Amzi! Prolog + Logic Server 3.3 -- Opening the "Closed World" of Prolog

by Arvindra Sehmi

[Used with the kind permission of PC AI Magazine, 602/971-1869, From the Sep/Oct 1995 issue.]

This review examines Amzi! Prolog, a compact, portable Edinburgh standard Prolog, and its Logic Server, a comprehensive and intuitive API for embedding Prolog components in host language environments.


Included in the theoretical foundation of Prolog is the assumption that nothing exists outside of Prolog's logic base of facts and rules. This is called the "closed world assumption." The phrase "closed world" also reminds me of how closed and inaccessible Prolog's logic-based computational paradigm has been for the mainstream programmer.

While strong procedurally, mainstream tools don't provide the power of Prolog's assertional database, symbolic pattern matching and search algorithms, and automatic memory management features.

Wouldn't it be better to use C/C++ and Prolog together--each one for things it is good at rather than fudge the programming paradigm of the one in the other? That may be the obvious thing to do, but on the PC platform, and until quite recently, all established Prolog systems presented their deductive proof machinery to the programmer in a way which was non-intuitive and quite difficult to use from an external procedural host language.

Most Prologs that implement host language interfaces do so in a way that forces the host language programmer to think of a Prolog program as just another procedural extension of the host language. In fact, what we actually need is the ability to treat a Prolog program as a logicbase which responds to queries just like Prolog, and why not? It's Prolog, after all, that we want to access in the first place!

For example, over the last year I was beginning to find that more than 50% of one of my Prolog applications was concerned with getting data into and out of the system, mostly via a graphical user interface, and that Prolog was becoming difficult to use efficiently. I seriously contemplated a port of my application to C/C++ under Windows, yet I bemoaned the loss of all of Prolog's wonderful features. Happily, I came across a pre-release version of Amzi! (then called Cogent) Prolog and its Logic Server API. The Logic Server hit the target. I could do all my procedural tasks in C/C++ yet use Prolog to manage declarative tasks.

Despite some problems with pre-release software (which Amzi! has since resolved) I was left convinced that Amzi!'s approach to interfacing Prolog and procedural languages was the right one. The Logic Server API recognizes that a Prolog program is fundamentally closer to a database than it is to a sequential program. As such, the API presents an interface to C/C++ to interact with the Logic Server that is similar to database interfaces. It lets the C/C++ programmer interact with Prolog in much the same way a Prolog programmer interacts with Prolog at the standard '?-' prompt of a Prolog listener. In my mind this is as intuitive as one can get in an interface to Prolog.

Application Building Tools

The full Professional Amzi! Prolog system includes an Edinburgh-standard Prolog, Prolog development tools and the Logic Server API. It supports 16/32-bit DOS and 16/32-bit Windows. A 16-bit only Professional Edition is available, as well as a Personal Edition that has restrictions on commercial distribution. (Amzi! places no distribution royalties or restrictions on the Professional Editions.) A Student Edition includes the Prolog pieces, but not the Logic Server API. (Non-PC versions are available as well. Contact Amzi! for details.) To keep customers up-to-date with rapidly evolving Windows development environments, the vendor also offers a subscription service that provides automatic updates for a year.

The development tools include the compiler, interpreter, Windows interactive development environment (IDE), interactive debugger, DCG, numerous example programs and technical papers, and hardcopy and on-line documentation. The documentation could stand some consolidation and perhaps a clearer distinction between technical and general aspects of the system.

The Logic Server API provides the tools for integrating Prolog and host languages. It includes static link libraries for a variety of C/C++ compilers and environments and 16- and 32- bit Windows dynamic link libraries (DLL) callable from any Windows development tool that can call a DLL. The vendor provides cover functions for Visual Basic, Access, Delphi, and Smalltalk, as well as a C++ wrapper around the Logic Server.

To implement embedded applications, a developer first uses the IDE to develop and test the Prolog components. Then the Logic Server is used to make that Prolog code available to the host language application.

The Prolog

Amzi! Prolog is close to a 'pure' Edinburgh-standard Prolog with relatively few vendor additions. Because it's based on the Warren Abstract Machine (WAM), supporting both compiled and interpreted (dynamic) Prolog code, you can use it to design 'serious' applications by reusing your own code and drawing upon the masses of freely available public domain Prolog libraries.

Amzi! moves you toward a unique view of its positioning in the Prolog market. It aims to be a component of an application written in other languages. In this vein, it's important to point out that the product doesn't have built-in predicates for constructing GUI/Windows interfaces and it doesn't try to compete with MSVC++ or VB or Delphi. Instead it's designed to work with these other tools.

Thus, I believe that Amzi! Prolog is not the Prolog to use for building large-scale stand-alone Prolog applications for Windows, but that it is the Prolog to use as a component of a larger programming effort. I've endeavoured to capture this philosophy in the design of the application described in the sidebar.


The IDE is the newest part of Amzi! Prolog. It offers some polish to the overall product and provides the essential toolset (listener, standard Edinburgh 4-port debugger, editor, compiler, and linker). These tools are also available in DOS command-line form. Figure 1 shows the IDE window with the sidebar application being debugged via the listener and debug dialog window.

Amzi! IDE Screen Shot

Figure 1

The IDE could use some work on usability aspects of the listener and debugger. Specifically, the listener could do with better use of cutting and pasting of queries from earlier parts of the session window and/or a query history mechanism. The '?-' prompt should be harder to delete. It would be helpful to be able to recover the prompt by hitting the Return key. Compiler error messages can be somewhat cryptic and difficult to interpret.

At present the IDE/debugger is suitable only for standalone Prolog program development, which means you develop the Prolog components of an embedded application separate from the host language development. The Prolog components are compiled and linked into a binary form for loading by the Logic Server.

As you read this review, Amzi! is working on additional tools to enable easier debugging of embedded components from a mixed language environment, and is continuing to improve the IDE.


To use the Logic Server from C/C++ under Windows, you need a Windows C/C++ compiler. You include the header file 'amzi.h' or 'amzi.hpp' in your C/C++ files where you reference API calls, add your Logic Server calls and link the application with one of several Logic Server dynamic or static libraries.

On running the application, API calls are quickly load the Logic Server and rapidly initialize it with the precompiled Prolog component of your application. Then, you call and redo any goals and manipulate returned bindings as you would if the Prolog program was loaded into a Prolog listener. You finish by closing down the Logic Server.

This minimal C program fragment will runs the main/0 predicate in the Prolog program (lsXXXX denotes API functions):

#include <amzi.h>
lsMain();      // calls main/0 in Prolog

Using the API is like using ODBC for databases. It has tools for any type of query and for easily mapping Prolog terms to and from host language variables. The API can

The foreign language extension aspects of Amzi! Prolog are powerful. They let you define new Prolog built-in predicates in the host language (e.g., C/C++) and they become transparent extensions to the underlying Prolog engine. Unlike foreign language extensions in other Prolog systems, Amzi!'s have no strongly typed mappings from Prolog to C/C++ functions. Instead, they can take any type and number of arguments and argument instantiations. The API easily determines argument-type information to direct host control flow. Dynamically typed argument bindings can be constructed and returned to Prolog.

The only disadvantage I see with Amzi!'s approach is that the extended Prolog predicate definitions in the host language have nil arguments (except for the Prolog Engine ID) and therefore some confusion can creep in when maintaining the code correspondence with the predicate/function written in Prolog. The best way around this is simply to make sure you properly document the functions.

Distributing Applications

You can distribute application executables royalty free. You distribute executables with your application's compiled binary Prolog file. You can optionally include Prolog source files intended for dynamic use. You also include a DLL for the Prolog engine, unless you chose to use one of the static link libraries which become part of your executable.

The Windows footprint of the running Prolog Logic Server is typically around 400K. The DLL takes up 180K of that and various Prolog runtime stacks, heaps, etc. might take up another 200K+. You can customize the runtime environment using an .INI file that will allow you tune the Prolog environment based on your application needs.

The Future

All API functions take a Prolog Engine ID which the API initialization function returns. At present this parameter has no use, but the idea is to eventually allow a single application to invoke multiple (possibly multi-tasking/threading) Prolog engines. If this does materialize, you can imagine a Pandora's box of smart applications ready to burst onto the market (e.g., in personal information management, collaborative systems, workflow and process re-engineering, distributed multiple agent systems, intelligent integration of legacy applications, flexible manufacturing systems, scheduling, data interpretation, monitoring and control, and more).


Amzi! Prolog is a solid, commercial grade, no-frills Prolog. Through its Logic Server API it's ideal for embedding. With a few more improvements to the IDE to support debugging embedded components, the product will be a breeze and a pleasure to use. Host language integration is easy.

I believe that few applications can do without a symbolic program component. You can do sophisticated things so quickly that arguments against using Prolog, say, in small specific doses, are very weak indeed. Weary programmers looking for new tricks, something to add pepper and spice to your applications, should look to the Amzi! Logic Server and enter the "opened world" of Prolog.

Arvindra Sehmi holds a Ph.D. in Biomedical Engineering from Leicester University, U.K. A user of Prolog for applied AI research for 8 years, he is investigating distributed multi-agent AI and nearing completion of an MBA in the Business School of the Open University.

A Blackboard Application

wxTinyBB is a small Blackboard (BB) Knowledge Base System that illustrates the use of Amzi! Prolog and its Logic Server. The application is a hybrid of C/C++ and Prolog. The BB logical inferencing is implemented under Prolog, whereas the GUI and procedural control is implemented using VC++ and the public domain wxWindows GUI C++ class library. The choice of these non-Prolog tools is for my own benefit. Choose any others to suit yourself--the Logic Server imposes no restrictions.

The BB system is composed of independent knowledge sources (KS), which cooperate in problem-solving with the other KSs, communicating via a central BB. Each KS is triggered by and responds to changes on the BB. An active KS will invoke its rules and procedures and post its conclusions on to the BB. The new state of the BB is then available to be read by any of the KSs (including the one that has just acted). These, in turn, make further modifications, and so on.

In this implementation, the user enters information through the GUI. It is then stored on the BB through Logic Server calls. The Prolog BB system is then activated and responds to the changed state and sends its current recommendations to the main output window. The output is implemented through extended predicates that let Prolog code talk directly to the window.

The Prolog predicate that activates the BB is

activate_ks :-

It is called from C like this

   tf = lsExecStr(engid, &term, "activate_ks");

As the code progresses information is written from Prolog code such as this


c_wxWrite/1 is a custom extended predicate, that takes its Prolog argument and writes it to a window using wxWindow functions. This particular implementation shows how wxWrite/1 can handle each Prolog type differently, although it isn't necessary--a Logic Server function converts any given term to a string.

/* define Prolog predicate to C function map */
PRED_INIT extPreds[] =
   {"c_wxWrite",      1,      p_wxWrite}, 
// c_wxWrite/1 : c_wxWrite(+Term)
TF p_wxWrite(ENGid eid) {
   extern char szBuffer[];
   pTYPE pt;
   int i;
   // get type of input parameter #1
   pt = lsGetParmType(CurEng, 1);
   switch (pt) {
      case pATOM:  // map Prolog atom to C string
         lsGetParm(eid, 1, cSTR, szBuffer);
      case pINT:   // map Prolog integer to C integer
         lsGetParm(eid, 1, cINT, &i);
         sprintf(szBuffer, "%d", i);
   // display parameter string in GUI textwindow
   wxPrintf("%s", szBuffer);

The list box in the figure below displays the observations. It illustrates the easy mapping between Prolog lists and GUI list boxes. The possible observations are held in Prolog as a list.

   'radar1(positive)', 'radar2(positive)', ...

The Prolog list is mapped to the listbox with these Logic Server calls. The first call queries the logic base, the second call gets the first argument, which is the desired list, and the while loop walks through the list, popping each element from the Prolog list into the wxWindows listbox.

   lsExecStr(eid, &t, "observations_list(ObsL)");
   while (OK == lsPopList(eid,&t2,cSTR,szBuffer))

I've posted the full wxTinyBB example on Amzi!'s FTP site. In addition to the Logic Server, the example illustrates blackboard systems, intelligent agents and truth maintenance. (Via the web, or via FTP, in directory /pub/demos/

Tiny BB Screen Shot

The listbox illustrates the easy mapping between Prolog lists and GUI listboxes.