SWI-Prolog 8.2.4 Reference Manual: Section 4.5

## 4.5 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 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 12.4.8.
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"`.62In 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, 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.63We 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 2.17.64The 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 2.17.