I wanted an IdentityT today, for extending xmonad. (The idea is to allower user-defined monad transformers, so users can plug in their own semantics easily). By default it would use IdentityT, which I note is not in mtl! Here's roughly what it would be: ----------------------------------------------------------------------------- -- | -- Module : Identity.hs -- License : BSD3-style (see LICENSE) -- module IdentityT where import Control.Monad.Trans -- -- IdentityT , a parameterisable identity monad, with an inner monad -- The user's default monad transformer -- newtype IdentityT m a = IdentityT { runIdentityT :: m a } instance (Functor m, Monad m) => Functor (IdentityT m) where fmap f = IdentityT . fmap f . runIdentityT instance (Monad m) => Monad (IdentityT m) where return = IdentityT . return m >>= k = IdentityT $ runIdentityT . k =<< runIdentityT m fail msg = IdentityT $ fail msg instance (MonadIO m) => MonadIO (IdentityT m) where liftIO = IdentityT . liftIO Any reasons why this shouldn't be in mtl? -- Don