lift always lifts from the “previous” layer. If you need to lift from the second layer, you would need lift . lift and so on. On the other hand, liftIO always lifts from the IO layer (which, when present, is always on the bottom of the stack). So, if you have more than 2 layers … Read more
A bunch of them are almost completely equivalent: mtl uses GHC extensions, but transformers is Haskell 98. monads-fd and monads-tf are add-ons to transformers, using functional dependencies and type families respectively, both providing the functionality in mtl that’s missing from transformers. mtl-tf is mtl reimplemented using type families. So essentially, mtl == transformers ++ monads-fd, … Read more
If we compare the types (<*>) :: Applicative a => a (s -> t) -> a s -> a t (>>=) :: Monad m => m s -> (s -> m t) -> m t we get a clue to what separates the two concepts. That (s -> m t) in the type of (>>=) … Read more