Introduction

If you have worked with object-oriented programming, then you must be aware of the properties (such as in C# or Python, or even in managed C++). Usually, we can access the properties inside an object and also set the property to some value:

    Point point = Point(1.0, 2.0);
    double x = point.x; // Should be 1.0
    point.x = 3.0;      // Now point x is changed to 3.0

Though the preceding code mutates the data, it is very convenient to get and set a property. Imagine doing the same with Haskell:

    data Point = Point Double Double

    x :: Point -> Double
    x (Point xv _) = xv

    setx :: Point -> Double -> Point
    setx (Point _ y) x = Point x y

We need to de-construct a type, and reconstruct it again. If we had some generic way of accessing a field inside the data, and then accessing it back, then we will get the lost convenience of getting and...