1. Prolog Built-in predicates
Dr. M. A. Pasha

2. Utility goals
• help(S)
S should be a symbolic atom, e.g.,
help(assert).
• halt
Stops Prolog, resume operating system.
• trace, notrace
Turns trace on and off, respectively.

3. Universals
• true
Always succeeds as a goal.
• fail
Always fails as a goal.

4. Loading Prolog programs
• consult(F)
Loads program from the file F. F should be bound to file
designator expression, e.g.,
F='/home/user/prolog/sample.pl', depending on the
file system.
• reconsult(F)
Like consult except that each predicate already defined
has its definition replaced by the new definition being
loaded.
• [F1,F2,...]
Bracket notation, meaning consult F1, then consult F2,
then ...

5. Testing types
• var(Term) succeeds if Term is currently uninstantiated (which therefore
has not been bound to anything, except possibly another uninstantiated
variable).
• nonvar(Term) succeeds if Term is currently instantiated (opposite of
var/1).
• atom(Term) succeeds if Term is currently instantiated to an atom.
• integer(Term) succeeds if Term is currently instantiated to an integer.
• float(Term) succeeds if Term is currently instantiated to a floating point
number.
• number(Term) succeeds if Term is currently instantiated to an integer or a
floating point number.
• atomic(Term) succeeds if Term is currently instantiated to an atom, an
integer or a floating point number.
• string(Term) Tests whether Term is bound to a string.

6. Arithmetic Predicates
• X is E Evaluate E and unify the result with X.
• X + Y When evaluated, yields the sum of X and Y.
• X - Y When evaluated, yields the difference of X and Y.
• X * Y When evaluated, yields the product of X and Y.
• X / Y When evaluated, yields the quotient of X and Y.
• X mod Y When evaluated, yields the remainder of X
divided by Y.
• X =:= Y Evaluate X and Y and compare them for
equality.
• X == Y Evaluate X and Y and succeed if they are not
equal. ...and similarly for >, <, >=, =<.

7. ?- X=2,Y=3,Z is X+Y.
X = 2,
Y = 3,
Z = 5.
?- X=3, Y=5, X =:= Y .
False
?- X=3, Y=5, X >= Y.
false.
X=3, Y=5, X =< Y.
X = 3,
Y = 5.

8. Equality of Prolog expressions
• X = Y, X =Y
Tests whether X and Y can be unified, or cannot,
respectively. For example, ?- [X,Y|R] = [a,b,c].
X = a, Y = b, R = [c]
?- [X,Y,Z] = [a,b].
No
• X ==Y, X == Y
Tests whether X and Y are currently co-bound, i.e.,
have been bound to or share same value, or not,
respectively. For example, ?- X = 3, Y = 1 * 3, X == Y.
no
?- X = a, [Y|_]= [a,b,c], X == Y.
X = a, Y = a

9. Testing for variables
• ground(G)
Tests whether G has unbound logical
variables.
• var(X)
Tests whether X is bound to a Prolog variable.

10. Database Manipulation P
• assert(X) Add X to the database. For syntactic reasons, if X
is not a base clause, use assert((X)).
• asserta(X) Add X to the database in front of other clauses
of this predicate.
• assertz(X) Add X to the database after other clauses of this
predicate.
• retract(X) Remove X from the database. For syntactic
reasons, if X is not a base clause, use retract((X)).
• abolish(F,A) Remove all clauses with functor F and arity A
from the database.
• clause(X,V) Find a clause in the database whose head (left
hand side) matches X and whose body (right hand side)
matches V. To find a base clause, use true for V.

11. Assert and Retract
• asserta(C)
Assert clause C into database above other clauses with
the same key predicate. The key predicate of a clause is
the first predicate encountered when the clause is read
from left to right.
• assertz(C), assert(C)
Assert clause C into database below other clauses with
the same key predicate.
• retract(C)
Retract C from the database. C must be sufficiently
instantiated to determine the predicate key.

12. Binding a logical variable to a numeric
value
• X is E
Binds logical variable V to the numerical value of
E. The expression E must either be a number or
be a number-valued expression, conventionally
parenthesized,
• ?- X is 22, Y is X * 3, Z is sqrt(Y).
X = 22 .
Y = 66 .
Z = 8.12404 .
• ?- X is sin(45).
X = 0.8509035245341184.

13. Control Predicates
• X ; Y X or Y. Try X first; if it fails (possibly after being backtracked
into), try Y.
• (X -> Y) If X, then try Y, otherwise fail. Y will not be backtracked into.
• (X -> Y ; Z) If X, then try Y, else try Z. X will not be backtracked into.
• not X (Sometimes written +X or not(X)) Succeed only when X fails.
• true Succeed once, but fail when backtracked into.
• repeat Always succeed, even when backtracked into.
• fail Never succeed.
• ! (Pronounced ``cut".) Acts like true, but cannot be backtracked
past, and prevents any other clauses of the predicate it occurs in
from being tried.
• abort Return immediately to the top-level Prolog prompt.

14. Negation as failure
• not(Q), +Q
Negation-as-failure, as if defined by:
• not(Q) :- call(Q), !, fail.
not(Q).

15. Prolog terms and clauses as data
• name(A,L)
Convert between atom and list.
• ?- name('pasha', P).
P = [112, 97, 115, 104, 97].
• ?- parent(a,X) = .. L.
L = [parent, a, _X001]
• ?- P=..[parent,pasha, abdul].
P= parent(pasha,abdul)

16. Associativity
• associativity (or fixity) of an operator is a
property that determines how operators of
the same precedence are grouped in the
absence of parentheses.
• Operators may be left-associative (meaning
the operations are grouped from the left),
right-associative (meaning the operations are
grouped from the right) or non-associative
(meaning there is no defined grouping).

17. Prolog operators
Operator Type
• xfx infix nonassociative
xfy infix right-associative
yfx infix left-associative
fx prefix nonassociative
fy prefix right-associative
xf postfix nonassociative
yf postfix left-associative

18. Prolog operators
• :- xfx, fx
• ?- fx
• ; xfy
• , xfy
• not fy
• is, =.. , < xfx
• +, - yfx, fx
• *, / yfx
• ^ xfy

19. • :- op(P,T,O).
Declare an operator symbol. For example, with source
program ...
• :- op(500,xfx,'has_color').
a has_color red.
b has_color blue.Then ...
• ?- b has_color C.
C = red
?- What has_color red.
What = aP is precedence, an integer. Larger P has less
precedence (ability to group). Precedence number values
for built-ins depend upon the actual Prolog system. User
needs to find out what these values are. (See the reference
materials or use the help facility with keyword 'operator').

20. Finding all answers
• findall(Things,GoalExpression,Bag)
Compute all Things which satisfy the GoalExpresssion and
collect them in the list Bag. If the GoalExpression fails, Bag
will be the empty list []. findall treats all variables in
GoalExpression as if they are existentially quantified.
• bagof(Things,GoalExpression,Bag)
Compute all Things which satisfy the GoalExpresssion and
collect them in the list Bag. bagof fails if GoalExpression
fails. Free variables in GoalExpression could be
bound, yielding many bags.
• setof(Things,GoalExpression,Bag)
Compute all Things which satisfy the GoalExpresssion and
collect them in the list Bag. Similar to bagof except that Bag
will not contain duplicates and it will be sorted.

21. Example
son(X,Y):- ?- findall(X, father(X,Y), Bag).
father(Y,X),
male(X). Bag = [afzal, afzal, nazir,
daughter(X, Y):- afzal, afzal].
father(Y, X),
female(X).
grandfather(X, Y):-
father(X, Z),
?- findall(Y, father(X,Y), Bag).
father(Z,Y).
Bag = [bilal, zubair, afzal,
father(afzal, bilal).
father(afzal, zubair). humara, sumara].
father(nazir, afzal).
father(afzal, humara).
father(afzal, sumara).
female(sumara).
female(humara).

22. p(1,3,5).
p(2,4,1).
p(3,5,2).
p(4,3,1).
p(5,2,4).
?- bagof(Z,p(X,Y,Z),Bag).
X = 1, Y = 3, Bag = [5] ; The predicates bagof and setof yield
X = 2,Y = 4,Bag = [1] ; collections for individual bindings of the
X = 3,Y = 5,Bag = [2] ; free variables in the goal.
X = 4,Y = 3,Bag = [1] ; setof yields a sorted version of the
X = 5,Y = 2,Bag = [4]. collection without duplicates.
?- findall(Z,p(X,Y,Z),Bag).
Bag = [5, 1, 2, 1, 4].

23. Output Predicates
• write(X) Write the single value X to the current
output file.
• writeq(X) Write X with quotes as needed so it can
be read in again.
• tab(N) Write N blanks to the current output file.
• nl Write a newline to the current output file.
• put(X) Write the character whose ASCII value is X
to the current output file.
• tell(File) Open File as the current output file.
• told Close the current output file.

24. Input predicates
• read(X) Read one clause from the current input
and unify it with X. If there is no further input, X
is unified with end_of_file.
• get(X) Read one printing character from the
current input file and unify the ASCII code of that
character (an integer) with X.
• get0(X) Read one character from the current
input file and unify the ASCII code of that
character with X.
• see(File) Open File as the current input file.
• seen Close the current input file.

25. To avoid binding variables, use an existential quantifier
expression. For example the goal bagof(Z,X^Y^p(X,Y,Z),Bag) asks
for "the Bag of Z's such that there exists an X and there exists a Y
such that p(X,Y,Z)".
?-setof(Z,X^Y^p(X,Y,Z),Bag).
Bag = [1, 2, 4, 5].
findall acts like bagof with all free variables automatically
existentially quantified. In addition findall returns an empty list []
there is no goal satisfaction, whereas bagof fails.
?- bagof(Z,(p(X,Y,Z),Z>5),Bag).
False.
?- findall(Z,(p(X,Y,Z),Z>5),Bag).
Bag = []

26. Listing and Debugging Predicates
• listing(P)
Display predicate P. P may be a predicate name, a structure of the form
Name/Arity, or a bracked list of the above.
• trace
Turn on tracing.
• notrace
Turn off tracing.
• spy P
Turn on tracing when predicate P is called. P may be a predicate name, a structure
of the form Name/Arity, or a non-bracked list of the above.
• nospy P
Turn off spying for P.
• nospyall
Turn off all spypoints.
• Debug
Enable spypoints (allow them to initiate tracing.).
• Nodebug
Disable spypoints (without removing them).