Developing Web Apps with Haskell and Yesod, 2nd Edition

Chapter 13. Yesod’s Monads

As you’ve progressed through this book so far, a number of monads have appeared: Handler, Widget, and YesodDB (for Persistent). As with most monads, each one provides some specific functionality: Handler gives access to the request and allows you to send responses; a Widget contains HTML, CSS, and JavaScript; and YesodDB lets you make database queries. In model-view-controller (MVC) terms, we could consider YesodDB to be the model, Widget to be the view, and Handler to be the controller.

So far, we’ve presented some very straightforward ways to use these monads: your main handler will run in Handler, using runDB to execute a YesodDB query and defaultLayout to return a Widget, which in turn was created by calls to toWidget.

However, if we have a deeper understanding of these types, we can achieve some fancier results.

Monad Transformers

Monads are like onions. Monads are not like cakes.

Variation on a quote from Shrek

Before we get into the heart of Yesod’s monads, we need to understand a bit about monad transformers. (If you already know all about monad transformers, you can likely skip this section.) Different monads provide different functionality: Reader allows read-only access to some piece of data throughout a computation, Error allows you to short-circuit computations, and so on.

Oftentimes, however, you’ll want to be able to combine a few of these features together. After all, why not have a computation with read-only access to some settings ...