1. Prolog Primarily for Symbolic (nonnumeric) Computation
Suited to Problems Involving Objects and Relationships
A Step Toward Declarative Programming.

2. Example: Family Tree always hold. parent(pam, bob). parent(tom, bob).
ann
jim
liz
pat
parent(pam, bob).
parent(tom, bob).
parent(tom, liz).
parent(bob, ann).
parent(bob, pat).
parent(pat, jim).
Facts: Relations that
always hold.

3. Yes/No Queries
?- parent(bob, pat).
yes
?- parent(liz, pat). no ?-

4. Queries with Variables
?- parent(X, bob).
X=pam;
X=tom; no
?- parent(X, Y).
X=pam
Y=bob
yes

5. Compound Queries ?- parent(Y, pat), parent(X,Y). X=pam Y=bob; X=tom
yes

6. Exercises 1. Who is Pat's parent? 2. Does Liz have a child?
3. Who is Pat’s grandparent?
4. Find a great-grandparent.

7. Adding Inference Rules
For all X and Y
Y is an offspring of X if
X is a parent of Y
offspring(Y,X):-
parent(X,Y).
Rules: Conclusion is True if Conditions Satisfied.
All Rules Have Two Parts:
Conclusion, Consequent or Head (Left-hand side).
Antecedents, Condtions or Body (Right-hand side).
A Fact is a Rule with no Conditions.

8. Exs: Sibling, Sister, Grandparent

9. Exs: Sibling, Sister, Grandparent
sibling(X,Y) :-
parent(P,X),
parent(P,Y),
X \= Y.

10. Exs: Sibling, Sister, Grandparent
sister(X,Y) :-
sibling(X,Y),
female(X).

11. Exs: Sibling, Sister, Grandparent
grandparent(X,Y) :-
parent(P,Y),
parent(X,P).

12. Summary So Far Prolog Program is a Set of Clauses.
Prolog Clauses: Facts, Rules, Queries.
Facts declare unconditional truths.
Rules declare truths based on conditions.
Queries are used to ask Prolog what is true.
Prolog Clauses have a head and body.

13. How Prolog Works p1(foo). ?- p1(X). p1(X) :- p2(X,Y), p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).
?- p1(X).

14. p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).
goal: p1(X)
return: X=foo;

15. How Prolog Works p1(foo). goal: p1(X). p1(X) :- return: X=foo;
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).
goal: p1(X).
return: X=foo;
REDO

16. How Prolog Works p1(foo). p1(X) :- goal: p1(X) goal: p2(X,Y) p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

17. How Prolog Works p1(foo). goal: p1(X) p1(X) :- goal: p2(X,Y) p2(X,Y),
X=foo, Y=bleen
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

18. How Prolog Works p1(foo). goal: p1(X) p1(X) :- goal: p2(X,Y) p2(X,Y),
X=foo, Y=bleen
goal: p3(bleen)
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

19. How Prolog Works p1(foo). goal: p1(X) p1(X) :- goal: p2(X,Y) p2(X,Y),
X=foo, Y=bleen
goal: p3(bleen)
FAIL
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

20. How Prolog Works p1(foo). goal: p1(X) p1(X) :- goal: p2(X,Y) p2(X,Y),
X=foo, Y=bleen
REDO
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

21. How Prolog Works p1(foo). goal: p1(X) p1(X) :- goal: p2(X,Y) p2(X,Y),
X=bar, Y=baz
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

22. How Prolog Works p1(foo). goal: p1(X) p1(X) :- goal: p2(X,Y) p2(X,Y),
X=bar, Y=baz
goal: p3(baz)
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

23. goal: p1(X)
goal: p2(X,Y)
X=bar, Y=baz
goal: p3(baz)
yes
return: X=bar ;
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

24. How Prolog Works goal: p1(X) p1(foo). goal: p2(X,Y) p1(X) :-
X=bar, Y=baz
goal: p3(baz)
REDO
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

25. How Prolog Works goal: p1(X) p1(foo). goal: p2(X,Y) p1(X) :-
X=bar, Y=baz
goal: p3(baz)
FAIL
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

26. How Prolog Works goal: p1(X) p1(foo). goal: p2(X,Y) p1(X) :-
X=bar, Y=baz
REDO
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

27. How Prolog Works goal: p1(X) p1(foo). goal: p2(X,Y) p1(X) :- FAIL
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

28. How Prolog Works goal: p1(X) p1(foo). REDO p1(X) :- p2(X,Y), p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

29. How Prolog Works goal: p1(X) p1(foo). return: X=snork ; p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

30. How Prolog Works goal: p1(X) p1(foo). return: X=snork p1(X) :- REDO
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

31. How Prolog Works goal: p1(X) p1(foo). FAIL p1(X) :- p2(X,Y), p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

32. How Prolog Works goal: p1(X) p1(foo). return: no p1(X) :- p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

33. How Prolog Works ?- p1(X). p1(foo). X=foo; p1(X) :- p2(X,Y), X=bar;
X=snork; no ?-
p1(foo).
p1(X) :-
p2(X,Y),
p3(Y).
p1(snork).
p2(foo,bleen).
p2(bar,baz).
p3(baz).

34. Is That All There is to It?
Yes.
But there is more to know:
Built-in predicates
Designing Algorithms (programming)
Predicates are sub-programs
Decisions (ifs) are Predicates with multiple clauses
Loops via recursion or Repeat/Fail
Variables don't vary.
Data Structures: Complex terms

35. Ex: Temperature Conversion
Conversion “Function”
% convert.pl
convert(Cel, Fahr) :-
Fahr is 9/5*Cel+32.

36. Interactive Conversion
?- [convert]. (Loads the code)
% convert compiled
?- convert(100, X).
X = 212
yes ?-

37. Adding Input/Output run :- write('Enter a Celsius temp: '), read(C),
convert(C, F),
write('The temp is '),
write(F),
write(' degrees Fahrenheit.').

38. Running the Enhanced Version
Enter a Celsius temp: 100.
The temp is 212 degrees Fahrenheit.
Yes ?-

39. Adding a Decision If temp is greater than 90: print a heat warning
if temp is less than 30:
print a cold warning
warn(T, 'It''s really hot') :- T > 90.
warn(T,'Brass monkey danger') :- T < 30.
warn(T, '') :- T >= 30, T <= 90.

40. Enhanced main program run :- write('Enter a Celsius temp: '), read(C),
convert(C, F),
write('The temp is '), write(F),
write(' degrees Fahrenheit.'),
nl,
warn(F, Warning),
write(Warning).

41. Version with Decision ?- run. Enter a Celsius temp: 100.
The temp is 212 degrees Fahrenheit.
It's really hot out!
Yes ?-

42. Retricting Backtracking
warn(T, 'It''s really hot') :- T > 90.
warn(T,'Brass monkey danger') :- T < 30.
warn(T, '') :- T >= 30, T <= 90.
! (called "cut") causes Prolog to commit to choices.
warn(T, 'It''s really hot) :-
T > 90, !.
warn(T, 'Brass monkey danger') :-
T < 30, !.
warn(_,'').

43. Adding a Loop Sentinel Loop As a Decision Get an Input
While input is valid number
Convert to Fahrenheit
Output temp and warnings
Get next input
Get an Input
if input is a number:
Do the conversion
Get another input
Do the loop again
if input is 'quit' just do it. 11

44. Using Recusion to Loop run2 :- getInput(C), convert_loop(C).
number(C),
convert(C,F),
...
getInput(C1), convert_loop(C1).
convert_loop(quit).
If input is a number:
Do the conversion
Get another input
Do the loop again
if input is 'quit' just do it.

45. Repeat-Fail Loop run3 :- repeat, write('Enter a Celsius Temp: '),
read(C),
process(C),
C = quit.
process(C) :-
number(C),
convert(C,F),
write('The temp is '), write(F),
warn(F, W), write(W), nl.
process(quit).

46. Structured Data