[This article was originally published in PC AI magazine, Jan/Feb 2001. The magazine can be reached at PC AI, 3310 West Bell Rd., Suite 119, Phoenix AZ, USA 85023 Tel: (602) 971-1869, FAX: (602) 971-2321, E-Mail: email@example.com, Web: http://www.pcai.com]
With the spread of computers and the Internet, the opportunities for capitalizing on intelligent systems has expanded tremendously. All sorts of different applications beg for the embedded expertise that sets them apart from the competition.
The problem is, its just not that easy to build intelligent components. Why? because the intelligence from one domain is invariably different from the intelligence in another. And the people with the intelligence to encode are not, in general, programmers.
Ideally, for any given domain you would want:
The encoding lets the expert work with the knowledge in a comfortable, familiar format; the reasoning engine ensures there are no surprises when the knowledge is applied; and the interface lets the programming staff integrate the component where it needs to be.
Now it might not seem practical to implement this type of domain-specific expert system tool, but it is. Prolog is an extremely powerful tool for implementing exactly such systems, especially if the common parts of the system are factored out into reusable libraries of code.
Consider a couple of examples.
Cole Davis (firstname.lastname@example.org) is an organizational psychologist building a jobs advisor based on a combination of psychological personality trait profiling and conventional constraints. He has a way of representing jobs and their attributes that is the base knowledge of his system. There is a specific way he would like to see that knowledge applied and wants this as a component of a larger VB application.
Ken Burres is a Doctor and top endurance athlete. His company, FitCentric (www.fitcentric.com), offers, as well as some really neat virtual reality products, a coaching tool for serious athletes. They want to add to the system an expert advisor that makes workout recommendations based on a large number of physiological factors. The purpose is to help an athlete get to his or hers optimal performance. The knowledge for each factor is stored in tables, and draws on a database of athlete information. The tables are combined to make a composite recommendation. The advisor initially will be part of a VB application and might make it to the Web in the future.
Amzi! is working on a technical support system. Conventional rule-based tools have been used in this context before, but they are often difficult for technical support people to program because they do not map directly to the problem. A solution object that encapsulates the environments, conditions and actions is easier to maintain and fits better to the domain. And a reasoning engine that searches for solutions is whats expected. A Web interface would be nice on this one.
Prolog is unique among programming languages in that it has, built into the language:
These features are ideally suited for the type of domain-specific tool we are talking about.
Prolog patterns, called terms, are built from simple components, but can be arbitrarily complex. It is these that can be used to model the knowledge representation of the domain.
Now you probably know that Prolog stands for programming in logic. It is a logic programming language, which is made possible by the features mentioned above. The result is that Prolog code is very close to a logical specification of a program.
And thus the reasoning engine falls as a straight forward implementation in Prolog. The knowledge is stored in Prolog terms, and the logical specification of how the knowledge is to be used maps directly into Prolog code.
Put a callable interface on it, and were done from the top level view. Lets expand some of these ideas, and see what sorts of tools we can implement to make the task as easy as possible.
Frames are object-like structures that can be used to hold any sort of information. A frame has a type (class) and slots or properties with values, which can be any sort of thing we would like. They are a perfect generalization for implementing the domain specific knowledge we are talking about.
The jobs advisor would need frames something like:
personality = artistic, investigative, practical
constraints = masters degree, age > 21
job social worker
personality = social, enterprising, artistic
constraints = college degree, age > 21, no criminal record
A technical support solution frame might look like:
environment = windows NT, version 4
conditions = system hangs and new software recently installed
explanation = software installations can cause instabilities
in Windows NT 4
action = reinstall service pack 4
Notice that the solution frame recognizes that, for tech support, it is important to segregate solutions by environment. This lets solutions be upgraded as necessary for different operating system releases, product releases and the like. The conditions then can refer to the particular problem at hand.
Prolog terms are made up of atomic words and numbers, structures with arguments in parentheses, lists in square brackets, and operators. These can be combined in arbitrarily complex ways and lend themselves perfectly to implementing frames.
So here is the architect frame represented as a pure Prolog term:
personality = [art,investigative,practical],
constraints = [ degree = masters, age > 21 ]
And heres the tech support solution frame:
environment = [operating_system = Windows NT, version = 4],
conditions = [ symptom = system hangs and recent_software_install = yes], explanation = Software installations can cause instabilities in Windows NT 4, action = Reinstall service pack 4
As you can see these are pretty close to how the experts might want to work with the knowledge. Now these could be entered in a text file, and that works, but one does have to be careful of the punctuation. Using a generic frame editor as a front-end on the Prolog terms makes maintenance of the knowledge a lot easier.
Figure 1 A frame editor showing two types of frames from the FitCentric prototype application. (The example is illustrative for a prototype, not accurate.)
Note that the factor frame in the FitCentric application encapsulates all of the knowledge about a particular factor. This includes the conditions under which the factor applies, and rules for determining the weight to be given this factor when combined with others. The table shows the relationship between the factors values and the workout recommendations, which can also have weighting factors applied to them.
The key point here is that all of this knowledge might have been encoded using conventional expert system rules, but the knowledge of a factor would be dispersed among several smaller chunks of rule knowledge, thus making for more difficult maintenance. The single frame brings it all together for the knowledge base developer.
The specifications for the reasoning engine are really the expectations of how the domain expert expects the knowledge to be used.
The job advisor expert expects to find all the jobs suited for a person, where suited means that first the job matches their personality profile, and second it meets the constraints.
The tech support person wants the system to look for a solution, first filtering by the environment, and then by the particular conditions. The support seeker would then be given an explanation of the problem and the actions to take.
The FitCentric expert expects all of the factor tables to be combined according to the weights and conditions into an aggregate recommendation. It is also expected that the system will go out and automatically gather the bits of information it needs, such as current data from the athlete and past data from the database.
We can use the job advisor to get a sense for how easily Prolog can take the knowledge structures and use them in implementing the reasoning specification.
Rephrasing slightly the spec: find all of the jobs where the users personality attributes are a subset of the attributes for the job, and the constraints are all met.
And writing the Prolog code:
This code is compact and close to the logical specification because it uses unification to find jobs, and backtracking to get all of the jobs that match the constraints. It will behave as the domain experts expects because it will only call the constraints_match if the personality_match succeeds. This all happens automatically in Prolog, which is why the language is so powerful and productive for this type of application.
The two matching functions (predicates in Prolog jargon) are similarly compact and straight forward. They use recursive list processing techniques to walk the lists checking for conditions under which they apply. In the case of personality, the function looks for a subset relationship; in the case of constraints, the function checks each constraint against information gathered from the user.
As to the expressions in the constraints, it is trivial to add to the application any sorts of operations desired, in addition to the = and > operators shown.
Each of the examples require asking questions of the user, to get information such as degree or operating system or current_weight. The prompts and menu choices for these interactions are really part of the knowledge, and can be stored in frames as well.
prompt = 'What is your highest degree?',
type = single_value,
menu = [ high_school, college, masters, phd ]
Clearly the code to use these question frames can be applied to many different expert systems.
Similarly, code to process if-then rules and goal-seeking logic can also be reused for many types of problems, as can other frames and code to define database queries or e-mail actions to be sent.
The FitCentric has its customized factors, but they draw heavily on existing code for processing rules, conditions, date arithmetic, and database queries.
Most intelligent components will need to carry on some type of dialog with the calling program. In general, this dialog will be of the form:
This pattern would certainly work for the job advisor and tech support systems described here. Standardizing on an API that follows this convention allows calling programs to interact with intelligent components in the same way, whether they are highly customized for a problem domain or not.
We mentioned the possibility of a frame-based graphical editor. Well that same tool could make use of a standard API, and thus be able to provide both editing and test capabilities for any customized tool that used frames for knowledge and the API for communicating with its caller.
This was done with the FitCentric prototype, which allows the knowledge base to tested from the editing tool, but run in production from VB.
Weve seen variations on these ideas, where conventional databases were used to store the knowledge frames, so knowledge creators could work using standard database tools. Another customer implemented a Web-based front-end that walked knowledge creators through the construction of the knowledge frames for that domain.
With Prolog handling the tricky bits, and a clean interface to other user interface tools, it is possible and practical to deploy expert systems that can be directly maintained by the experts themselves.