Hi Edgar You might want to look at Data.Sequence... The transform function in your original code did both a cons (add to the front) and an append / snoc (add one to the end). transform :: BF -> [BF] transform (Loop bs) = Jmpz (2 + lengthbs bs) : (bs >>= transform) -- cons ++ [Jmpb (1 + lengthbs bs)] --snoc / append transform a = [a] snoc-ing is expensive on standard lists because you have to implement it as an appending a singleton list (which you did). Append performs a full traversal of the first list - here is the implementation copied from the GHC's libraries: (++) :: [a] -> [a] -> [a] (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys In contrast, Data.Sequence has cheap operations for cons (<|), snoc (|>) and append (><). Best wishes Stephen