CSC/ECE 517 Fall 2010/ch4 4e ms - Revision history

Spatil: /* Programming Constructs */

2010-10-30T03:18:46Z

Programming Constructs

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	==Programming Constructs==		==Programming Constructs==
	Prototype based languages are varied in terms of their implementation patterns. We will consider the features of JavaScript, one of the most popular prototype based languages, for deeper insight into the typical implementation of programming constructs in prototyping.		Prototype based languages are varied in terms of their implementation patterns. We will consider the features of [http://en.wikipedia.org/wiki/JavaScript JavaScript], one of the most popular prototype based languages, for deeper insight into the typical implementation of programming constructs in prototyping.

	===Creating Objects===		===Creating Objects===

Spatil: /* Cloning */

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Cloning

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	There are 2 levels at which an object can be cloned. These are [http://en.wikipedia.org/wiki/Object_copy ''deep cloning''] and [http://en.wikipedia.org/wiki/Object_copy ''shallow cloning'']. In shallow cloning, a copy of an object is created wherein just the references of the sub-objects are copied. In deep cloning, a complete duplicate of the original is created, i.e. it creates not only the primitive values of the original object but also copies all its sub objects as well.		There are 2 levels at which an object can be cloned. These are [http://en.wikipedia.org/wiki/Object_copy ''deep cloning''] and [http://en.wikipedia.org/wiki/Object_copy ''shallow cloning'']. In shallow cloning, a copy of an object is created wherein just the references of the sub-objects are copied. In deep cloning, a complete duplicate of the original is created, i.e. it creates not only the primitive values of the original object but also copies all its sub objects as well.

	~~[http://en.wikipedia.org/wiki/~~JavaScript ~~JavaScript]~~ supports shallow cloning. The copy(), clone() functions are used to clone an object.		JavaScript supports shallow cloning. The copy(), clone() functions are used to clone an object.

	i) clone() uses JavaScript's built-in prototype mechanism to create a cheap, shallow copy of a single Object.		i) clone() uses JavaScript's built-in prototype mechanism to create a cheap, shallow copy of a single Object.

Spatil: /* Cloning */

2010-10-30T03:17:50Z

Cloning

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	=====Cloning=====		=====Cloning=====
	There are 2 levels at which an object can be cloned. These are ''deep cloning'' and ''shallow cloning''. In shallow cloning, a copy of an object is created wherein just the references of the sub-objects are copied. In deep cloning, a complete duplicate of the original is created, i.e. it creates not only the primitive values of the original object but also copies all its sub objects as well.		There are 2 levels at which an object can be cloned. These are [http://en.wikipedia.org/wiki/Object_copy ''deep cloning''] and [http://en.wikipedia.org/wiki/Object_copy ''shallow cloning'']. In shallow cloning, a copy of an object is created wherein just the references of the sub-objects are copied. In deep cloning, a complete duplicate of the original is created, i.e. it creates not only the primitive values of the original object but also copies all its sub objects as well.

	JavaScript supports shallow cloning. The copy(), clone() functions are used to clone an object.		[http://en.wikipedia.org/wiki/JavaScript JavaScript] supports shallow cloning. The copy(), clone() functions are used to clone an object.

	i) clone() uses JavaScript's built-in prototype mechanism to create a cheap, shallow copy of a single Object.		i) clone() uses JavaScript's built-in prototype mechanism to create a cheap, shallow copy of a single Object.

Spatil: /* Primitives of virtual machine */

2010-10-30T03:16:02Z

Primitives of virtual machine

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	Another parallel and overlapping classification of prototype based languages is based on group oriented constructions i.e. classification according to the level of abstractness of the constructions. Following are the high level groups:		Another parallel and overlapping classification of prototype based languages is based on group oriented constructions i.e. classification according to the level of abstractness of the constructions. Following are the high level groups:
	<br/>1. Purely prototype based: Self, Kevo, Taivalsaari, Agora, Omega, Obliq, NewtonScript		<br/>1. Purely prototype based: Self, Kevo, Taivalsaari, [http://en.wikipedia.org/wiki/Agora_%28programming_language%29 Agora], Omega, [http://en.wikipedia.org/wiki/Obliq Obliq], [http://en.wikipedia.org/wiki/NewtonScript NewtonScript]
	<br/>2. Not Strictly prototype based: Object-Lisp, Yafool		<br/>2. Not Strictly prototype based: [http://en.wikipedia.org/wiki/Object_Lisp Object-Lisp], Yafool
	<br/>3. Languages mixing prototypes and classes.		<br/>3. Languages mixing prototypes and classes.

Spatil: /* Slots or methods and variables */

2010-10-30T03:10:15Z

Slots or methods and variables

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	2. By amalgamating attributes and methods into something known as "slots".		2. By amalgamating attributes and methods into something known as "slots".

	Thus, slots hold data values as well as methods. Some languages differentiate between methods and variables, while others treat them in the same way. ''Self '' treats methods and variables as same, which allows to over-ride an attribute with a method and vice versa. In prototype based languages one can add or change not only data but also methods. For this reason, most prototype-based languages refer to both data and methods as "slots". Slots are defined to answer ''messages''. Either data or code can be found in a slot as the result of sending a message. Data is just returned, and code is actually executed. If a prototype does not contain a requested slot or does not know how to answer a specific message it may delegate the request to another prototype through an inheritance relation which is discussed next. [8]		Thus, slots hold data values as well as methods. Some languages differentiate between methods and variables, while others treat them in the same way. [http://en.wikipedia.org/wiki/Self_%28programming_language%29 ''Self''] treats methods and variables as same, which allows to over-ride an attribute with a method and vice versa. In prototype based languages one can add or change not only data but also methods. For this reason, most prototype-based languages refer to both data and methods as "slots". Slots are defined to answer ''messages''. Either data or code can be found in a slot as the result of sending a message. Data is just returned, and code is actually executed. If a prototype does not contain a requested slot or does not know how to answer a specific message it may delegate the request to another prototype through an inheritance relation which is discussed next. [8]

Spatil: /* Introduction */

2010-10-30T02:52:30Z

Introduction

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	==Introduction==		==Introduction==
	Prototype based programming is an object oriented programming methodology. It differs from the other classes of object oriented languages due to its main characteristic feature that no classes are present in the language constructs. Behavior reuse or inheritance is performed by cloning some instance or prototype objects. Thus, it is also known as 'instance-based programming' or 'class-less based programming' and such languages are known as 'prototype based languages'.		Prototype based programming is an [http://en.wikipedia.org/wiki/Object-oriented_programming object oriented programming] methodology. It differs from the other classes of object oriented languages due to its main characteristic feature that no classes are present in the language constructs. Behavior reuse or inheritance is performed by cloning some instance or prototype objects. Thus, it is also known as 'instance-based programming' or 'class-less based programming' and such languages are known as 'prototype based languages'.

	Objects used in prototype based programming are referred as ''prototypes'', which resemble an instance of a class in class-based programming. It differs from a class instance as one can add or remove variables and methods at any level of a single object, without worrying if the object still fits in the system. Typically, new objects are created by copying existing objects, which is called as ''cloning''.		Objects used in prototype based programming are referred as ''prototypes'', which resemble an instance of a class in [http://en.wikipedia.org/wiki/Class-based_programming class-based programming]. It differs from a class instance as one can add or remove variables and methods at any level of a single object, without worrying if the object still fits in the system. Typically, new objects are created by copying existing objects, which is called as ''cloning''.

	~~Programming~~ languages are classified based on following criteria:		Object Oriented programming languages are classified based on following criteria:
	* Class based : Object instances are created fro skeleton or blueprint of the class.		* Class based : Object instances are created fro skeleton or blueprint of the class.
	* Prototype based : Object instances are created from concrete examples and other objects are created with such a prototype reference.		* Prototype based : Object instances are created from concrete examples and other objects are created with such a prototype reference.
	* Automata based : The program is thought of as a model of a finite state machine or any other formal automata.		* [http://en.wikipedia.org/wiki/Automata-based_programming Automata based] : The program is thought of as a model of a finite state machine or any other formal automata.
	* Based on separation of concerns e.g. aspect oriented languages		* Based on separation of concerns e.g. aspect oriented languages

Mnarang: /* Delegation */

2010-10-27T02:40:55Z

Delegation

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	<br/>2. Form.onClick()		<br/>2. Form.onClick()
	<br/>3. Window.onClick()		<br/>3. Window.onClick()

			The above propagation description is illustrated in the diagram below.
	<center>		<center>
	[[Image:formButton.jpg]]		[[Image:formButton.jpg]]

Mnarang: /* Criticism */

2010-10-27T02:38:51Z

Criticism

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	==Criticism==		==Criticism==
	* Although class based programming seems to be very restrictive in nature, it provides better type-safety and predictability. This is not the case with prototype based languages wherein this violation is particularly observed in case of the delegation mechanism which allows the parent to be modified during runtime. Any unintended changes will affect the parent and alter the behavior of system which might be undesirable.		* Although class based programming seems to be very restrictive in nature, it provides better type-safety and predictability. This is not the case with prototype based languages wherein this violation is particularly observed in case of the delegation mechanism which allows the parent to be modified during runtime. Any unintended changes will affect the parent and alter the behavior of system which might be undesirable.
	* Prototype based languages were developed incrementally based on specific platforms or implementation requirements. Thus, the larger community of developers is not well versed with its concepts.		* Prototype based languages were developed incrementally based on specific platforms or implementation requirements. Thus, the larger community of developers is not well versed with its concepts. [11]

	==Applications==		==Applications==

Mnarang: /* References / External Links */

2010-10-27T02:38:30Z

References / External Links

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	# C. Dony, J. Malenfant, D. Bardon, [http://www.lirmm.fr/~dony/postscript/proto-book.pdf ''Classifying Prototype Based Languages'']		# C. Dony, J. Malenfant, D. Bardon, [http://www.lirmm.fr/~dony/postscript/proto-book.pdf ''Classifying Prototype Based Languages'']
	# [http://www.learn-javascript-tutorial.com/Prototype-Based-Inheritance.cfm Prototype-Based Inheritance]		# [http://www.learn-javascript-tutorial.com/Prototype-Based-Inheritance.cfm Prototype-Based Inheritance]
	# ~~Criticism of prototype based programming languges~~ [http://en.wikipedia.org/wiki/Prototype-based_programming]		# [http://en.wikipedia.org/wiki/Prototype-based_programming Criticism of prototype based programming languages]

Mnarang: /* References / External Links */

2010-10-27T02:37:23Z

References / External Links

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	# C. Dony, J. Malenfant, D. Bardon, [http://www.lirmm.fr/~dony/postscript/proto-book.pdf ''Classifying Prototype Based Languages'']		# C. Dony, J. Malenfant, D. Bardon, [http://www.lirmm.fr/~dony/postscript/proto-book.pdf ''Classifying Prototype Based Languages'']
	# [http://www.learn-javascript-tutorial.com/Prototype-Based-Inheritance.cfm Prototype-Based Inheritance]		# [http://www.learn-javascript-tutorial.com/Prototype-Based-Inheritance.cfm Prototype-Based Inheritance]
			# Criticism of prototype based programming languges [http://en.wikipedia.org/wiki/Prototype-based_programming]