Working with lenses

In this recipe, we will be working with the lens library. This library provides a whole battery of functions. We will be using some of those functions. We will also create lenses for our own data type.

How to do it...

1. Create a new project working-with-lenses, with a simple stack template:

        stack new working-with-lenses simple

2. Add a dependency on lens library in the build-depends sub-section of the executable section:

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

3. Open src/Main.hs. We will...