If you recall the days of Borland Turbo Prolog (there were books on Turbo Prolog in bookstores and libraries back then) you may not be aware that it lived on as PDC Prolog (Prolog Development Center)
Much like the first home of Prolog at prologia.fr, this is a web site that does no appear to be the site of a language implementation, let alone Prolog.
Prologia IV is a language for Constraint Logic Programming in which Prolog is a limiting case. The company now focuses on industrial applications. The same is true at PDC. The other notable company with a Prolog more widely used is ILog with JRules.
PDC has focused on domain specific languages (DSL). Visual Prolog itself has fostered another company, EZY Prolog which looks to simplify using VIP.
Visual Prolog for Windows GUI has always required as much knowledge of Windows GUI programming as would be required of a C++ developer although much better suited to RAD.
What makes Visual Prolog worth exploring is the way in which OOP has been brought to a declarative language with hooks to user interaction. It is an implementation which continues to evolve as an industrial strength framework. It is worth comparing to the Logtalk framework which brings a remarkably flexible OOP option to almost any Prolog implementation (PDC and Strawberry are notable exceptions.)
Logtalk can be employed to specify a prototype-based framework: it lives in symbiosis with a prolog implementation. PDC Prolog is fundamentally class-based, but its specification of interfaces is so flexible that nothing much is lost: a interface can be used in an implementation of a class even if not declared by the class - which provides almost as much as working with objects as prototypes. There are other type-based OOP projects with a similar strength: TiScript and the XOTcl extension to Tcl which allows an object of a class to also declare its own attributes.
Some years ago I built a dynamic aerobatic squadron generator for a flight simulator in PDC Prolog without objects ( takeoff from any airport, perform manoeuvres, land at an airport. It used to run at the home airfield museum display of an aerobat team. That was done in Prolog without an object-framework or constraints (but at least did not require XML, being in Prolog.) If I ever find time or a sponsor to re-write it, it would be hard-sell to move to Oz if that meant losing the object-flexibility of PDC7.1 or Logtalk with any Prolog with constraint resolution.
The PDC install ran flawlessly (an installer for a smart language should be as smart) which certainly is not always the case with interesting languages, e.g., Python or its CMS, Plone.
PDC also comes with its own external database framework which has always been a favorite feature with me and worked very well for my dynamic squadron generator.
Even if you don't remember Turbo Prolog, check out PDC. If you use Eclipse, checkout AMZI! Both sites have great instructional resources. If you need open-source, there is, of course, Logtalk+SWI+java
halt.
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