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The basic approach would be to rewrite your <code>Op</code> monad as a monad transformer. This would allow you to use it in a "stack" of monads, the bottom of which might be <code>IO</code>. Here's an example of what that might look like: <pre><code>import Data.Array import Control.Monad.Trans data Registers = Reg { foo :: Int } data ST = ST {registers :: Registers, memory :: Array Int Int} newtype Op m a = Op {runOp :: ST -> m (ST, a)} instance Monad m => Monad (Op m) where return a = Op $ \st -> return (st, a) (>>=) stf f = Op $ \st -> do (st1, a1) <- runOp stf st (st2, a2) <- runOp (f a1) st1 return (st2, a2) instance MonadTrans Op where lift m = Op $ \st -> do a <- m return (st, a) getState :: Monad m => (ST -> a) -> Op m a getState g = Op $ \st -> return (st, g st) updState :: Monad m => (ST -> ST) -> Op m () updState g = Op $ \st -> return (g st, ()) testOpIO :: Op IO String testOpIO = do x <- lift getLine return x test = runOp testOpIO </code></pre> The key things to observe: <ul> <li>The use of the <code>MonadTrans</code> class</li> <li>The use of the <code>lift</code> function acting on <code>getLine</code>, which is used to bring the <code>getline</code> function from the <code>IO</code> monad and into the <code>Op IO</code> monad.</li> </ul> Incidentally, if you don't want the <code>IO</code> monad to always be present, you can replace it with the <code>Identity</code> monad in <code>Control.Monad.Identity</code>. The <code>Op Identity</code> monad behaves exactly the same as your original <code>Op</code> monad.
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1. COI realize (now) that the solution presented is standard, but this is a very interesting (and elegant) solution. Your rewrite of my code using a monad transformer was very helpful, since it provided a concrete example. Thanks!
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