Working with Traversal

In this recipe, we will work with traversals, where we can use lens for traversing many fields.

How to do it...

1. Create a new project working-with-traversal with a simple stack template.

        stack new working-with-traversal simple

2. Add a dependency on the lens library in the build-depends sub-section of the executable section. Also add containers, as we will be using Map in this recipe:

        executable working-with-traversal
         hs-source-dirs:      src
         main-is:             Main.hs
         default-language:    Haskell2010
         build-depends:       base >= 4.7 && < 5
                              , containers
                              , lens

3. Open src/Main.hs. We will be editing this file for this recipe. Add the module definition for Main. Also import the required modules. Enable TemplateHaskell  at the top, as we will be creating lenses using the template Haskell:

        {-# LANGUAGE TemplateHaskell #-}        moduleMainwhere   ...