4.6 Verify Type of a Term

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 an integer or floating point number.^{44As rational numbers are not atomic in the current implementation and we do not want to break the rule that number/1 implies atomic/1, number/1 fails on rational numbers. This will change if rational numbers become atomic.}

[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 10.4.7.

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 version 7, the syntax for a string object is text between double quotes, such as "hello".^{45In 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) 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 and call/2.. 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.^{46We 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.

cyclic_term(@Term)

True if Term contains cycles, i.e. is an infinite term. See also acyclic_term/1 and section 2.16.^{47The 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.