Skip to Main Content
Developing Web Applications with Haskell and Yesod
book

Developing Web Applications with Haskell and Yesod

by Michael Snoyman
April 2012
Intermediate to advanced content levelIntermediate to advanced
294 pages
6h 27m
English
O'Reilly Media, Inc.
Content preview from Developing Web Applications with Haskell and Yesod

Chapter 13. Yesod’s Monads

As you’ve read through this book, there have been a number of monads that 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 let’s 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. Shrek, more or less

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 would like 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 variable, ...

Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Start your free trial

You might also like

Developing Web Apps with Haskell and Yesod, 2nd Edition

Developing Web Apps with Haskell and Yesod, 2nd Edition

Michael Michael Snoyman Snoyman

Publisher Resources

ISBN: 9781449336868Supplemental ContentErrata Page