Other monad transformers

With the notable exception of the IO monad, which is a special case because it is built into the language, most other monads have a corresponding monad transformer. As they mostly follow the same approach as ReaderT, we’ll only review them briefly.

The StateT and WriterT transformers

The transformers for the state and writer effects have the closest resemblance to ReaderT:

Control.Monad.Trans.State
newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }
Control.Monad.Trans.Writer
newtype WriterT w m a = WriterT { runWriterT :: m (a, w) }

As monad transformers, they have appropriate Monad instances:

Control.Monad.Trans.State
instance Monad m => Monad (StateT s m)
Control.Monad.Trans.Writer
instance (Monad m, Monoid w) => Monad (WriterT w m)

We haven’t shown the implementation because it is very similar to the ones for State s and Writer w from the previous chapter. Instead, we will show the full MonadTrans instances...