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Built-in Predicates

Explore a detailed breakdown of Prolog built-in predicates for efficient program development, including file I/O, debugging, term manipulation, and database interaction. Learn how to interpret terms as goals, access database clauses, and implement advanced functions like findall. Master Prolog's powerful features to enhance your programming skills.

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Built-in Predicates

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  1. Built-in Predicates t.k.prasad@wright.edu http://www.knoesis.org/tkprasad/ L8BuiltIns

  2. Example Categories • Program Updates • File I/O • Opening/closing, character I/O, term I/O • Value Classification using type predicates • Manipulating terms and programs • Debugging predicates L8BuiltIns

  3. Database Manipulation Predicates q(b). ?-asserta(q(a)). ?-listing(q). q(a). q(b). ?-assertz(q(c)). ?-listing(q). q(a). q(b). q(c). L8BuiltIns

  4. (cont’d) ?-retract(q(a)). ?- q(a). no ?-abolish(q/1). ?-retractall(p). ?- q(X). no • These are extra-logical predicates that change the “program” on the fly. • Useful for updating databases or simulating persistence of values through backtracking. L8BuiltIns

  5. Input Prolog program from files ?-consult(eg). ?-[eg]. ?-[‘eg.pl’]. ?-reconsult(eg). • From keyboard … ?-consult(user). … ^D L8BuiltIns

  6. Communication with files • At anytime during the execution of a Prolog program, only two files are active: current input stream and current output stream. • Opening see(fileName). tell(fileName). • Closing seen. told. • Currently active stream seeing(X). telling(user). L8BuiltIns

  7. Character I/O • get(X) : read next, non-blank character • get0(X) : read next character (ISO Std.) • put(X) : write the character (given X is bound to character encoding) ?-get(X). :e X = 101 ?-put(101). e ?-put(‘e’). e ?-put(“e”). e L8BuiltIns

  8. Term I/O • read(X) • write(X) • display(X) • nl • tab(N) ?-read(X). :a + b X = a+b ?-write(a+b). a+b ?-display(a+b). +(a,b) L8BuiltIns

  9. Term to/from list ?- f(a,b) =.. L. L = [f,a,b] ?- Z =.. [p,a,f(X,Y)]. Z = p(a,f(X,Y)) L8BuiltIns

  10. Term construction and inspection • functor(Term, FunctionSymbol, Arity) ?- functor(f(a,b,c), F, N). F = f N = 3 • arg(Number, Term, Argument) ?- arg(3, f(a,b,g(c,d)), T). T = g(c,d) ?- functor(T,g,2), arg(1,T,a). T = g(a,_) L8BuiltIns

  11. Atom to/from list • name(Atom, List) ?- name(abc,L). L = [97,98,99] ?- name(N,[66,67,68]). N = ‘ABC’ ?- name(N,”abc”). N = abc ?- name(123,[49,50,51]). true L8BuiltIns

  12. Debugging Predicates ?- trace. ?- notrace. ?- spy(p). ?- spy(q/2). • Trace stops at every goal. • <RETUTN> takes to the next goal. • l (leap) goes to next spy-point. L8BuiltIns

  13. Interpretation of term as a goal • call meta-predicate • cf. eval function in LISP • as predicate formula ?- p(X). • as object term ?- call(p(X)). • Call interprets a “data structure” as a piece of “program”. • Requires dynamic compilation and execution L8BuiltIns

  14. Accessing “database” clauses clause(Head, Body). • Iterates over term representations of head and body of clauses of the loaded program • Fundamental to meta-programming, specifically, for writing meta-interpreters L8BuiltIns

  15. Defining basic call-predicate call( true ) :- !. call( (G1, G2) ) :- !, call(G1), call(G2). call( G ) :- clause(G,B), call(B). L8BuiltIns

  16. Implementing findall findallB(X, Goal, Xlist) :- call(Goal), assertz(queue(X)), fail ; assertz(queue(bottom)), collect(Xlist). L8BuiltIns

  17. (cont’d) collect(L) :- retract(queue(X)), !, (X == bottom,!, L = [] ; L = [X | Rest], collect(Rest) ). L8BuiltIns

  18. Alternative Implemention findallCM(X, Goal, _) :- asserta(queue(bottom)), call(Goal), asserta(queue(X)), fail. findallCM(_, _, L) :- collect([],M), !, L = M. L8BuiltIns

  19. (cont’d) collect(S,L) :- getNext(X), !, collect([X|S],L). collect(L,L). getNext(S,L) :- retract(queue(X)), !, X \== bottom. L8BuiltIns

  20. Database e(happy). e(sad). m(tom,happy). m(bev,sad). m(amy,happy). L8BuiltIns

  21. Queries • Both definitions agree on the following query. ?- findallB(em(E,P),m(P,E),EC). ?- findallCM(em(E,P),m(P,E),EC). EC = [ em(happy,tom), em(sad,bev), em(happy,amy)] L8BuiltIns

  22. (cont’d) • Both definitions do not agree on the following query. ?- findallB(EC, (e(E), findallB(em(E,P),m(P,E),EC)),Ans). EC = [[[em(happy,tom), em(happy,amy)], em(sad,bev)]] L8BuiltIns

  23. (cont’d) • Both definitions do not agree on the following query. ?- findallCM(EC, (e(E), findallCM(em(E,P),m(P,E),EC)),Ans). EC = [[em(happy,tom), em(happy,amy)], [em(sad,bev)]] L8BuiltIns

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