Haskell in Depth

6 Structuring programs with monad transformers

This chapter covers

• Using monad transformers to extend monad functionality
• Describing an application structure with monad stacks
• Defining monad transformers
• Overview of the most common monad transformers provided by the Haskell libraries

We’ve seen many monads already. We know that every monad specifies a way to combine two computations in a sequence by implementing the >>= operator. We also know that in a monadic setting we have a result of a computation with some additional effects: computations may fail (as in the Maybe monad) or give an opportunity to communicate with the real world by doing input and output (as in the IO monad), or we may mutate the state or consult an environment in some ...