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2010-09-04T10:25:46.160
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Use liftIO You're already very close! Your suggestion <pre><code>newtype Op a = Op {runOp :: ST -> IO (ST, a)} </code></pre> is excellent and the way to go. To be able to execute <code>getLine</code> in an <code>Op</code> context, you need to 'lift' the <code>IO</code> operation into the <code>Op</code> monad. You can do this by writing a function <code>liftIO</code>: <pre><code>liftIO :: IO a -> Op a liftIO io = Op $ \st -> do x <- io return (st, x) </code></pre> You can now write: <pre><code>runOp (do x <- liftIO getLine; ... </code></pre> Use class MonadIO Now the pattern of lifting an IO action into a custom monad is so common that there is a standard type class for it: <pre><code>import Control.Monad.Trans class Monad m => MonadIO m where liftIO :: IO a -> m a </code></pre> So that your version of <code>liftIO</code> becomes an instance of <code>MonadIO</code> instead: <pre><code>instance MonadIO Op where liftIO = ... </code></pre> Use StateT You've currently written your own version of the state monad, specialised to state <code>ST</code>. Why don't you use the standard state monad? It saves you from having to write your own <code>Monad</code> instance, which is always the same for the state monad. <pre><code>type Op = StateT ST IO </code></pre> <code>StateT</code> already has a <code>Monad</code> instance and a <code>MonadIO</code> instance, so you can use those immediately. Monad transformers <code>StateT</code> is a so-called monad transformer. You only want <code>IO</code> actions in your <code>Op</code> monad, so I've already specialized it with the <code>IO</code> monad for you (see the definition of <code>type Op</code>). But monad transformers allow you to stack arbitrary monads. This what intoverflow is talking about. You can read more about them <a href="http://blog.sigfpe.com/2006/05/grok-haskell-monad-transformers.html" rel="noreferrer">here</a> and <a href="http://en.wikibooks.org/wiki/Haskell/Monad_transformers" rel="noreferrer">here</a>.
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1. POCombine state with IO actions
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1. USMartijn
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1. COYour suggestions are all excellent, and I do realize that my 'op' monad is a specialized version of the state monad. But at this stage, I prefer to write the code myself, since it cements the structure and concepts in my head. However, thank you for bringing MonadIO, StateT, and so forth, to my attention.
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