Haskell Cookbook

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 the instance of Monoid. The Monoid class needs to define at minimum two functions, mempty and mappend: 

        instance Monoid Option where        mempty = Option False []       (Option b1 s1) `mappend` (Option b2 s2) = Option (b1 || b2) (s1        ++ s2)

4. Use the Option data type and its Monoid instance in the main function:

        main :: IO ()        main = do        putStrLn "Define default options"        let defaultOptions = mempty :: Option putStrLn (show defaultOptions) ...