Language Reference/Objects and Polymorphism - Revision history

Kasper B H Petersen: /* Subsumption polymorphism */

2021-10-12T09:57:39Z

Subsumption polymorphism

← Older revision		Revision as of 11:57, 12 October 2021
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	A very important property of predicate values is that they can be '''''object''''' predicates, and that these have access to the object state to which they belong.		A very important property of predicate values is that they can be '''''object''''' predicates, and that these have access to the object state to which they belong.

	We shall not describe the event notification scheme in details here, though it is good to understand. The reader is encouraged to study the concept in PFC (look for ''addXxxxListener''). However, the use of predicate values will be exercised ~~in the example~~ in the job-scheduling example in the end of this paper.		We shall not describe the event notification scheme in details here, though it is good to understand. The reader is encouraged to study the concept in PFC (look for ''addXxxxListener''). However, the use of predicate values will be exercised in the job-scheduling example in the end of this paper.

	==== Parametric Polymorphism ====		==== Parametric Polymorphism ====

Thomas Linder Puls: version number

2017-07-15T21:45:25Z

version number

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Thomas Linder Puls: Reference to old version removed

2017-07-15T21:13:03Z

Reference to old version removed

← Older revision		Revision as of 23:13, 15 July 2017
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	Visual Prolog has been extended with many language features that support the goals above, but it is still up to the programmer to apply these features such that the goals are achieved. Here we will focus on understanding and using polymorphism.		Visual Prolog has been extended with many language features that support the goals above, but it is still up to the programmer to apply these features such that the goals are achieved. Here we will focus on understanding and using polymorphism.

	The object-system in Visual Prolog gives so called '' subsumption polymorphism'', and ~~from Visual Prolog 7.0 you~~ also have '' parametric polymorphism''. Polymorphism is especially well-suited for '' divide and conquer'' that supports ''sharing'' of code.		The object-system in Visual Prolog gives so called ''subsumption polymorphism'' and also have ''parametric polymorphism''. Polymorphism is especially well-suited for ''divide and conquer'' that supports ''sharing'' of code.

	==== Subsumption polymorphism ====		==== Subsumption polymorphism ====

SergeMukhin: /* Parametric Polymorphism */

2017-07-10T09:36:38Z

Parametric Polymorphism

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	<vip>X = [1,2,3]		<vip>X = [1,2,3]
	X = [7,7]		X = [7,-7]
	X = [9,4,5]</vip>		X = [9,4,5]</vip>

Thomas Linder Puls: header level

2017-07-04T09:14:52Z

header level

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	The programming language Visual Prolog has gone through a huge development in the latest years, and this will also continue in future. Most noticeable is the shift to object-orientation, and the introduction of parametric polymorphism. In this paper we will explain the reason for introducing these features. Examples will be used to illustrate how these facilities can help to tackle the increasing complexity and size of software.		The programming language Visual Prolog has gone through a huge development in the latest years, and this will also continue in future. Most noticeable is the shift to object-orientation, and the introduction of parametric polymorphism. In this paper we will explain the reason for introducing these features. Examples will be used to illustrate how these facilities can help to tackle the increasing complexity and size of software.

	==Motivation==		=== Motivation ===

	While Prolog has many great virtues, one must remember that it is from the IT-bronze-age, and it would be silly to think that it would stay contemporary after three decades or more, especially considering the speed with, which the IT-world has evolved since then. Back then you could write an amazing Prolog program that described how to get a cabbage, a goat, a wolf and a farmer across a river in a little boat without anybody eating anybody. You can still write such programs just as easily (see [[Farmer, Wolf, Goat and Cabbage]]), but they are just not that amazing anymore. Show it to your children and they will immediately ask why the solution is not an animation, and explain that there are millions of much more advanced programs one click away on the Internet already.		While Prolog has many great virtues, one must remember that it is from the IT-bronze-age, and it would be silly to think that it would stay contemporary after three decades or more, especially considering the speed with, which the IT-world has evolved since then. Back then you could write an amazing Prolog program that described how to get a cabbage, a goat, a wolf and a farmer across a river in a little boat without anybody eating anybody. You can still write such programs just as easily (see [[Farmer, Wolf, Goat and Cabbage]]), but they are just not that amazing anymore. Show it to your children and they will immediately ask why the solution is not an animation, and explain that there are millions of much more advanced programs one click away on the Internet already.
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	Many attempts has been made to help tackle these challenges; we have chosen to add the elements that we believe are the winners to Visual Prolog. These are especially object-orientation and parametric polymorphism, but there are also other new elements especially taken from the functional programming world.		Many attempts has been made to help tackle these challenges; we have chosen to add the elements that we believe are the winners to Visual Prolog. These are especially object-orientation and parametric polymorphism, but there are also other new elements especially taken from the functional programming world.

	==Goals and Means==		=== Goals and Means ===

	The goals of the language update is to provide better means for creating and maintaining more complex programs for more complex surroundings.		The goals of the language update is to provide better means for creating and maintaining more complex programs for more complex surroundings.

	===Simplicity===		==== Simplicity ====

	Humans are better to dealing with simple things than complex things, so one important way to deal with complexity is to reduce it to something simpler, especially by using "structure" to organize the complexity.		Humans are better to dealing with simple things than complex things, so one important way to deal with complexity is to reduce it to something simpler, especially by using "structure" to organize the complexity.
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	The main "trick" is the ''divide and conquer '' principle: Divide the original problem into separate sub-problems that can be solved individually. I.e., the solution to the overall problem consists of a number of sub-solutions and something that combines these into the overall solution.		The main "trick" is the ''divide and conquer '' principle: Divide the original problem into separate sub-problems that can be solved individually. I.e., the solution to the overall problem consists of a number of sub-solutions and something that combines these into the overall solution.

	===Maintainability===		==== Maintainability ====

	For software it is often not enough to solve a problem, we must also be able to maintain and extend the solution in an evolving world. So it is important that the used methods not only solve the problem, but also result in a program that can be maintained. Here it is important that the program is understandable and especially that the ''divides'' are visible and clear in the program.		For software it is often not enough to solve a problem, we must also be able to maintain and extend the solution in an evolving world. So it is important that the used methods not only solve the problem, but also result in a program that can be maintained. Here it is important that the program is understandable and especially that the ''divides'' are visible and clear in the program.

	===Reuse and sharing===		==== Reuse and sharing ====

	When you divide problems into sub-problems, perhaps repeatedly, you will sometimes see similar sub-problems. Then it would of course be nice to short cut by reusing a similar solution. Such a solution can be adopted into the new context, but there are two ways this can happen. We can make a copy and update it to its new purpose, or we can try to share the code between the two problems.		When you divide problems into sub-problems, perhaps repeatedly, you will sometimes see similar sub-problems. Then it would of course be nice to short cut by reusing a similar solution. Such a solution can be adopted into the new context, but there are two ways this can happen. We can make a copy and update it to its new purpose, or we can try to share the code between the two problems.
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	The latter is clearly the hardest and only makes sense if both pieces of software are still maintained. But in that case you will have the advantage that maintenance made in one context is also shared with the other context.		The latter is clearly the hardest and only makes sense if both pieces of software are still maintained. But in that case you will have the advantage that maintenance made in one context is also shared with the other context.

	===Flexibility===		==== Flexibility ====

	It is also desirable that your software is flexible. You may for example need to deliver several variants of your software to different customers. Or you may need to reconfigure the software to a changed context. Also some of your shared sub-solutions may need to go into contexts that are not completely the same.		It is also desirable that your software is flexible. You may for example need to deliver several variants of your software to different customers. Or you may need to reconfigure the software to a changed context. Also some of your shared sub-solutions may need to go into contexts that are not completely the same.

	===Power===		==== Power ====

	Finally, it is of course desirable if language extensions add additional programming power. For example, by simplifying the code necessary to tackle certain standard "issues".		Finally, it is of course desirable if language extensions add additional programming power. For example, by simplifying the code necessary to tackle certain standard "issues".

	==Polymorphism==		=== Polymorphism ===

	Visual Prolog has been extended with many language features that support the goals above, but it is still up to the programmer to apply these features such that the goals are achieved. Here we will focus on understanding and using polymorphism.		Visual Prolog has been extended with many language features that support the goals above, but it is still up to the programmer to apply these features such that the goals are achieved. Here we will focus on understanding and using polymorphism.
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	The object-system in Visual Prolog gives so called '' subsumption polymorphism'', and from Visual Prolog 7.0 you also have '' parametric polymorphism''. Polymorphism is especially well-suited for '' divide and conquer'' that supports ''sharing'' of code.		The object-system in Visual Prolog gives so called '' subsumption polymorphism'', and from Visual Prolog 7.0 you also have '' parametric polymorphism''. Polymorphism is especially well-suited for '' divide and conquer'' that supports ''sharing'' of code.

	===Subsumption polymorphism===		==== Subsumption polymorphism ====

	An object is a closed entity that carries state and code. The object provides an interface through which the surroundings can interact with the object. The object can also interact with other objects through their interfaces.		An object is a closed entity that carries state and code. The object provides an interface through which the surroundings can interact with the object. The object can also interact with other objects through their interfaces.
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	We shall not describe the event notification scheme in details here, though it is good to understand. The reader is encouraged to study the concept in PFC (look for ''addXxxxListener''). However, the use of predicate values will be exercised in the example in the job-scheduling example in the end of this paper.		We shall not describe the event notification scheme in details here, though it is good to understand. The reader is encouraged to study the concept in PFC (look for ''addXxxxListener''). However, the use of predicate values will be exercised in the example in the job-scheduling example in the end of this paper.

	===Parametric Polymorphism===		==== Parametric Polymorphism ====

	Like for subsumption polymorphism the purpose of ''parametric'' polymorphism is to use the same code for many things. However, where subsumption polymorphism is mainly about supplying different implementations to the same context, parametric polymorphism is mainly about using the same implementation in different contexts.		Like for subsumption polymorphism the purpose of ''parametric'' polymorphism is to use the same code for many things. However, where subsumption polymorphism is mainly about supplying different implementations to the same context, parametric polymorphism is mainly about using the same implementation in different contexts.
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	However, the world is not black and white in this respect, there are many graduations and clear exceptions to this.		However, the world is not black and white in this respect, there are many graduations and clear exceptions to this.

	==Example: Job Scheduler==		=== Example: Job Scheduler ===

	Now let us try to combine some of the things from above to produce a simple, but yet powerful '''''job scheduler'''''.		Now let us try to combine some of the things from above to produce a simple, but yet powerful '''''job scheduler'''''.
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	In the scheduler we have clearly used the (parametric) polymorphic priority queue. We have also used the (subsumption) polymorphism that comes from predicate values, such that we can register jobs with different implementation in the scheduler. The parametric polymorphism has exploited the homogeneous treatment of prioritizing things, no matter which kind of things we are dealing with. The subsumption polymorphism has been used to deal with the heterogeneous nature of jobs themselves.		In the scheduler we have clearly used the (parametric) polymorphic priority queue. We have also used the (subsumption) polymorphism that comes from predicate values, such that we can register jobs with different implementation in the scheduler. The parametric polymorphism has exploited the homogeneous treatment of prioritizing things, no matter which kind of things we are dealing with. The subsumption polymorphism has been used to deal with the heterogeneous nature of jobs themselves.

	~~==Conclusion==~~

	We believe that parametric polymorphism can help you share general solutions between many different problems, and thus reducing your total development time and especially your maintenance time. We know that it makes it possible for us to create more library software to use off-the-shelf, actually to share off-the-shelf.

	We believe that subsumption polymorphism can help you to deal with the heterogeneous world that surrounds your software. Both on the inner level as in the scheduler, and on the outer level as in the salary example.


	~~[[Category:Object system]]~~
	~~[[Category:Data types and handling]]~~
	[[ru:Объекты и полиморфизм. Ч.1]]		[[ru:Объекты и полиморфизм. Ч.1]]

Thomas Linder Puls: move to lang ref

2017-07-04T09:08:23Z

move to lang ref

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			{{languageReferenceNavbar\|Generic Interfaces and Classes}}

	The programming language Visual Prolog has gone through a huge development in the latest years, and this will also continue in future. Most noticeable is the shift to object-orientation, and the introduction of parametric polymorphism. In this paper we will explain the reason for introducing these features. Examples will be used to illustrate how these facilities can help to tackle the increasing complexity and size of software.		The programming language Visual Prolog has gone through a huge development in the latest years, and this will also continue in future. Most noticeable is the shift to object-orientation, and the introduction of parametric polymorphism. In this paper we will explain the reason for introducing these features. Examples will be used to illustrate how these facilities can help to tackle the increasing complexity and size of software.

Thomas Linder Puls: moved Objects and Polymorphism to Language Reference/Objects and Polymorphism

2017-07-04T09:04:10Z

moved Objects and Polymorphism to Language Reference/Objects and Polymorphism

← Older revision	Revision as of 11:04, 4 July 2017
(No difference)

Thomas Linder Puls: Parametric Polymorphism

2017-07-04T08:58:45Z

Parametric Polymorphism

← Older revision		Revision as of 10:58, 4 July 2017
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	===Parametric Polymorphism===		===Parametric Polymorphism===

	~~In Visual Prolog 7.0 parametric polymorphism is introduced.~~ Like for subsumption polymorphism the purpose of parametric polymorphism is to use the same code for many things. However, where subsumption polymorphism is mainly about supplying different implementations to the same context, parametric polymorphism is mainly about using the same implementation in different contexts.		Like for subsumption polymorphism the purpose of ''parametric'' polymorphism is to use the same code for many things. However, where subsumption polymorphism is mainly about supplying different implementations to the same context, parametric polymorphism is mainly about using the same implementation in different contexts.

	Let us start with a very simple example, to introduce the concepts. We will implement a function, which takes the list as argument and returns the elements one by one (non-deterministically).		Let us start with a very simple example, to introduce the concepts. We will implement a function, which takes the list as argument and returns the elements one by one (non-deterministically).

Thomas Linder Puls: No longer new

2017-07-04T08:55:30Z

No longer new

← Older revision		Revision as of 10:55, 4 July 2017
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	In the scheduler we have clearly used the (parametric) polymorphic priority queue. We have also used the (subsumption) polymorphism that comes from predicate values, such that we can register jobs with different implementation in the scheduler. The parametric polymorphism has exploited the homogeneous treatment of prioritizing things, no matter which kind of things we are dealing with. The subsumption polymorphism has been used to deal with the heterogeneous nature of jobs themselves.		In the scheduler we have clearly used the (parametric) polymorphic priority queue. We have also used the (subsumption) polymorphism that comes from predicate values, such that we can register jobs with different implementation in the scheduler. The parametric polymorphism has exploited the homogeneous treatment of prioritizing things, no matter which kind of things we are dealing with. The subsumption polymorphism has been used to deal with the heterogeneous nature of jobs themselves.

	~~==Other New Constructions==~~

	This paper only describes some of the new language features, but it also uses the new foreach and if-then-else constructions, without making any special remarks about them. These language constructions are easily understandable, and thus needs little explanation. However, you may notice that they sometimes make strange constructions like "catch all" clauses and cuts superfluous. Though they look less like "old" Prolog, they actually have quite clearly Prolog-style semantics, and at the same time they can make code clearer.

	==Conclusion==		==Conclusion==

Thomas Linder Puls: /* Future Extensions */ Remove

2017-06-07T12:51:48Z

Future Extensions: Remove

← Older revision		Revision as of 14:51, 7 June 2017
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	We believe that subsumption polymorphism can help you to deal with the heterogeneous world that surrounds your software. Both on the inner level as in the scheduler, and on the outer level as in the salary example.		We believe that subsumption polymorphism can help you to deal with the heterogeneous world that surrounds your software. Both on the inner level as in the scheduler, and on the outer level as in the salary example.

	~~==Future Extensions==~~

	~~Visual Prolog will still develop. Many features are already planned and the implementation is in progress. Especially two things are worth noticing.~~

	Parametric polymorphism will be extended to cover classes and interfaces as well. This simply adds more power of the same kind as we have discussed here. It is also natural to have such power on all data types, rather than on a limited subset. Later again, the parametric polymorphism will become F-bounded, but it is beyond this paper to talk about this. Interested users can search the literature and of course the Internet for this.

	The other interesting feature is anonymous predicate values. These greatly simplify the code that is otherwise necessary to write to obtain certain "effects", for example when creating jobs for the job scheduler discussed in this paper. The thing is that it is a bit cumbersome to transfer parameters into such jobs: The job itself does not take any user defined parameters, so the parameters must be "stored" somewhere. Typically in facts in the object to which the job predicate belongs. With anonymous predicates, this is not necessary.

	[[Category:Object system]]		[[Category:Object system]]
	[[Category:Data types and handling]]		[[Category:Data types and handling]]
	[[ru:Объекты и полиморфизм. Ч.1]]		[[ru:Объекты и полиморфизм. Ч.1]]