Example of free monad usage in Haskell

free.hs

{-# LANGUAGE DeriveFunctor #-}

module Main where

import Control.Monad
import Control.Monad.Free

data InteractionF f
    = Get (String -> f)
    | Put String f
    deriving (Functor)

type Interaction = Free InteractionF

put :: String -> Interaction ()
put s = liftF (Put s ())

get :: Interaction String
get = liftF (Get id)

hello :: Interaction String
hello = do
    put "What's your name?"
    name <- get
    put ("Hello, " ++ name)
    pure name

interpret :: InteractionF (IO a) -> IO a
interpret (Get f) = getLine >>= f
interpret (Put s f) = putStrLn s >> f

main :: IO ()
main = void (iterM interpret hello)