Haskell Cookbook

How to do it...

1. Create a new project state-monad using the stack command with the simple template:

        stack new state-monad simple

2. Open the file src/Main.hs; we will add our code here after the initial module declaration.
3. Import the following modules:

        import Prelude hiding (Either(..))        import Data.Functor        import Control.Applicative        import Control.Monad

4. Now, add the definition for the State Monad. A State Monad will store state s with the monad:

        data State s a = State { runState :: s -> (a, s) } 

5. Now, we will write the Functor instance for the state. Writing the Functor instance is easy; we need to transform output a with the function f :: a -> b and produce b:

        instance Functor (State s) where        fmap f (State stateFunc) =  let nextStateFunction ...