type Index = Integer

data Namegen a = Namegen (Index -> (a, Index))

runNameGen :: Index -> Namegen a -> a
runNameGen i (Namegen f) = fst (f i)

supply :: Namegen Integer
supply = Namegen $ \i -> (i, i + 1)

makeName :: String -> Namegen String
makeName base = supply >>= \i -> return $ base ++ show i

makeTempName :: Namegen String
makeTempName = makeName "t"

instance Functor Namegen where
    fmap f (Namegen g) = Namegen $ \i -> let (x, i2) = g i in (f x, i2)

instance Applicative Namegen where
    pure x = Namegen $ \i -> (x, i)

    (Namegen fg) <*> (Namegen xg) =
        Namegen $ \i -> let (f, i2) = fg i
                            (x, i3) = xg i2
                        in (f x, i3)

instance Monad Namegen where
    (Namegen xg) >>= f =
        Namegen $ \i -> let (x, i2) = xg i
                            (Namegen resg) = f x
                        in resg i2


funString :: Int -> Namegen String
funString i = do
    n1 <- makeTempName
    n2 <- makeTempName
    return $ "Fun String " ++ show i ++ ", made with '" ++ n1 ++ "' and '" ++ n2 ++ "'"

main :: IO ()
main = do
    putStrLn $ runNameGen 1 $ do
        n1 <- makeName "xyz"
        s <- funString 42
        n2 <- makeName "abc"
        return $ s ++ "\nGenerated names " ++ n1 ++ " and " ++ n2