Built-in Predicates

1
Built-in Predicates

• t.k.prasad_at_wright.edu
• http//www.knoesis.org/tkprasad/

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

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).

4
(contd)

• ?-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.

5
Input Prolog program from files

• ?-consult(eg).
• ?-eg.
• ?-eg.pl.
• ?-reconsult(eg).
• From keyboard
• ?-consult(user).
• D

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).

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

8
Term I/O

• read(X)
• write(X)
• display(X)
• nl
• tab(N)

• ?-read(X).
• a b
• X ab
• ?-write(ab).
• ab
• ?-display(ab).
• (a,b)

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))

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,_)

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

12
Debugging Predicates

• ?- trace.
• ?- notrace.
• ?- spy(p).
• ?- spy(q/2).
• Trace stops at every goal.
• ltRETUTNgt takes to the next goal.
• l (leap) goes to next spy-point.

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

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

15
Defining basic call-predicate

• call( true ) - !.
• call( (G1, G2) ) - !,
• call(G1), call(G2).
• call( G ) -
• clause(G,B), call(B).

16
Implementing findall

• findallB(X, Goal, Xlist) -
• call(Goal),
• assertz(queue(X)),
• fail
• assertz(queue(bottom)),
• collect(Xlist).

17
(contd)

• collect(L) -
• retract(queue(X)), !,
• ( X bottom,!, L
• L X Rest,
• collect(Rest) ).

18
Alternative Implemention

• findallCM(X, Goal, _) -
• asserta(queue(bottom)), call(Goal),
• asserta(queue(X)),
• fail.
• findallCM(_, _, L) -
• collect(,M), !, L M.

19
(contd)

• collect(S,L) -
• getNext(X), !,
• collect(XS,L).
• collect(L,L).
• getNext(S,L) -
• retract(queue(X)), !,
• X \ bottom.

20
Database

• e(happy).
• e(sad).
• m(tom,happy).
• m(bev,sad).
• m(amy,happy).

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)

22
(contd)

• 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)

23
(contd)

• 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)