{-# LANGUAGE GeneralizedNewtypeDeriving #-}

Hello Mike If the is a principal as such, I'd suggest that's working out the return type that you want your run function to have. Or more plainly - work out what you want the result to be. Thats a bit gnomic of course, so here are two examples with ErrorT (for failure) and WriterT (for logging), the ErrMsg type is contrived slightly to be a distinct type. This message should be literate Haskell if my mail service likes me:

module Transforming where

import Control.Monad.Error import Control.Monad.Identity import Control.Monad.Writer

type Log = String newtype ErrMsg = ErrMsg { getMsg :: String } deriving Show

newtype EWI a = EWI { getEWI :: ErrorT ErrMsg (WriterT Log Identity) a } deriving (Functor, Monad, MonadWriter Log, MonadError ErrMsg)

The run functions are pretty /natural/ just the run functions of the monad transformer stack in the reverse order. Note the /outer tupling/ over the Either type in the run function - runEWI always returns a log regardless of whether the computation fails with an error... [ without embellishments: (Either _ _,_) ]

runEWI :: EWI a -> (Either ErrMsg a, Log) runEWI ma = runIdentity (runWriterT (runErrorT (getEWI ma)))

newtype WEI a = WEI { getWEI :: WriterT Log (ErrorT ErrMsg Identity) a } deriving (Functor, Monad, MonadWriter Log, MonadError ErrMsg)

Note the Either type has an inner tuple in the run function - runWEI returns a log /only/ when the computation succeeds otherwise it fails with just an error... [ without embellishments: Either _ (_,_) ]

runWEI :: WEI a -> Either ErrMsg (a, Log) runWEI ma = runIdentity (runErrorT (runWriterT (getWEI ma)))

Support code

instance Error ErrMsg where noMsg = ErrMsg "" strMsg = ErrMsg