Working with Prism

Lens gives you the ability to focus on a particular field in a data type. Traversal will do the same thing for a traversable (something that you can traverse and collect). But these data types were product types.

In this recipe, we will work with Prism, where we will work with sum type data.

How to do it...

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

        stack new working-with-prism simple

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

     executable working-with-prism
      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 be adding our source here. Enable TemplateHaskell, as we will produce Prism using the template Haskell. Add the Main module. Import the template haskell module for lens, alongwith other imports:

        {-# LANGUAGE...