Developing Web Applications with Haskell and Yesod by Michael Snoyman

Author:Michael Snoyman [Michael Snoyman]
Language: eng
Format: epub
Tags: COMPUTERS / Programming Languages / General
ISBN: 9781449316969
Publisher: O'Reilly Media
Published: 2012-04-22T16:00:00+00:00

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, which could error out at any time? One approach to this would be to write a new monad like ReaderError, but this has the obvious downside of exponential complexity: you’ll need to write a new monad for every single possible combination.

Instead, we have monad transformers. In addition to Reader, we have ReaderT, which adds reader functionality to any other monad. So we could represent our ReaderError as (conceptually):

type ReaderError = ReaderT Error

In order to access our settings variable, we can use the ask function. But what about short-circuiting a computation? We’d like to use throwError, but that won’t exactly work. Instead, we need to lift our call into the next monad up. In other words:

throwError :: errValue -> Error lift . throwError :: errValue -> ReaderT Error

There are a few things you should pick up here:

A transformer can be used to add functionality to an existing monad.

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