Prolog - check if list is empty []

Question

I am just trying to figure out how to check if a list is empty, I put together something that checks the length of the list, and additionally should check to see if the list isn't empty.

% Gives the length of a list.
listlength([]     , 0 ).
listlength([_|Xs] , L ):- 
    listlength(Xs,N), 
    L is N+1. 

% checks to see that the length of the list is greater than or equal to 3 and not empty.
check_length( [] ).
check_length( L  ):-
    listlength(L, Len),
    Len >= 3,
    L \== [].    %This is the bit of code I'm having problems with 
                  it should be false if the list is just [] or empty.

I am a student so I don't necessarily need a straight answer I'm just trying to figure out what I'm doing wrong.

Answer 1

You don't need to explicitly test for L being an empty list; it having a length > 0 establishes that.

And you already know how to test for a list not being empty, since you used it in listLength: L=[_|_] (that is, L is a list with at least 1 element).

Answer 2

The question is "how to check if a list is empty". By definition, an empty list is a list with no elements, i.e., the [] list. You could simply check if your list unifies with that.