1. 28-4-2008P-Phunck - Prolog1 Introduction to Prolog Brian Paden

2. 28-4-2008P-Phunck - Prolog2 History Creators: Alain Colmerauer & Phillipe Roussel  University of Aix-Marseille 1972 Robert Kowalski  University of Edinburgh Named by: Philippe Roussel  programmation en logique First compiler: David H. D. Warren

3. 28-4-2008P-Phunck - Prolog3 More History 1984 – Began working on standard 1987 – ISO Standard Group Formed 1995 – ISO Standard published  Most current New version expected “by early 2000”

4. 28-4-2008P-Phunck - Prolog4 Predicate Logic First order predicate calculus Propositions  Atomic propositions & compound terms Resolution  Inference rule Horn clauses  Single atomic proposition on left side  Empty left side

5. 28-4-2008P-Phunck - Prolog5 Data Types Atom  Letters or numbers'x', 'longConstant17', '“Var”' Variable  Start with uppercase 'X', 'NotanAtom'  Anonymous variable - '_' Fact  'doesntKnow( “Prolog”, brian ).'

6. 28-4-2008P-Phunck - Prolog6 Variables Two states  Instantiated Variable has a set value  Uninstantiated Has not been assigned a value Closed-world assumption  Only true if it can be proven

7. 28-4-2008P-Phunck - Prolog7 Data Structures Compound term  Functor and arguments  'weird( prolog, yes ).' Queries  '?- good( presentation, brian )'  Only returns 'yes' or 'no' Conjunction  Multiply queries and facts

8. 28-4-2008P-Phunck - Prolog8 Special Compound Terms List  Head First element in list  Tail List containing everything else in list except the head Last element is empty list - '[]'  '[]', '.' - '[a,[b,[c]]]' == 'a.b.c.[]' String  Special list – all elements are atoms '[“This”,is,a,string,in,”Prolog”]'

9. 28-4-2008P-Phunck - Prolog9 More Lists Special list operators  '[]' - Empty list  '[X|Y]' – X is head, Y is tail  '””' - convert list to list containing ascii values Recursive search for element of list member(X,[X|_]). member(X,[_|Y]) :- member(X,Y). ?- member(c,[a,b,c,d,e,f]). yes

10. 28-4-2008P-Phunck - Prolog10 Built in Operators Compound construction  ',' - conjunction, ';' - disjunction Comparison  'X = Y' – equality, 'X \= 2' – inequality  'X == Y' – strict equality, 'X \== 2' – strict inequality  'X < Y', 'X =< Y', - less than, less than equal to  'X > Y', 'X >= Y' – greater than, greater than equal to Arithmetic  'X + Y', 'X - Y', 'X * Y', 'X / Y', 'X mod Y'

11. 28-4-2008P-Phunck - Prolog11 Arithmetic Operators that stand out  '\' - not, '\=' - not equal  '= =' - greater equal Perform arithmetic  'is' ' Y = Y/X' – tests equality 'Y is Y/X' – assigns numeric value to Y Right hand side of 'is' must be algebraic

12. 28-4-2008P-Phunck - Prolog12 Looping No iteration, only recursion factorial(0,1). factorial(N,F) :- N>0, N1 is N-1, factorial(N1,F1), F is N * F1. ?- factorial(3,X). X=6

13. 28-4-2008P-Phunck - Prolog13 I/O Special file  'user' Output  'tell(X).' - creates/opens file  'telling(X).' - gets open file name  'told' – closes file Input  see, seeing, seen Misc  'tab(X)', 'nl'

14. 28-4-2008P-Phunck - Prolog14 Backtracking User query  Searches database for fact declaring it true Matching fact (head rule) found  Mark place in database, move on to next goal No matching fact found  Attempt to re-satisfy goal Undo last change and try again Cut  Shortcuts backtracking '!' symbol

15. 28-4-2008P-Phunck - Prolog15 Backtracking in Action father(mary,george). father(john,george). father(sue,harry). father(george,edward). father(X) :- father(_,X). Database Query ?- father(Y). Result Y=george; Y=harry; Y=edward; no

16. 28-4-2008P-Phunck - Prolog16 Lists and Backtracking reverse([],[]). reverse([Head|Tail],List) :- reverse(Tail,Result), append(Result,[Head],List). trace. ?- reverse([a,b,c],Q).

17. 28-4-2008P-Phunck - Prolog17 Reverse Example (1) 1 Call: reverse([a,b,c],_6)? (2) 2 Call: reverse([b,c],_65636)? (3) 3 Call: reverse([c],_65646)? (4) 4 Call: reverse([],_65656)? (4) 4 Call: Exit: reverse([],[])‏ (5) 4 Call: append([],[c],_65646)? (5) 4 Exit: append([],[c],[c])‏ (3) 3 Exit: reverse([c],[c])‏ (6) 3 Call: append([c],[b],_65636)? (7) 4 Call: append([],[b],_25)? (7) 4 Exit: append([],[b],[b])‏ (6) 3 Exit: append([c],[b],[c,b])‏ (2) 2 Exit: reverse([b,c],[c,b)‏ (8) 2 Call: append([c,b],[a],_6)? (9) 3 Call: append([b],[a],_32)? (10) 4 Call: append([],[a],_39)? (10) 4 Exit: append([],[a],[a])‏ (9) 3 Exit: append([b],[a],[b,a])‏ (8) 2 Exit: append([c,b],[a],[c,b,a])‏ (1) Exit: reverse([a,b,c],[c,b,a])‏

18. 28-4-2008P-Phunck - Prolog18 How to Cut sum_to(1,1) :- !. sum_to(N,Res) :- N1 is N – 1, sum_to(N1,Res), Res is Res + N. Database Query ?- sum_to(5,X). Result X = 15; no

19. 28-4-2008P-Phunck - Prolog19 Useful Built in Predicates Conditional  'true', 'fail' – Always their namesakes  'atom(X)', 'integer(X)' – true if X is an atom or int  'atomic(X)' – X is not a variable  'asserta(X)' – adds clause X to start of database  'assertz(X)' – adds clause to end of database  'X =.. L' - Univ X is variable L is list where head is functor and tail is arguments

20. 28-4-2008P-Phunck - Prolog20 Example: Binary Tree lookup(H,w(H,G,_,__,G) :- !. lookup(H,w(H1,_,Before,_),G) :- aless(H,H1), lookup(H,Before,G). lookup(H,w(H1,_,_,After),G) :- not(aless(H,H1)), lookup(H,After,G). ?- lookup(key,X,582), lookup(val,X,356). ImplementationUsage

21. 28-4-2008P-Phunck - Prolog21 Example: Quicksort partition([], _, [], []). partition([X|Xs], Pivot, Smalls, Bigs) :- ( X @ Smalls = [X|Rest], partition(Xs, Pivot, Rest, Bigs)‏ ; Bigs = [X|Rest], partition(Xs, Pivot, Smalls, Rest)‏ ). quicksort([]) --> []. quicksort([X|Xs]) --> { partition(Xs, X, Smaller, Bigger) }, quicksort(Smaller), [X], quicksort(Bigger).

22. 28-4-2008P-Phunck - Prolog22 Example: Turing Machine turing(Tape0, Tape) :- perform(q0, [], Ls, Tape0, Rs), reverse(Ls, Ls1), append(Ls1, Rs, Tape). perform(qf, Ls, Ls, Rs, Rs) :- !. perform(Q0, Ls0, Ls, Rs0, Rs) :- symbol(Rs0, Sym, RsRest), once(rule(Q0, Sym, Q1, NewSym, Action)), action(Action, Ls0, Ls1, [NewSym|RsRest], Rs1), perform(Q1, Ls1, Ls, Rs1, Rs). symbol([], b, []). symbol([Sym|Rs], Sym, Rs). action(left, Ls0, Ls, Rs0, Rs) :- left(Ls0, Ls, Rs0, Rs). action(stay, Ls, Ls, Rs, Rs). action(right, Ls0, [Sym|Ls0], [Sym|Rs], Rs). left([], [], Rs0, [b|Rs0]). left([L|Ls], Ls, Rs, [L|Rs]).

23. 28-4-2008P-Phunck - Prolog23 Compilers ISO Compliant  GNU Prolog http://www.gprolog.org/  SWI-Prolig http://www.swi-prolog.org/  YAP http://www.ncc.up.pt/yap/ Visual Prolog (Turbo Prolog)‏  http://www.visual-prolog.com/ http://www.visual-prolog.com/

24. 28-4-2008P-Phunck - Prolog24 Sources Clocksin, W.F and Mellish, C.S. Programming in Prolog. Berlin. Springer-Verlag: 1984 Sebesta, Robert W. Concepts of Programming Languages: Eighth Edition. 2007 http://en.wikipedia.org/wiki/Prolog