Working with Monoid

Monoid is an important and very useful type class. A Monoid assumes two behaviors:

1. There is a default or empty value of the data type.
2. Given two values of the data type, they can be combined to create a single value. 

The simplest example of a Monoid is Integer. We can define an empty value of an Integer as 0. We can then use addition as an operation to combine two Integers. In this recipe, we will define a data type Option and define an instance for Monoid.

Getting ready

Create a new project called working-with-monoid with the simple template using Stack:

    stack new working-with-monoid simple

Change into the newly created project directory.

How to do it...

1. Start editing src/Main.hs. Add import Data.Monoid at the top. This module contains the definition of the Monoid type class. 
2. Define a data type Option. The data contains a Boolean field and a list of String:

        data Option = Option { boolOption :: Bool, selections :: 
        [String] }
            deriving Show

3. Define...