Haskell Cookbook

How to do it...

1. Create new project state-monad-trans using the simple Stack template:

        stack new state-monad-trans simple

2. Open src/Main.hs. We will be adding our state transformer here.
3. Add the following imports after the initial module declaration:

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

4. Define the State Monad transformer. Note how we embed middle m (monad) in the type:

        newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }

5. Define the Functor instance for our state transformer:

        instance Functor m => Functor (StateT s m) where        fmap f (StateT func) =         let stateFunc s = (\(xa,xs) -> (f xa, xs)) <$> func s        in StateT stateFunc

6. Now, define an Applicative instance for our state transformer. We make ...