The Design of Class Mechanism for MOBY

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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-gtInt
• meth moveInt -gtPoint
• Objtype CPoint extends Point with
• meth getC Uint-gtColor
• meth shadeColor -gtCPoint

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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 lt type

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• Module PointTypes
• objtype Point
• meth getX Uint-gtInt
• meth move Int -gtPoint
• Type CPoint lt 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 inMOBY

• 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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• JAVAs interfaces and Cs 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 JAVAs package scope
• MOBY uses the combination of modules and
• partial type revelation