Tutorial 1


Key Features of Prolog

Defining Relations with Facts

Here is a simple Prolog database comprising some facts. Each fact each states a certain pair of individuals are in the "likes" relation.
Note that


Once a database has been written in a file and loaded into Prolog, the system issues a prompt


and awaits a query. At this point the user can type in a query terminated with a period. In the examples below, user input appears in italics.

Simple Queries

?- likes(john, fish).
Prolog searches the database for a matching clause. If it finds one, the query succeeds, as it does here, and as indicated by the "yes" response. Note that a simple query is just another kind of clause and has the same general shape as a fact (that's why it has to be terminated with a period).

If there is no matching clause, the query fails. This is indicated by the response "no".

?- likes(john, money).

Queries with Variables

Most of the time we issue queries in order to associate values with variables. Variables are symbols that begin with an uppercase letter. The next query corresponds to the English question "What does John like?", or more precisely, "Is there an X such that John likes it?". When a query contains a variable, Prolog searches the database for matching clauses, as before. The difference is that when it finds one it reports the corresponding variable binding. At this point the user can enter a carriage return (written as [return] below), whereupon the query succeeds as before.
?- likes(john,X).
X=book [return]

Alternatively, the user can enter a semicolon, thereby instructing Prolog to continue the search for other matching clause, which, if it exists, is reported in the same way.
?- likes(john,X).
Note that

Matching Queries Against Facts

Clearly, the success of a query depends upon there being a match between query and fact. To summarise the above:

Conjoined Queries

So far we have looked at simple queries. A more complex kind of query can be formed with one or more simple queries separated by a comma, as shown in the next example, which corresponds to the English question "Does John like Mary and Mary like John?".
?- likes(john,mary), likes(mary,john). 
Note that The latter point is best illustrated by means of the following example:
?- likes(john,X), likes(mary,Y).


Note carefully the steps followed to yield these solutions:
  1. Prolog first attempts to satisfy likes(john,X). It finds the first matching clause with X=fish.
  2. It then proceeds to likes(mary,Y), which succeeds with Y=book.
  3. Both subgoals are satisfied and the variable bindings are printed.
  4. The user types a semicolon. Internally, this initiates a failure and a subsequent attempt to resatisfy the last goal.
  5. There is no way to resatisfy likes(mary,Y) so the failure is propagated backward to the moment when immediately preceding goal left off. Can that subgoal be successfully resatisfied?
  6. It can - a second matching clause is found, with X=mary. We have now succeeded in resatisfying the first subgoal.
  7. Next Prolog attempts the second subgoal - afresh, since all record if the previous attempt was "forgotten" when we went back in time.
  8. The second subgoal is satisfied - again Y=book.
  9. The second solution is printed, and the user types a semicolon, provoking the third solution.
  10. When the user types a semicolon for the third time, there is no way to resatisfy the first goal, so the failure is propagated right back to the user level.
  11. The failure is reported with "no"

Multiple Occurrences of the Same Variable

The last example involves the two distinct variables X and Y whose values are independent of each other. Clearly, the range of expressible queries is enhanced if we envisage multiple occurrences of the same variable. For example, the English query "What is it that John and Mary both like?" can be expressed by
?- likes(john,X), likes(mary,X).
Note that the mechanics of attempting to satisfy this query is essentially similar to that of the last example. The only difference is that the contents of the second goal is now linked to the first through the value of the variable X. Prolog thus attempts to solve the second goal with X=fish, X=mary, and X=book (in that order), but it only succeeds with the last of these.
Contents of this tutorial
Exercise on the contents of this tutorial
How to run Prolog
Tutorial 2
Mike Rosner (mros@cs.um.edu.mt)
Last modified: Wed May 21 16:44:12 MET DST