A Simple Expert System in Prolog Recently, I had to write an "intelligent" tourist advisor in Prolog. It was only supposed to be very simple and basically help me learn the basics of Prolog beyond the typical family tree-type programs. In this article we will look at slightly more complicated rules and comparisons. What I am aiming at is to allow you to write programs with an integrated i ntegrated "database" that will allow some relatively powerful queries. Note that since this article was originally published published as the Prolog Tutorial Part II, it is assumed that your knowledge of Prolog is very limited. If you have never worked with Prolog before, please read the first installment of the Prolog tutorials first.
The Facts Obviously, the first part of the program consisted of all the facts. Now the registerEvent fact was quite long, since it consisted of a name, place, the starting month, the starting date, the end date (assuming same month), a rough classification and an event genre. I only created a 16 places in the database. /******************************************************* * registerEvent(Name, Place, Start-Month, Start-Date, * * End-Date, Classificati Classification, on, Genre) * *******************************************************/ registerEvent(grungefest2000, bradford, 9, 12, 14, adolescent, music). registerEvent(moti-curry-house, bradford, -1, -1, -1, cheap, restaurant). registerEvent(the-national-russian-jazz-ensemble, bradford, 1, 1, 1, family, music). registerEvent(horrorfest-2000, leeds, 6, 4, 6, adult, cinema). registerEvent(kiddyland-gokarting-championships, leeds, 10, 4, 4, family, sport). registerEvent(dust, leeds, -1, -1, -1, adult, nightclub). registerEvent(the-old-b registerEvent(the-old-bar, ar, leeds, -1, -1, -1, cheap, restaurant). registerEvent(est-est-e registerEvent(est-est-est, st, leeds, -1, -1, -1, expensive, restaurant). registerEvent(corn-exch registerEvent(corn-exchange-mall, ange-mall, leeds, -1, -1, -1, family, shopping). registerEvent(manchester-city-zoo, manchester, -1, -1, -1, family, animal). registerEvent(art-erotica-exhibition, manchester, 4, 10, 14, adult, art). registerEvent(bbc-radio-5-live, manchester, 6, 7, 8, family, music). registerEvent(steak-house, manchester, -1, -1, -1, medium, restaurant). registerEvent(york-equine-festival, york, 5, 7, 14, family, sport). registerEvent(york-science-museum, york, -1, -1, -1, family, science). registerEvent(the-flying-horse-pub, york, -1, -1, -1, medium, restaurant). Obviously, the first thing that you'll notice is that all the place names look horrible. This is an unfortunate byproduct of Prolog's syntax, since anything with a capital letter is a variable. While they could have been encompassed in quotation marks, I decided to keep things simple. foodAvailableAt(grungefest2000, cheap). foodAvailableAt(corn-ex foodAvailableAt(corn-exchange-mall change-mall, , medium). foodAvailableAt(manchester-city-zoo, expensive). foodAvailableAt(york-eq foodAvailableAt(york-equine-festiv uine-festival, al, medium). foodAvailableAt(york-science-museum, medium). distance(york, leeds, 50). distance(york, manchester, 100). distance(leeds, manchester, 75). distance(leeds, bradford, 25). distance(manchester, bradford, 90). distance(york, distance(york, bradford, 60).
The other facts are pretty self explanatory. One thing to note at this juncture is that the distance fact is not reflective. In that, if you typed "distance(leeds, york, X)", SWIProlog would answer "No". What I didn't want to have to do, though, when designing this program is have to create two facts for every distance between each city. So, moving swiftly on...
The Rules We now have to introduce some new things. In Prolog, when you separate rules we've used the comma (,) so far. This makes the rules inclusive (AND). What do we do if we want the rules to be exclusive (OR)? We use a semicolon (;). A simple example of this will be shown soon, but not before I introduce one more thing. Comparisions are an intrinsic part of any programming language, and Prolog is no exception. Prolog has all the standard comparisions, but they tend to differ slightly from most languages: Comparision
Prolog Syntax
Greater than
>
Less than
<
Greater than or equal to
>=
Less than or equal to
<=
Equal
=:=
Not equal
=\=
Now we can look at our first rule: /************************************* * U T I L I T Y F U N C T I O N S * *************************************/ near(X,Y)
:- X = Y ; (distance(X,Y,D), D =< 50) ; (distance(Y,X,D), D =< 50). near will return 'Yes' if two cities are close. Firstly, we check whether the two cities we are checking for are in fact the same city (leeds is close to leeds!). Don't ask me why we have to use the standard equal sign for constants, it is rather confusing, I know. Notice the semicolon (OR), now we want to check whether the distance between the two cities is close. If the distance between X and Y OR Y and X is less than or equal to 50, the cities are close. We check both for X and Y and Y and X because of that non-reflective property I discussed above. getDates(E,M,Ds,De) :- registerEvent(E,_,M,Ds,De,_,_). allYearEvent(E) :- registerEvent(E,_,-1,-1,-1,_,_). suitableForKids(E) :- registerEvent(E,_,_,_,_,family,_). These rules are relatively easy. Note that the underscore tells the Prolog system that we don't care about that information. Note that if all the date information is set at -1, then the event is an all-year event. The suitableForKids rules basically aids readability of the program further down. /************************************* * E N D U S E R F U N C T I O N S * *************************************/ getEvent(E,P,C,G) :- registerEvent(E,P,_,_,_,C,G). partyEvents(E,M,D,P) :- getEvent(E,Ep,family,G), near(P,Ep), G \= shopping,
getDates(E,Em,Ds,De), (allYearEvent(E) ; (Ds =< D, De >= D, Em =:= M)). The partyEvents rule is fairly complicated. Firstly, we get a family event (assuming E is passed as a variable), then we check whether it is near the place P that was passed. Now, kids hate shopping as a party exercise, so make sure the genre is "shopping". Then, we need to check the dates, so we retrieve the necessary dates. Finally, the event either needs to be an allYearEvent or we need to make sure the date passed resides between the dates of the event, and the month passed is equal to the event month. Phew... nearEvents(E,P) :- near(P,X), getEvent(E,X,_,_). foodAvailable(E,T) :- foodAvailableAt(E,T) ; (getEvent(E,_,T,G), G = restaurant). ...and that's a wrap.
Example Queries Here are some examples of how you would use the program: Is there anything suitable for a party for someone whose birthday is on the 4th October in or near Leeds. •
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? partyEvents(X,10,4,leeds). Is there food available at the York Science Museum, if so how expensive is it?
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? foodAvailable(york-science-museum, P). List all events in or near York.
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? nearEvents(E,york). Get all adult events in Leeds ? getEvents(E,leeds,adult,_).
Conclusion If you would like to download the program in its entirety, you may do so here! So now you should be able to create relatively useful programs that can use inclusive and exclusive rules and comparisions
