[Haskell-beginners] foldl' vs seq

Hi I am trying to understand the behavior of foldl' and seq. Why is the following code is much slower than the foldl' version ? (which is foldl' (+) 0) I thought the the below code is equivalent to foldr' but apparently I was wrong. mySum :: [Integer] -> Integer mySum[] = 0 mySum (x:xs) = seq x (mySum xs + x)