Creating lenses

In this recipe, we will look at how we can define a generic property getter and setter. We will write a data type and we will write a generic type that will achieve both getting and setting a field inside the data type.

How to do it...

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

        stack new creating-lenses simple

2. Open src/Main.hs. We will be adding our source here. Define the Main module. Enable extension Rank2Types  before the Main module. Also, add StandaloneDeriving and DerivingFunctor.  We will use DerivingFunctor to automatically derive the Functor definition:

        {-# LANGUAGE Rank2Types, StandaloneDeriving, DeriveFunctor #-}
         module Main where

3. Define a data type Point, which represents a two-dimensional point:

        data Point = Point Double Double deriving Show

4. Now define a generic structure s, and we need to get a field of type a from the structure. Its type would be s -> a. Now imagine we need to change some property of structure...