Type tests are semi-deterministic predicates that succeed if the
argument satisfies the requested type. Type-test predicates have no
error condition and do not instantiate their argument. See also library
library(error)
.
- [ISO]var(@Term)
- True if Term currently is a free variable.
- [ISO]nonvar(@Term)
- True if Term currently is not a free variable.
- [ISO]integer(@Term)
- True if Term is bound to an integer.
- [ISO]float(@Term)
- True if Term is bound to a floating point number.
- rational(@Term)
- True if Term is bound to a rational number. Rational numbers
include integers.
- rational(@Term,
-Numerator, -Denominator)
- True if Term is a rational number with given Numerator
and
Denominator. The Numerator and Denominator
are in canonical form, which means Denominator is a positive
integer and there are no common divisors between Numerator
and Denominator.
- [ISO]number(@Term)
- True if Term is bound to a rational number (including
integers) or a floating point number.
- [ISO]atom(@Term)
- True if Term is bound to an atom.
- blob(@Term,
?Type)
- True if Term is a blob of type Type. See
section 12.4.10.
- string(@Term)
- True if Term is bound to a string. Note that string here
refers to the built-in atomic type string as described in section
5.2. Starting with versionĀ 7, the syntax for a string object is
text between double quotes, such as
"hello"
.63In
traditional Prolog systems, double quoted text is often mapped to a list
of character codes. See also the Prolog flag
double_quotes.
- [ISO]atomic(@Term)
- True if Term is bound (i.e., not a variable) and is not
compound. Thus, atomic acts as if defined by:
atomic(Term) :-
nonvar(Term),
\+ compound(Term).
SWI-Prolog defines the following atomic datatypes: atom (atom/1),
string (string/1),
integer (integer/1),
floating point number (float/1),
rational (rational/1)
and blob (blob/2).
In addition, the symbol
[]
(empty list) is atomic, but not an atom. See
section 5.1.
- [ISO]compound(@Term)
- True if Term is bound to a compound term. See also functor/3
=../2, compound_name_arity/3
and compound_name_arguments/3.
- [ISO]callable(@Term)
- True if Term is bound to an atom or a compound term. This was
intended as a type-test for arguments to call/1, call/2
etc. Note that callable only tests the surface term. Terms such
as (22,true) are considered callable, but cause call/1
to raise a type error. Module-qualification of meta-argument (see meta_predicate/1)
using
:
/2
causes callable to succeed on any
meta-argument.64We think that callable/1
should be deprecated and there should be two new predicates, one
performing a test for callable that is minimally module aware and
possibly consistent with type-checking in call/1
and a second predicate that tests for atom or compound.
Consider the program and query below:
:- meta_predicate p(0).
p(G) :- callable(G), call(G).
?- p(22).
ERROR: Type error: `callable' expected, found `22'
ERROR: In:
ERROR: [6] p(user:22)
- [ISO]ground(@Term)
- True if Term holds no free variables. See also nonground/2
and term_variables/2.
- cyclic_term(@Term)
- True if Term contains cycles, i.e. is an infinite term. See
also acyclic_term/1
and section 2.16.65The
predicates cyclic_term/1
and acyclic_term/1
are compatible with SICStus Prolog. Some Prolog systems supporting
cyclic terms use is_cyclic/1 .
- [ISO]acyclic_term(@Term)
- True if Term does not contain cycles, i.e. can be processed
recursively in finite time. See also cyclic_term/1
and section 2.16.