 f4;
	set t4 = send o getf4();
	list(t1,t2,t3,t4)
end
=> (20 5 15 30)

Explanation of the above example

• We merge all the fields and methods of class c1 and c2, then add to class c4. Now, we create an instance o with all properties of class c4 which inherits from class c1 and c2.

• When field 1 in class c1 is set to 5, and f1 in instance o is set to 20, and set f4 in class c2 to 15, and then set f4 in the instance o to 30, we get the list of result as follows (20 5 15 30). This means that the instance o has all properties of class c4 which is inherited from class c1 and c2.