Prolog Quick Start

On the CS departmental machines, the prolog interpreter is /usr/bin/pl. It should automatically be in your PATH environment variable.

You first need to write a prolog program. Cut and paste the following into a file called test.pl (your programs must end in .pl):

% Percent signs comment out the rest of the line

% parent(X,Y) = "X is the parent of Y".
% this prolog program has no rules (clauses with bodies) only facts
parent(bob, joe).  %fact - all prolog predicates end in a period
parent(bob, tim).  %fact
parent(sue, joe).  %fact
parent(joe, sally).   %fact

To start prolog, type: pl

To load your program, type: [test].

Note that you leave off the .pl from the end.
Note also that you must end each line with a period.
Note also that there may be warnings about singleton variables when you load your files -- which you can ignore.

If you forget the period, you will be given a prompt that is a vertical line. This is just a continuation of the previous line. Type a period (.) to end it. For example:

?- parent(bob,X)		%oops forgot the period
|    .				% | means I'm waiting for more of the previous line
				% I typed a period here
X = joe ;			% first answer.  I typed a semicolon here
				%    (which says, "look for more answers")
X = tim ;			% second answer.  I typed a semicolon here
				%    (which says, "look for more answers")
No				% no more answers found

?-				%ready for next query
?- parent(bob,X).		%same query again

X = joe 			%this time I don't type ;  instead I press return
				%  the system stops looking and returns true (Yes)
Yes
?-				%ready for next query

Remember that if first letter is lowercase, the parameter is a constant, eg bob above. If the first letter is uppercase, eg X above, the parameter is a variable that you are asking prolog to CONSTRUCT a value/constant for that makes the predicate true. You can pass in multiple variables if they make sense for the predicate. Commonly only a single variable is sent in (in addition to multiple constants). Constants can be string names (in lowercase) or constant values, or lists, eg, [1,4,3]
The syntax: [X|Y] (using any variable names) means to take the list that is passed in and to break it up into two pieces. X is instantiated with a value, in particular the first element of the list passed in. Y is instantiated with a list, the list X without its first element. The empty list has no head and no tail. A list with one element has a head and the empty list as the tail. For example:
```
%getsecond is a predicate that takes a list and returns the second element of
%the list passed in.  The second parameter should be a variable - which prolog
%will construct to be the first element of the list passed in.
getsecond([H|T],Y) :- gethead(T,Y).
gethead([H|T],H).  % no need for a body, simply assign H to the return value
```
type:
```
?- halt.
```
to exit prolog.
You can read in a file again say if you changed things by just doing what you did above:
```
?- [fname].
```

If you want to see the steps that the prolog interpreter takes as it executes type trace. For example:

[ckrintz@ella ~]$ pl
Welcome to SWI-Prolog (Multi-threaded, Version 5.2.13)
Copyright (c) 1990-2003 University of Amsterdam.
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

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

?- [tmp2].
Warning: (/cs/faculty/ckrintz/tmp2.pl:5):
        Singleton variables: [X]
% tmp2 compiled 0.00 sec, 696 bytes

?- trace.

Yes
[trace]  ?- len([1,2],X).
   Call: (7) len([1, 2], _G290) ? creep    	%orig call
   Call: (8) len([2], _L206) ? creep		%first recursive call
   Call: (9) len([], _L225) ? creep		%second recursive call
   Exit: (9) len([], 0) ? creep			%base case
^  Call: (9) _L206 is 0+1 ? creep		%second recursive call using return from base case
^  Exit: (9) 1 is 0+1 ? creep			 
   Exit: (8) len([2], 1) ? creep		%first recursive call using return from second recurs. call
^  Call: (8) _G290 is 1+1 ? creep		
^  Exit: (8) 2 is 1+1 ? creep			
   Exit: (7) len([1, 2], 2) ? creep		%orig call

X = 2 ;

No