module FoldlFoldr where

{- foldl can be expressed as foldr too! It is difficult, but it's less difficult
   if we change the parameter order to put the "accumulator" as the last
   parameter.

   E.g., newFoldl (-) [1,2,3] 10 = ((10-1)-2)-3 = 4

   Here is the actual code of newFoldl. To reduce name clashes, I'm renaming
   "op" to "f", "z" to "b".
-}
newFoldl :: (b -> a -> b) -> [a] -> b -> b
newFoldl f [] b = b
newFoldl f (x : xs) b = newFoldl f xs (f b x)

{- Below, fill in the new version that uses foldr.

   Note: Please stick to the given format. More details below.

   Note 2: Both the list parameter and the accumulator parameter are
   deliberately absent!  You absolutely don't need them.

   Hint: "newFoldl f xs" and "myFoldl f xs" produce a "b->b" function. Aim for
   that.
-}

-- Please don't change the following 3 lines.
myFoldl :: (b -> a -> b) -> [a] -> (b -> b)
myFoldl f = foldr op z
  where
-- Please don't change the 3 lines above.
-- Below, changing the format is OK, e.g., "op x r = ...".
    z = error "TODO z"
    op = error "TODO op"