Haskell Cookbook

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 be adding our source here. Add a TemplateHaskell extension for creating lenses for the user-defined data types. Define the Main module, and import the necessary imports:

        {-# LANGUAGE TemplateHaskell #-}

        module Main where

        import Control.Lens.TH
        import Control.Lens

4. Define a data type, Line, which is composed of two end Points. Note that we used *_* for naming ...