SWI-Prolog -- atomic/1

Availability:built-in

[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.