Hi Ben, I recently wrote a package with the same intent, called monad-classes https://github.com/feuerbach/monad-classes First, I am astonished with the simplicity of your approach. My library is much more complicated, and uses closed type families to find the right layer. I honestly didn't expect anything as simple as that to work — and yet is seems to do well, at least in simple cases that I checked. I am not sure about more complex ones — the use of IncoherentInstances somewhat bothers me, and I wonder if it's going to backfire in more complex settings. monad-classes also allows more flexibility regarding which transformers can handle given effects: * @MonadState s@ constraint can be handled by both lazy and strict StateT * @MonadReader r@ can be handled by @StateT r@ (and similarly for MonadWriter) * Given a lens from s' to s, @MonadState s@ can be handled by @StateT s'@ Roman * Ben Foppa [2014-07-09 19:52:17-0400]

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

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