Folds can be left-associative or right-associative. (l or r suffix). Folds can be eager or lazy (prime suffix for eager). Folds can work with at least 1 eleme

Folds can be left-associative or right-associative. (l or r suffix). Folds can be eager or lazy (prime suffix for eager). Folds can work with at least 1 element to avoid the empty case ('1' suffix) (e.g: a function like `maximum`). This gives 2^3 cartesian space of folds. Each and every one may be useful in some cases. For ordinary lists, though, foldr and foldl' cover virtually all uses (just use 'error' for the empty case when you need to). Monad transformers don't require O(n^2) anything. You're talking about the 'mtl' package on top of the 'transformers' package. The 'transformers' package is useful on its own and doesn't have this problem. The 'mtl' automatic lifter framework does require something in the order of O(n^2) instances (for n=6 or so). However, this is not really just 6 copies of the same code over and over. The threading of monadic state through different monads varies. For example: `Reader.local` is implemented very differently for `WriterT` and for `ContT`. So there really are in the order of O(n^2) different interactions to write code to account for.