Hints on Prolog

SWI Prolog is the default prolog on the IT and the Grad suns.

To start Prolog type module load prolog; prolog' at the unix prompt. You need to type in the command 'module load prolog' only the first time you use prolog in each login. If you prefer, you can make this step automatic by adding the module 'prolog' to the list of modules that are added to you path when you login. This is done by modifying the module command line in your .cshrc file.
To get out of Prolog type followed by the letter e or use the Prolog predicate halt. interrupts the execution of the program. Type h to see what commands are available.
When Prolog starts it is expecting a goal (the prompt is ?-) not facts or clauses. You should write facts and clauses into a file and then consult the file by typing [filename] to the prolog prompt. Conventionally prolog files use the suffix .pl, which is assumed by default when the file is consulted.
A % indicates that the rest of the line (up to the end of line) is a comment. Alternatively, everything bewteen /* and */ is considered a comment.
Upper case names are used for variables, lower case for constants.
Do not forget to end each clause or goal by a period.
The typing format is free, you can write clauses in as many lines as you want, using a . to terminate the clause.
Prolog will give you the first soilution. You can obtain additional solutions by typing ; after each solution is returned.

A simple prolog example is in the file parent. In this example the file name is parent and not parent.pl to avoid problems with Netscape. The command to consult the file would have been the same even if the file had the .pl extension. Here is a sample run.

dante> pl
Welcome to SWI-Prolog (Version 4.0.0)
Copyright (c) 1990-2000 University of Amsterdam. 
Copy policy: GPL-2 (see www.gnu.org)

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- [parent].
% parent compiled 0.01 sec, 1,792 bytes

Yes
2 ?- grandparent(X,Y).

X = john
Y = bill ;

X = john
Y = dan ;

No
3 ?- grandparent(john,X).

X = bill ;

X = dan ;

No
4 ?- parent(X,Y).

X = sonja
Y = mary ;

X = sonja
Y = jane ;

X = john
Y = jim ;

X = john
Y = bob ;

X = bob
Y = bill ;

X = bob
Y = dan ;

No
5 ?- listing.

mother(sonja, mary).
mother(sonja, jane).

father(john, jim).
father(john, bob).
father(bob, bill).
father(bob, dan).

parent(A, B) :-
	mother(A, B).
parent(A, B) :-
	father(A, B).

grandparent(A, B) :-
	parent(A, C),
	parent(C, B).

Yes
6 ?- halt.
dante>

Prolog assumes that all predicates with the same head appear together in the program, and if they are not together there could be a typing error. Prolog also assumes that if you use a variable in the head of a clause and not in the body of the same clause it must be a typing error. Prolog issues warn- ings that can be safely ignored.
Comparison of numbers can be performed using >=, =<, >, < as relational operators in infix form. These operators require both their arguments to be bound to a numerical value. For instance if you write something like:
```
commencement(Date):- Date >= 1982.
```
the variable Date needs to be bound when the goal commencement(Date) is executed. Otherwise the comparison Date>=1982 generates an error message because >= expects to compare two numbers.
To assign a value to a variable you need to use the operator is, which is used in infix notation, as in the example shown here:
```
age(X,Age,now):- born(X,Date), Age is now-Date.
```
The operator 'is' assigns the value of the expression now-Date to the variable Age, so when the goal succeeds Age is bound.
The negation is called \+ and it is used as a prefix operator without parentheses, as shown here:
```
married(X):- \+single(X).
```
Since negation can be expensive (because of the close world assumption) is better to avoid using it, mainly when the negated predicate has unbound arguments.
To enter new facts during the execution you can use the special directive [user] as shown here:
```
    | ?- [user].
    | age(john,70).
    | single(john).
```
.... etc ... The system types the | to remind you that you are entering information, not trying to prove a goal. When you are done you type control-D.
To debug programs you can use trace or set spy points. The built-in predicate trace/0 switches the debugger on, untrace/0 switches it off. The built-in predicate spy/1 allows you to specify which predicates you want to spy when they are used. nospy/1 removes the spy points. The built-in predicate debugging/0 will show current spypoints and other degugging options.
```
    ?- spy(parent).
    ?- spy(parent/2).
    ?- spy([parent/2,born_in_UK]).
```
The reference manual for is available on-line. The relevant commands are: help, which explain the help-system briefly, help(Topic) which shows the help-page on the topic, and apropos(String), which show all topics holding String in their summary documentation.