Haskell Cookbook

How to do it...

1. Create a new project working-with-monad using the simple Stack template:

        stack new working-with-monad simple

2. Open src/Main.hs and edit it.

3. After the initial module definition, add the following imports:

        import Prelude hiding(Maybe(..))

Note that we are importing otherwise implicitly imported module Prelude explicitly by hiding Maybe. This is because we are defining our own Maybe data type.

4. Import other required headers now. The header for monad is Control.Monad:

        import Data.Functor
        import Control.Applicative
        import Control.Monad

5. Now, define the Maybe data type. Also, define the instance for Functor and Applicative for it:

 data Maybe a = Nothing | Just a deriving Show instance Functor Maybe where fmap f (Just ...