Haskell Cookbook

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 TemplateHaskell #-}

        module Main where

        import Control.Lens
        import Control.Lens.TH

4. Define the sum data type:

    data Point = Point { _x :: Double ...