Question

For example, list = [1,2,3,4]. listProduct list returns [1,2,3,4,6,8,9,12,16] i.e [(1*1),(1*2),(1*3),(1*4),(2*3),(2*4),(3*3),(3*4),(4*4)]

I remember seeing that there was something that did this, but I can’t find that resource any more.

Answer 1

Why does your example not include 2*2 in the result?

If it’s because it’s the same as 1*4 — that is, you don’t want duplicates — then

listProduct xs = nub [x * y | x <- xs, y <- xs]

On the other hand, if you do want duplicates — if you want to multiply each number by each subsequent number in the list, and include duplicates in the result — then

listProduct' xs = triangularAutoZipWith (*)

triangularAutoZipWith op = concatMap f . tails
  where f [] = []
        f xs @ (x : _) = map (op x) xs

You could use triangularAutoZipWith in a more efficient version of the first solution:

listProduct = nub . triangularAutoZipWith (*)