concept monad stack in category haskell

This is an excerpt from Manning's book Haskell in Depth MEAP V12. Login to get full access to this book.

Figure 6.2. Building monad stack from a monad transformer and a monad.

Every monad provides some functionality (effect) to our program. If we need a functionality of several monads at once, it’s a job for monad transformers. Monad transformers suggest the following way of thinking about our application or its independent components. An application is a monad stack built from a base monad with added various monadic functionality via monad transformers. A monad stack is a monad too, but it is rich in terms of the functionality provided.

We call such exception mechanisms programmable meaning that everything is done programmatically by the programmers themselves or the libraries they use. As we often want to have other effects to happen while doing computations, it is usually the case that programmable exceptions are provided by a monad transformer adding one layer to our monad stacks. The ExceptT monad transformer helps us to create monadic stacks with this ability. It is the best for describing computations that could fail with some known set of exceptional conditions. These conditions are specified by data constructors of an algebraic data type. This is quite often the case with pure computations. It’s a good idea to use ExceptT for monad stacks based on anything except for the IO monad. We’ll get back to the ExceptT monad transformer later in the next section.

type Stack = [Integer]
type EnvVars = [(Text, Integer)]

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