SWI-Prolog -- is

Documentation
- Reference manual
  - Built-in Predicates
    - Built-in list operations
      - is_list/1
      - memberchk/2
      - length/2
      - sort/2
      - sort/4
      - msort/2
      - keysort/2
      - predsort/3
- Packages

Availability:built-in

is_list(+Term)

True if Term is bound to the empty list ([]) or a compound term with name‘[|]’^{125The
traditional list functor name is the dot (’.'). This
is still the case of the command line option --traditional
is given. See also section
5.1.} and arity 2 and the second argument is a list.^{126In versions before 5.0.1, is_list/1
just checked for [] or [_|_] and
proper_list/1 had the role of the current is_list/1.
The current definition conforms to the de facto standard. Assuming
proper coding standards, there should only be very few cases where a
quick-and-dirty is_list/1
is a good choice. Richard O'Keefe pointed at this issue.} This predicate acts as if defined by the definition below on acyclic terms. The implementation safely fails if Term represents a cyclic list.

is_list(X) :-
        var(X), !,
        fail.
is_list([]).
is_list([_|T]) :-
        is_list(T).