
is_list(?Term)

   Succeeds if Term is a proper list.

Arguments
   Term                Prolog term.

Type
   Type Testing

Description
   Used to test whether Term is a proper list, i.e. either the empty list,
   of a list element whose tail is itself a proper list.  The predicate
   could be recursively defined as:

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

   The complexity of this operation is linear in the length of the list.


Modes and Determinism
   is_list(?) is semidet

Fail Conditions
   Fails if Term is not a proper list

Examples
   
    ?- is_list([]).
    Yes (0.00s cpu)

    ?- is_list([1,2,3]).
    Yes (0.00s cpu)

    ?- is_list([1,2|3]).   % illegal tail
    No (0.00s cpu)

    ?- is_list([1,2|_]).   % open-ended list
    No (0.00s cpu)

    ?- is_list(list).
    No (0.00s cpu)

    ?- is_list(42).
    No (0.00s cpu)

    ?- is_list(X).
    No (0.00s cpu)


See Also
   atom / 1, compound / 1, ground / 1
