queens :: Int -> [[Int]]
queens n = filter test (generate n)
where generate 0 = [[]]
generate k = [q : qs | q <- [1..n], qs <- generate (k-1)]
test [] = True
test (q:qs) = isSafe q qs && test qs
isSafe try qs = not (try `elem` qs || sameDiag try qs)
sameDiag try qs = any (\(colDist,q) -> abs (try - q) == colDist) $ zip [1..] qs