Решение задачи №63 от UniLecs (на этот раз верное)

least_common_multiple_of_array_of_ints.hs

import qualified Data.Vector.Unboxed as V

--gcd=greatest common divisor
gcd' x y 
    | x < y = gcd' y x
    | x == y = x
    | otherwise = let x' = x - y in x' `seq` gcd' x' y
    
--lcm=least common multiple
lcm' x y =
    let prod = x*y
    in prod `seq` prod `div` gcd' x y
    
lcmOfArr = V.foldl' lcm' 1

main = putStrLn . show . lcmOfArr . V.fromList $ [6 :: Int, 4] --12

Playground: https://paiza.io/projects/cZiHtM6g4s5uHpYm73cKVQ?language=haskell