Tuesday, 20 October 2020

Commands, queries and side-effects (object interface design)

Commands and queries

"A machine has 2 types of buttons, command and query buttons. When a command button is pressed, the machine performs work and its state is modified. When a query button is pressed, the machine returns some information about it." - Bertrand Meyer

Command corresponds to procedures, they do not return any results. For example:

class MyMachine {

public:

    void SetName(const char * name); // is a command/procedu

}

Query corresponds to functions or attributes, for example:

class MyMachine {

public:

    void SetName(const char * name);     // command/procedure

    const char * GetName() const;        // query/function        

    bool ProcessingReady() const;        // query/attribute

private:

    const char * name;

}


Side effects

 "A side effect on an object is an operation that may change at least one attribute of the object."

The above command and query view is important with relation to side effects since  according to Bertrand Meyer in his book XXX (Chapter 7.7.2 "Commands and queries", Page 134)...

"The clean seperation between procedures and functions averts many of the pitfalls of traditional programming. Side-effect-producing functions, which have been elevated by some langauges (C seems to be the most extreme example) to the status of an institution, conflict with the classical notion of function in mathematics. A mathematical function will always give the same result when applied to the same argument."

Conclusion

From this information, we can create better class interfaces and keep them "more true" to the mathmatical foundation of abstract data type theory. This means that our code is closer to "formal verification" than guesswork.



Wednesday, 7 October 2020

ADT, type theory and set theory.

Intro

Mathematical models underpin most (arguably all) software, to create models we need to understand the bits that make them. Sets, types, algebraic structures.

Type and set theory provide the formal definitions for programming today. I believe that understanding how these pieces fit together we can create "better" software; more readable, easier to debug.

There is a saying that "the ideal solution for a professor to a problem was to introduce 10 classes instead of creating 1 function" and i think that this blog post might shed light on it. By distinguishing Types from Sets from Algebraic Structures and ADTs.

Sets and types

Sets and types are similar concepts and can both be thought about at a high level.

  1. They are used in logic.
  2. They are used to create models.

Since programming is about representing concepts and creating models, sets and types are important.

An example of a type is PERSON, and using programming we can represent this type in many ways, the most common is by using classes. Therefore we can create a class called PERSON which represents the type PERSON. Also, we can view the PERSON type as a set, it is identical to the type view.

class PERSON {
public:
 void Op();
private:
 bool HasName();
private:
 int data;
};

Algebraic structure as ADT as set/type

An algebraic structure is defined as a carrier set, operations, axioms and distinguished members. The above classes are expressed as an Abstract Data Type. Where PERSON is the carrier set (or type), operations are Op(), and an example of an explicit axiom is HasName().

Since we can represent algebraic structures with an ADT. And algebraic structures can represent types/sets, therefore, we can represent types/sets with an ADT. This is recursive definition where we can represent a set of a set using two ADTs.

Conclusion

By using classes we have bound the operations to the type/set explicitally, this means we can more easily represent individual types than individual sets. However, the unification of the two means that while programming, "names run out" and we often find ourselves creating strange types to manage types. However, if we seperate the carrier set and the operations that perform on it, we can create more sensible programs. To summarise; by using classes to create individual types and favouring modules over types to perform functions. We might end up with better sematic and formalized programs.

Seperating OOP objects and modules using this as our reasoning gives us a huge amount of flexibility with how we structure our programs. On the one hand we can create descriptive real-life heirarchies by following the type view, where PERSON is-a HUMAN and using polymorphism for dynamic dispatch. On the other hand we can functionally decompose the programs system into modules which more directly represent what the software is doing.

I believe that a common error with modern OOP (where real life concept=object) is that everything is strictly a type, and never viewed as a algebraic structure. Functions are bound to the objects as types and then more types are created to hold functions that manage other types. Now this view may seem ideal, and represent real life perfectly where say; a chest holds and number of items, however as we can see above, type theory is not the only way to create programs. This is reflected with Java, with a heavy use of type theory the application can be overly verbose and make abstractions that are wasteful and make no sense.

References

  1. https://en.wikipedia.org/wiki/Abstract_data_type
  2. https://en.wikipedia.org/wiki/Algebraic_structure
  3. Discrete Mathematics in Computer Science - STANAT, DONALD F - 1977 - ISBM 0-13-2160528 - Chapter 7 Algebras, 7.1 Structure of algebras

Sunday, 20 September 2020

Computer Science Reference List

CS Reference List

This is a collection of papers, websites and books that I have found useful when researching into computer science. Mainly into programming paradigms and languages, focusing on understanding OOP

OOP



Phenomena and Concepts

Website

http://people.cs.aau.dk/~normark/oop-csharp/html/notes/intro-oop_themes-phen-concepts-sect.html

PDF

https://drive.google.com/file/d/1FQLDJC6Kj2r7SeLZK1YnLCcJP6tKvvrH

About

Learning how objects relate to philosophical concepts and phenomena, provides a starting point to model objects.

Archived

  • Wayback machine (20/09/2020)
  • Google drive (20/09/2020)

Specialization of classes

Website

http://people.cs.aau.dk/~normark/oop-csharp/html/notes/inheritance_themes-specialization-sect.html#inheritance_specialization-sect_section-title_1

PDF

https://drive.google.com/file/d/1FQLDJC6Kj2r7SeLZK1YnLCcJP6tKvvrH

About

A description of inhertiance using sets.

Archived

  • Wayback machine (20/09/2020)
  • Google drive (20/09/2020)

A Tour of C++

Blog post

https://www.informit.com/articles/article.aspx?p=25003

About

Bjarne stroustrup on Procedural, Modules, ADT, Abstract types and Objects.

Archived

  • Wayback machine (all pages available 20/09/2020)

On Understanding Data Abstraction

PDF

https://drive.google.com/file/d/1dYfehZ764TL_k_RpH-xTGr0Zt9TFOdTo/view?usp=sharing

About

This paper describes what the difference between an Object (OOP) and ADT (abstract data type) is.

Archived

  • https://drive.google.com/file/d/1dYfehZ764TL_k_RpH-xTGr0Zt9TFOdTo/view?usp=sharing




Modules



Criteria for modularity

PDF

https://drive.google.com/file/d/1_q3AWXMsl5jZjkS7C68czn8kNQkCXAoG/view?usp=sharing

About

A white paper describing how to modularize a program.

Archived

  • Google drive (20/09/2020)




Records



ALGOL bulletin No. 21 - AB21.3.6 "Record Handling"

PDF

https://drive.google.com/file/d/1dYfehZ764TL_k_RpH-xTGr0Zt9TFOdTo/view?usp=sharing

About

An old ALGOL bulletin paper that describes records and their relation to types.

Archived

  • Google drive (20/09/2020)

Sunday, 6 September 2020

OOP Quotes 1: ADT vs Object

From: "On Understanding Data Abstraction, Revisited. William R. Cook"

 "in modern object-oriented languages, the issue boils down to whether or not classes are used as types. In a pure object-oriented style, classes are only used to construct objects, and interfaces are used for types. When classes are used as types, the programmer is implicitly choosing to use a form of abstract data type."

 

"To summarize, when a class name is used as a type, it represents an abstract data type"

 

"Object-Oriented Programming in Java

While Java is not a pure object-oriented language, it is possible to program in a pure object-oriented style by obeying the following rules

Classes only as constructors: A class name may only be used after the keyword new.

No primitive equality: The program must not use primitive equality (==). Primitive equality exposes representation and prevents simulation of one object by another. "


"Objects work with interfaces as types"

"OOP is using dynamic dispatch, ADT does not"

"ADT work with class as types"



Example ADT:

C++

class Person {}

Person p = new Person();

C

struct set rep; // representation is not defined in header  

typedef struct set rep* set;
set empty();
bool isEmpty(set s);

set insert(set s, int i); 

bool contains(set s, int i); 

set union(set s1, set s2);

 

 

Example Objects:

class PersonManager {};

 
class PersonSystem {

    void printName(Person p) {}

    PersonManager m_manager;

};

Person p = new Person();

PersonSystem.printName(p);



Monday, 24 August 2020

C++ Interfaces Explained

When designing a program, using interfaces improves the program by drastically lowering coupling.

This first diagram, we can see that the EntityManager depends on the Renderer class. This is bad because programs should "depend upon abstractions not concretions", if the Renderer class changes then EntityManager will break too.

Therefore we should 'fix' the Renderers interface so that other class that use the Renderer, will not have to be changed when the renderer functions change. To do this we use what is called an "Interface" class (interface in Java and C# are natively supported but not in C++). The "Interface" class is in fact a PIMPL (pointer to implementation) used to create the boundary between implementation and abstraction.

An "Interface" class in C++ is a polymorphic base class with no implementation (code,data.etc) just function signatures. The reason why it holds nothing but function signatures. One reason why an interface has no implementation is because (see example double-dispatch below) the "Interface" classes can be forward declared and linked at link time, this means the "Interface" class can't contain any implementation, only function signatures, otherwise we receive "Duplicate Symbols" error from the linker when we try to use them.

Without interface
Without the IRenderer interface

This second diagram shows the Renderer concrete class, they are exactly the same other than Renderer is replaced with the abstract polymorphic class "IRenderer" to seperate the Renderer heirachy from the EntityManager.

With the IRenderer interface


IRenderer interface /PIMPL

Although it might seem redundant to duplicate the Renderer interface as IRenderer, it is infact a cornerstone of "Designing by contract", using interfaces you can define contracts between class heirachies.

Commands, queries and side-effects (object interface design)

Commands and queries "A machine has 2 types of buttons, command and query buttons. When a c...