Combining monad transformers

So far, we have seen different monad transformers dedicated to specific causes. What if we would like to work with more than one transformer at the same time? In this recipe, we will be doing exactly that! We will work with Reader and Writer transformers with IO monad.

We will revisit the cursor example that we wrote earlier and then transform it to use it with multiple monad transformers.

How to do it...

1. Create a new project combine-trans with the simple Stack template.
2. Add mtl to the build-depends subsection of the executable section:

        executable combine-trans
          hs-source-dirs: src
          main-is: Main.hs
          default-language: Haskell2010
          build-depends: base >= 4.7 && < 5
                 , mtl

3. Open src/Main.hs and add the following imports after the initial module declaration. Import Prelude as well to avoid a clash with some names:

        import Prelude hiding (Either(..))
        import Control.Monad.Reader
    ...