Developing Web Apps with Haskell and Yesod, 2nd Edition

Appendix A. monad-control

monad-control is used in a few places within Yesod, most notably to ensure proper exception handling within Persistent. It is a general-purpose package to extend standard functionality in monad transformers.

Overview

One of the powerful, and sometimes confusing, features in Haskell is monad transformers. They allow you to take different pieces of functionality—such as mutable state, error handling, or logging—and compose them easily. Though I swore I’d never write a monad tutorial, I’m going to employ a painful analogy here: monads are like onions. (Monads are not like cakes.) By that, I mean layers.

We have the core monad, also known as the innermost or bottom monad. On top of this core, we add layers, each adding a new feature and spreading outward/upward. As a motivating example, let’s consider an ErrorT transformer stacked on top of the IO monad:

newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }
type MyStack = ErrorT MyError IO

Now pay close attention here: ErrorT is just a simple newtype around an Either wrapped in a monad. Getting rid of the newtype, we have:

type ErrorTUnwrapped e m a = m (Either e a)

At some point, we’ll need to actually perform some I/O inside our MyStack. If we went with the unwrapped approach, it would be trivial, as there would be no ErrorT constructor in the way. However, we need that newtype wrapper for a whole bunch of type reasons I won’t go into here (this isn’t a monad transformer tutorial, after all). So ...