4.5 Verify Type of a Term

[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.^{24As 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.

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 4.23, Text in double quotes such as "hello" creates a list of character codes. We illustrate the issues in the example queries below.

?- write("hello").
[104, 101, 108, 108, 111]
?- string("hello").
No
?- is_list("hello").
Yes

[ISO]atomic(+Term)

True if Term is bound to an atom, string, integer or floating point number. Note that string refers to the built-in type. See string/1. Strings in the classical Prolog sense are lists and therefore compound.

[ISO]compound(+Term)

True if Term is bound to a compound term. See also functor/3 and =../2.

callable(+Term)

True if Term is bound to an atom or a compound term, so it can be handed without type-error to call/1, functor/3 and =../2.

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.^{25The predicates cyclic_term/1 and acyclic_term/1 are compatible to SICStus Prolog. Some Prolog systems supporting cyclic terms use is_cyclic/1 .}

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.