Main.hs

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleContexts #-}
import Control.Monad.State
import Control.Monad.Trans.Free

data Turtle n
    = MoveForward n
    | MoveBackwards n
    | TurnLeft n
    | TurnRight n
    | MarkSquare n
    | Echo String n
    | Stop n
    deriving (Show, Eq, Functor)

data TurtleState = TurtleState String

type Program = Free Turtle

echo :: String -> Program ()
echo x = liftF (Echo x ())

program :: Program ()
program = do
    echo "foobar"

run :: Program a -> IO ()
run prg =
    case runFree prg of
        Free (Echo s n) -> putStrLn s >> run n

main :: IO ()
main = undefined