Hi cafe, I whipped up extensible-transformers ( https://github.com/RobotGymnast/extensible-transformers) this afternoon. The idea is to make Monad transformer code more like extensible-effects code (http://hackage.haskell.org/package/extensible-effects). Here's a sample: {-# LANGUAGE FlexibleContexts #-} module Main(main) where import Control.Monad.Trans.Flexible import Control.Monad.Trans.List import Control.Monad.Trans.State.Strict -- A flexible transformer stack built from existing transformers using `liftT`. bar :: (In (StateT Int) t, In ListT t) => t () bar = do n <- liftT get liftT $ ListT $ return $ replicate n () -- A flexible transformer stack built from existing transformers using `liftT`. baz :: In (StateT Int) t => t () baz = do liftT $ state $ \i -> ((), i + (1 :: Int)) -- A flexible transformer monad stack composed of two other flexible -- transformer monad stacks. foo :: (In (StateT Int) t, In ListT t) => t () foo = do bar baz main :: IO () main = do evalStateT (runListT foo) (1 :: Int) >>= putStrLn . show runListT (evalStateT foo (2 :: Int)) >>= putStrLn . show Any feedback on this? Does such a package already exist? Thanks, Ben