Availability:built-in
[][|]’.’).
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.136In versions before 5.0.1, is_list/1
just checked for [][_|_] 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).