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Built-in Predicates. [email protected] http://www.knoesis.org/tkprasad/. 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. - PowerPoint PPT Presentation
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cs774 (Prasad) L8BuiltIns 1
Built-in Predicates
http://www.knoesis.org/tkprasad/
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
cs774 (Prasad) L8BuiltIns 2
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).
cs774 (Prasad) L8BuiltIns 3
(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.
cs774 (Prasad) L8BuiltIns 4
Input Prolog program from files?-consult(eg).
?-[eg].
?-[‘eg.pl’].
?-reconsult(eg).
• From keyboard …
?-consult(user).
…
^D
cs774 (Prasad) L8BuiltIns 5
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).
cs774 (Prasad) L8BuiltIns 6
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
cs774 (Prasad) L8BuiltIns 7
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)
cs774 (Prasad) L8BuiltIns 8
Term to/from list
?- f(a,b) =.. L.
L = [f,a,b]
?- Z =.. [p,a,f(X,Y)].
Z = p(a,f(X,Y))
cs774 (Prasad) L8BuiltIns 9
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,_)
cs774 (Prasad) L8BuiltIns 10
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
cs774 (Prasad) L8BuiltIns 11
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.
cs774 (Prasad) L8BuiltIns 12
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
cs774 (Prasad) L8BuiltIns 13
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
cs774 (Prasad) L8BuiltIns 14
Defining basic call-predicate
call( true ) :- !.
call( (G1, G2) ) :- !,
call(G1), call(G2).
call( G ) :-
clause(G,B), call(B).
cs774 (Prasad) L8BuiltIns 15
Implementing findall
findallB(X, Goal, Xlist) :-
call(Goal),
assertz(queue(X)),
fail ;
assertz(queue(bottom)),
collect(Xlist).
cs774 (Prasad) L8BuiltIns 16
(cont’d)
collect(L) :-
retract(queue(X)), !,
( X == bottom,!, L = [] ;
L = [X | Rest],
collect(Rest) ).
cs774 (Prasad) L8BuiltIns 17
Alternative Implemention
findallCM(X, Goal, _) :-
asserta(queue(bottom)), call(Goal),
asserta(queue(X)),
fail.
findallCM(_, _, L) :-
collect([],M), !, L = M.
cs774 (Prasad) L8BuiltIns 18
(cont’d)
collect(S,L) :-
getNext(X), !,
collect([X|S],L).
collect(L,L).
getNext(S,L) :-
retract(queue(X)), !,
X \== bottom.cs774 (Prasad) L8BuiltIns 19
Database
e(happy).
e(sad).
m(tom,happy).
m(bev,sad).
m(amy,happy).
cs774 (Prasad) L8BuiltIns 20
Queries
• Both definitions agree on the following query.
?- findallB(em(E,P),m(P,E)m(P,E),EC).
?- findallCM(em(E,P),m(P,E)m(P,E),EC).
EC = [ em(happy,tom), em(sad,bev),
em(happy,amy)]
cs774 (Prasad) L8BuiltIns 21
(cont’d)• Both definitions do not agree on the
following query.
?- findallB(EC, (e(E), findallBfindallB((em(E,P),m(P,E),ECem(E,P),m(P,E),EC))),
Ans).
EC = [[[ em(happy,tom),
em(happy,amy)], em(sad,bev)]]
cs774 (Prasad) L8BuiltIns 22
(cont’d)• Both definitions do not agree on the
following query.
?- findallCM(EC, (e(E), findallCMfindallCM((em(E,P),m(P,E),ECem(E,P),m(P,E),EC))),
Ans).
EC = [[ em(happy,tom),
em(happy,amy)], [em(sad,bev)]]
cs774 (Prasad) L8BuiltIns 23
