Haskell Cookbook

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               , mtl

3. Open src/Main.hs. We will edit this file for our purpose.
4. Add the following imports after initial module declaration for the WriterT monad transformer:

        import Data.Monoid        import Control.Monad.Writer

5. Add the data type Transaction. We will also add a monoid instance for the Transaction: 

       newtype Transaction = Transaction Double deriving Show

Add the monoid instance. We will define an empty transaction as a transaction ...