Strict foldr function

StrictFoldr.hs

module StrictFoldr where

-- A deep strict foldr, very memory inefficient
dsFoldr :: (a -> b -> b) -> b -> [a] -> b
dsFoldr f z []     = z
dsFoldr f z (x:xs) = seq below $ f x $ below
  where below = dsFoldr f z xs

-- The lazy value will be `"abc" ++ undefined` whilst the strict will be just `undefined`
valueLazy, valueStrict :: String
valueLazy   = foldr   (:) undefined "abc"
valueStrict = dsFoldr (:) undefined "abc"

main = do
    print $ head valueLazy
    print $ head valueStrict