Transcript The Design of Class Mechanism for MOBY 1

The Design of Class Mechanism
for MOBY
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Earlier Software designers have to
choose between the following schemes
for the development
– Class rich languages like C++ or JAVA.
– Module rich languages like Standard ML.
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Paper describes the design of the class mechanism
with a richer module system
– C++ and JAVA has the rich class mechanisms but week
module system
– Standard ML has rich Module System but lack
Class Mechanism
– Moby provides both rich Class and Module Mechanism
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Class in the class mechanism has two
roles:
• Implementing Objects
• Supporting inheritance
Based on the above roles class have two
type of Clients:
•Object Client
•Class Client
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MOBY : It is a ML-like language that
supports Class bases mechanisms.
MOBY has three mechanism to cover the
features of class based languages.
– Objects and Object Types.
– Classes and Class Interfaces.
– Modules and Module Signature.
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Objects
Object in MOBY are collection of fields
and methods.
Member of Objects: fields + Methods
Operations on Objects:
» Select a field
» Update a field
» Invoke a method
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Object Type
Object Type specifies which object members
client may use.
Object Types are nothing but are interfaces
of JAVA.
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Objtype Point{
meth getX : Uint->Int
meth move:Int ->Point
}
Objtype CPoint extends Point with {
meth getC : Uint->Color
meth shade:Color ->CPoint
}
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Typing
Typing in the MOBY is structural rather than
Name Subtyping as in JAVA.
Structural Subtyping has several variants:
– Width Subtyping
– Depth Subtyping
– Full Subtyping
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Modules
Modules in Moby are like ML-Style languages.
 Signature Controls the Visibility of the
definitions outside the Modules.
 Modules may be nested .
 Partial type revelation can be defined in
signatures.
type type-name <: type

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Module PointTypes:{
objtype Point{
meth getX : Uint->Int
meth move : Int ->Point
}
Type CPoint <: Point
}{
………body of the PointTypes
}
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MOBY Class
Defines the implementation of object
 It contains the declaration of:

– fields
– Method
– Makers (which are constructors in C++ or
JAVA)
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Class interface
It is a signature of the class
 It is not interfaces as in JAVA.
 It defines both object and class view.

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Visibility
Internal Class View
 External Class View
 Internal Object View
 External Object View

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Class based features are realized in
MOBY:

Static Class Members
MOBY relies on Module system to support global
definition.

C++ private inheritance :
In MOBY Object Type implemented by subclass
is independent of the Object Type implemented by
its super class.
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
JAVA’s interfaces and C++’s multiple
inheritance with abstract base classes:
MOBY has the Structural sub typing relationship.

Final Classes:
We can have this by hiding the class in a module and
controlling the visibility of its methods

.
C++ friends and JAVA’s package scope:
MOBY uses the combination of modules and
partial type revelation
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