MonadBase is a generalized MonadIO

When you work with monad transformers, you may lift a function using lift. For example, this code lifts print from the base IO monad.

test = runMaybeT go
  where
    go = do lift $ print "test"
            return 3

But with lift you need to lift multiple times. For example, this code doesn’t compile.

test5 = runMaybeT (runStateT go 0)
    where
      go = do lift $ print "test"
              return 3

You need to lift two times explicitly.

test5 = runMaybeT (runStateT go 0)
    where
      go = do lift $ lift $ print "test"
              return 3

With MonadIO, you can lift a function in the base IO monad through arbitrary levels. Now you can write:

test5 = runMaybeT (runStateT go 0)
    where
      go = do liftIO $ print "test"
              return 3

MonadBase is a generalized version of MonadIO that allows you to lift a function in a base monad through arbitrary levels. The base monads include List, Maybe, ST, STM, IO and so on.

For example, you can access STRef in the base monad directly using liftBase.

test = runST go
    where
      go = do r