Check if all numbers in a list are different in prolog

Question

I want to create a rule in prolog that checks if there\'s a repeated number in a list.

For example:

• for [1,2,3,4] it will return tru

Answer 1

The rule provided in the question is very close to a correct answer with minimal library usage. Here's a working version that required only one change, the addition of \+ in the third row:

uniqueList([]).
uniqueList([H|T]):-
     \+(member(H,T)),
     uniqueList(T).

Explanation of the code for Prolog beginners: The member(H,L) predicate checks if element H is a member of the list L. \+ is Prolog's negation function, so the above code amounts to:

uniqueList([H|T]) returns true if: (H doesn't have a copy in T) and uniqueList(T)

Whereas the code by the original asker didn't work because it amounted to:

uniqueList([H|T]) returns true if: (H has a copy in T) and uniqueList(T)

*I renamed Different() to uniqueList() because it reads better. Convention is to reserve capital letters for variables.