:- use_module(library(clpfd)).

eightQueens(Queens) :-
    length(Queens, 8),
    Queens ins 1..8,
    canPlaceQueens(Queens).

canPlaceQueens([]).
canPlaceQueens([Queen|Queens]) :-
    arentThreatened(Queens,Queen,1),
    canPlaceQueens(Queens).

arentThreatened([], _, _).
arentThreatened([Queen|Queens], NewQueen, ColumnsApart) :-
    NewQueen #\= Queen,
    abs(Queen - NewQueen) #\= ColumnsApart, %%diagonallySafe
    NewApart #= ColumnsApart+1,
    arentThreatened(Queens, NewQueen, NewApart).