Determining if a list is empty or not

Question

I want to do some if | else stuff in prolog. Below is my code, Prolog will return 'R = false' if my input list is not empty, it will return 'false' if my list is empty. What do I miss?

Code:

isEmpty([H|T],R) :- 
    length([H|T],L),
    ( L > 0 -> R = 'false'
    ;
    L =:= 0 -> R = 'true'
    ).

Prolog Output:

1 ?- isEmpty([],R).
false.

2 ?- isEmpty([1],R).
R = false.

3 ?- isEmpty([1,2,3],R).
R = false.

Solution 1

I know it's old thread but still this might be useful for those using SWI-Prolog. Instead of defining their own rules, you can try:

nth0(0, List, _).

It is used to return the first element on the list, which in case of empty list simply fails.

Solution 2

Since you are only dealing with lists of the form [H|T] your clauses will only match for lists that have at least one list element. In other words, you get the answer false for the empty list because there is not a matching clause - not because the length is 0.