1	"Visual Prolog Object System" Visual Prolog Object System APPSEM II 2005 Thomas Linder Puls Prolog Development Center Denmark
2	Thomas Linder Puls Masters: Compiling λ-prolog PhD: Higher order logic (type theory) Computer Resources International: AI system for shock absorber design (constraint resolution engine) Oersted satellite on-board software (Ada) Prolog Development Center Staff Planning & Scheduling Visual Prolog (6 years)
3	Prolog Development Center Founded in 1985 Prolog Compiler for MS-DOS (masters project) Copenhagen, Denmark (~50 employees) Staff Planning & Scheduling Aviation Planning & Scheduling Production Planning & Scheduling Hospital Booking Emergency Management Speech Recognition St. Petersburg, Russia (~15 employees) Visual Prolog Agile Team: Team/Project Management
4	Language Characteristics Logical programming language based on Prolog not ISO-Prolog compliant Successor of Turbo Prolog and PDC Prolog Compiled for Microsoft Windows (i386) Object-Oriented Non-determinism/backtracking Unification, pattern matching Algebraic data types Fact databases Functions Statically typed Multithreaded Garbage Collected Industrial Strenght
5	Object System Class based object system Separate: Interfaces Class declarations Class implementations Support of multiple interfaces Multiple inheritance State is completely hidden Object callbacks (seamless C# delegates)
6	Interfaces – Object Types Interfaces define object types interface person predicates getName : () -> string Name. setName : (string Name). end interface person person is an object type All objects of the person type has predicates getName and setName person is not bound to any particular class: any class can support the person interface
7	Interface Support An interface can support other interfaces (hierarchically) The support hierarchy is a subtype/”is a” hierarchy interface user supports person predicates getLoginName : () -> string Name. end interface user user is an object type user is subtype of person A user object can be used as a person object (subsumption) A user object has predicates getLoginName, getName and setName Multiple supports is supported Support does not imply any code inheritance
8	Class Declarations A class can construct objects of some object type A class does not define a type class userClass : user constructors newUser : (string LoginName, string Name). end class userClass userClass is a class that constructs objects of type user userClass has a constructor named newUser To construct an object newUser is invoked as a function User = userClass::newUser(”thomas”, ”Thomas Puls”), User:setName(”Thomas Linder Puls”) Class declarations are declarative (for the programmer, not for the compiler) Implementation details not mentioned (inheritance, override, etc)
9	Implementation & Inheritance A class must have an implementation implement userClass inherits personClass facts loginName : string. clauses newUser(LoginName, Name) :- loginName := LoginName, personClass::newPerson(Name). clauses getLoginName() = loginName. end interface person State is hidden in the implementation, and can only be accessed there Inheritance is hidden in the implementation
10	Specialities (1) Objects are completely abstract ”This” is the only exception Objects can only be accesseed through their interfaces This also goes for objects of the same class classes do not define types This also goes for inherited objects seamless multiple inheritance Classes do not define object types Liberty to re-implement any object type No access modes (no: protected, no: friends, etc) Class declaration is separate from implementation Separate version control (declaration is a ”contract”) More files for the same class
11	Specialities (2) Support (subtype) is separate from inheritance Conceptually clear Liberty to ”be something” without being forced to inherit anything Inheritance is hidden/private Natural to hide information that has no ”interest” for users Complicates implementation (compiler lacks information) State is hidden (non-optional) In line with ”best practice”: hide representation Sometimes to elaborate Information for programmer, not for compiler
12	Additional Object Callback (object in predicate closure) Pred = User:setName, Pred(”W.A.M.”) Invocation of Pred acts on the User object Delegate (dynamic inheritance) implement someClass delegate interface person to currentPerson facts currentPerson : person. The person interface is delegated to the currentPerson fact variable I.e., ”inherit” person behavior from the currentPerson object The effective ”inheritance” can be changed dynamically
13	Future Generics (parametric polymorphism) List comprehension Structured language constructs (if-then-else, foreach) Name spaces Anonymous predicates (λ-expressions) Properties F-bounded polymorphism Guarded monitors .net CLR (managed code)
14	More info Visual Prolog: http://www.visual-prolog.com Discussion forum: http://discuss.visual-prolog.com Prolog Development Center: http://www.pdc.dk CD: Visual Prolog 6.3 Personal Edition including Visual Prolog 7.0 alpha