Actually, this isn't true.  In the case of ReaderT Config IO (), liftIO is the same as lift.

 

liftIO exists because it is more general; it works for any monad that is an instance of "MonadIO", which includes IO, ReaderT r IO, StateT s IO, ReaderT r (StateT s (ErrorT e IO))), etc.

 

In the last case, you could write

    lift $ lift $ lift $ hPutStrLn h "You lose"

but it's much simpler to write

    liftIO $ hPutStrLn h "You lose"

 

Similarily, "ask" is defined in the MonadReader typeclass, which includes Reader, ReaderT IO, and all sorts of other monads that include "Reader" at some level; only the real "reader" monad has a full implementation; the rest are defined in terms of "lift" or simpler operations, like so:

 

instance MonadReader r m => MonadReader r (StateT s m) where

    ask = lift ask

    local f m = StateT $ \s -> local f (runStateT m s)

 

  -- ryan