Answers

Here are the answers to this chapter’s questions:

1. A monad transformer is a type constructor, T, that transforms any monad, M, into a new monad, T M. Moreover, T must be an instance of the MonadTrans type class:

   class MonadTrans t where
           lift :: Monad m => m a -> t m a

   Monad transformers are typically used to augment existing monad with the functionality of an additional effect.

2. The important monad transformers are:

...

Effect	Monad Transformer	Key Operations
Reader/environment	ReaderT r	ask, local
Writer	Monoid w => WriterT w	tell
State	StateT s	put, get