Working with the Writer monad transformer

In the previous recipe, we worked with the Reader monad transformer. In this recipe, we will work with the Writer monad transformer. Like the Reader monad transformer, the Writer monad transformer is also a restricted version of a state transformer, in which we can only write (but cannot read).

Getting ready

In this example, we will use the Writer monad transformer to keep updating a balance sheet with transactions. We will keep pushing transactions to the Writer monad, finally yielding the balance after all transactions are processed.

We will work with the mtl library in this recipe.

How to do it...

1. Create a new project write-trans using the simple Stack template.

2. Open write-trans.cabal and add the mtl dependency in the build-depends subsection of the executable section:

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