generate all combinations of given length for [1..k]

> selections 2 4
[[1,1],[1,2],[1,3],[1,4],[2,1],[2,2],[2,3],[2,4],[3,1],[3,2],[3,3],[3,4],[4,1],[4,2],[4,3],[4,4]]

selections :: Int -> Int -> [[Int]]
selections 1 k = fmap pure [1..k]
selections n k = liftA2 (:) [1..k] (selections (n-1) k)

total number of selections = k ^ n