Bramer M. Logic Programming with Prolog
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172 Logic Programming With Prolog
Name of Predicate
member/2
Syntax
member(Term,List )
Description
Gets or checks a member of a list (see Chapter 9).
Name of Predicate
name/2
Syntax
name(Atom,List)
Description
Converts between an atom and a list of characters (see Chapter 10).
Name of Predicate
nl/0
Syntax
nl
Description
Outputs a carriage return and line feed to the current output stream, to complete a
line of output.
Name of Predicate
op/3
Syntax
op(Precedence,Type,Name)
Description
Used to set, reset or clear the definition of an operator, using the given Precedence,
Type and Name.
Built-in Predicates 173
Name of Predicate
put/1
Syntax
put(Integer)
Description
Outputs the character corresponding to Integer to the current output stream.
Name of Predicate
read/1
Syntax
read(Var)
Description
Reads a term from the current input stream and attempts to assign the value to Var,
which should previously be unbound.
Name of Predicate
reconsult/1
Syntax
reconsult(Filename)
Description
Loads the program contained in the named disk file. Any clauses for the same
predicates already in the database are first deleted.
Name of Predicate
repeat/0
Syntax
repeat
Description
Always succeeds when called, both when called and on backtracking. Used to
provide a looping facility.
174 Logic Programming With Prolog
Name of Predicate
retract/1
Syntax
retract(Clause)
Description
Deletes the first matching clause from a predicate (see Chapter 8).
Name of Predicate
retractall/1
Syntax
retractall(Head)
Description
Deletes all clauses whose heads match the given Head (see Chapter 8).
Name of Predicate
reverse/2
Syntax
reverse(List,Reverse)
Description
Reverses the order of elements in a list (see Chapter 9).
Name of Predicate
see/1
Syntax
see(Stream)
Description
Sets Stream to be the current input stream. Stream may be the name of a disk file
or the atom user (referring to the console input device). If Stream refers to a disk
file which is not open, it is automatically opened for reading. If the file is already
open, input continues from the point immediately after the previously-read
character.
Built-in Predicates 175
Name of Predicate
seeing/1
Syntax
seeing(Stream)
Description
Stream is bound to the name of the current input stream, which may be a disk file
or the atom user (referring to the console input device).
Name of Predicate
seen/0
Syntax
seen
Description
Closes the file associated with the current input stream, and resets the current input
stream to user.
Name of Predicate
statistics/0
Syntax
statistics
Description
Displays statistics about the current session.
Name of Predicate
tell/1
Syntax
tell(Stream)
Description
Sets the current output stream, which may be a disk file or the atom user (referring
to the console output device). If Stream refers to a disk file which is not yet open, it
is automatically opened for writing. If the file was already open, output continues
from the point immediately after the previously written character.
176 Logic Programming With Prolog
Name of Predicate
telling/1
Syntax
telling(Stream)
Description
Gets the current output Stream, which may be a disk file or the atom user (referring
to the console output device). Stream is bound to the name of the current output
stream.
Name of Predicate
told/0
Syntax
told
Description
Closes the file associated with the current output stream, and resets the current
output stream to user.
Name of Predicate
true/0
Syntax
true
Description
Always succeeds.
Name of Predicate
write/1
Syntax
write(Term)
Description
Writes Term to the current output stream, in unquoted syntax.
Built-in Predicates 177
Name of Predicate
writeq/1
Syntax
writeq(Term)
Description
Writes Term to the current output stream, in quoted syntax.
Appendix 2
Built-in Operators
This appendix gives a brief description of each built-in operator mentioned in this
book and some others. They are all 'standard' operators, which should be available
in every version of Prolog, but inevitably there may be some exceptions to this. It
is also possible that in some implementations of Prolog the definitions may vary
slightly from those given here. In cases of disagreement, the definitions given in
the supplier's documentation should always be taken as definitive.
Name of Operator
, [comma]
Type of Operator
infix
Syntax
Goal1,Goal2
Description
Succeeds if and only if Goal1 and Goal2 are both true.
Name of Operator
; [semicolon]
Type of Operator
infix
Syntax
Goal1;Goal2
Description
Succeeds if either Goal1 or Goal2 is true (or both).
180 Logic Programming With Prolog
Name of Operator
=
Type of Operator
infix
Syntax
Term1 = Term2
Description
Succeeds if terms Term1 and Term2 unify (see Chapter 4).
Name of Operator
\=
Type of Operator
infix
Syntax
Term\=Term2
Description
Succeeds if Term1 does not unify with Term2 (see Chapter 4).
Name of Operator
==
Type of Operator
infix
Syntax
Term1 == Term2
Description
Succeeds if Term1 is identical to Term2 (see Chapter 4).
Name of Operator
\==
Type of Operator
infix
Syntax
Term1 \== Term2
Description
Succeeds if Term1 is not identical to Term2 (see Chapter 4).
Built-in Operators 181
Name of Operator
=:=
Type of Operator
infix
Syntax
Exp1 =:= Exp2
Description
Succeeds if the arithmetic expressions Exp1 and Exp2 evaluate to the same
numerical value (see Chapter 4).
Name of Operator
=\=
Type of Operator
infix
Syntax
Exp1 =\= Exp2
Description
Succeeds if the arithmetic expressions Exp1 and Exp2 do not evaluate to the same
numerical value (see Chapter 4).
Name of Operator
=.. [pronounced 'univ']
Type of Operator
infix
Syntax
Term=..List
Description
Converts from a list to a term or vice versa (see Chapter 11).
182 Logic Programming With Prolog
Name of Operator
<
Type of Operator
infix
Syntax
Exp1<Exp2
Description
Succeeds if the value of arithmetic expression Exp1 is less than the value of
arithmetic expression Exp2.
Name of Operator
=<
Type of Operator
infix
Syntax
Exp1=<Exp2
Description
Succeeds if the value of arithmetic expression Exp1 is less than or equal to the
value of arithmetic expression Exp2.
Name of Operator
>
Type of Operator
infix
Syntax
Exp1>Exp2
Description
Succeeds if the value of arithmetic expression Exp1 is greater than the value of
arithmetic expression Exp2.