foldl-foldr.hs

-- foldl using only foldr & lambdas

-- increase every number in a list by one except when it's not preceded by one
-- e.g. inc [1, 2, 1, 3, 4, 5] == [2, 2, 2, 3, 5, 6]

-- folding function
folder :: ([Int], Int) -> Int -> ([Int], Int)
folder (acc, u) item = (acc ++ [item + u], if item == 1 then 0 else 1)

-- initial acc value
initAcc :: ([Int], Int)
initAcc = ([], 1)

-- inc using native foldl
inc :: [Int] -> [Int]
inc = fst . foldl folder initAcc

-- inc using foldr-based foldl
inc' :: [Int] -> [Int]
inc' = fst . foldlE folder initAcc
  where foldlE f z xs = foldr (\item fs -> fs . (`f` item)) id xs z