# 10.1. Built-in predicates#

Term manipulation

Term1 = Term2

Term1 and Term2 are unified.

Term1 \= Term2

Term1 and Term2 cannot be unified.

Term1 == Term2

Term1 and Term2 are bound to the same term.

Term1 \== Term2

Term1 and Term2 are bound to different terms.

var(V)

V is an unbound variable.

arg(N,T,A)

A is the N-th argument of term T.

functor(T,F,N)

T is a term with functor F and arity N.

Term =.. List

List is a list starting with the functor of Term, followed by its arguments.

varsin(Term,Vs)

Vs is a list of the variables in Term.

numbervars(T,N,M)

the variables in term T are instantiated to terms of the form '\$VAR'( n ), where n is an integer which has a different value for each distinct variable. The variables will be numbered starting from N, and M is the next unused number.

Database manipulation

assert(Clause)

Clause is added at the end of the database (can also be a single atom, without a body).

asserta(Clause)

Clause is added at the top of the database.

clause(H,B)

H:-B is a clause in the database (H must be instantiated).

retract(Clause)

the first clause unifying with Clause is removed from the database.

retractall(Head)

all clauses with head unifying with Head are removed from the database.

kill(Predicate)

all clauses defining Predicate are removed from the database.

save(File)

save the clauses in the database to File.

consult(File)

load the clauses in File into the database.

op(P,Type,Name)

declare an operator Name with priority P (a number between 0 and 1200, lower priority binds stronger); Type is fx or fy for prefix, xfx, xfy or yfx for infix, and xf or yf for postfix.

Control

call(Goal)

call Goal (must be instantiated to a goal).

not(Goal)

Goal is not provable (must be instantiated to a goal).
not/1 could be defined as

• not(Goal):-call(Goal),!,fail.
• not(Goal).

fail, false

forces failure.

true, otherwise

always succeeds.
true/0 could be defined as

• true.

repeat

succeeds indefinitely many times on backtracking.
repeat/0 could be defined as

• repeat.
• repeat:-repeat.

findall(X,G,L)

L is a list of X’s, one for each solution of G (succeeds with the empty list if no solutions are found).

bagof(X,G,L)

L is a list of X’s, one for each solution of G, which may be preceded by existential variables (fails if no solutions are found).

setof(X,G,L)

as bagof/3, but L is a sorted list without duplicates.

forall(G,C)

for all the solutions of G, C is true.
forall/2 could be defined as

• forall(G,C):-not((G,not(C))).

Interaction

read(Term)

Term is instantiated to the next line typed by the user (must be a Prolog term).

write(Term)

write Term to the screen.

tab(N)

write N spaces to the screen.

nl

write a newline to the screen.

get(C)

C is ASCII code of next character typed by the user.

put(C)

write character with ASCII code C to the screen.

tell(File)

redirect output to File.

told

stop redirecting output.

see(File)

redirect input from File.

seen.

stop redirecting input.