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    <p>Let's imagine we were to <code>IO</code>-ify the <code>State</code> monad. What would that look like? Our pure <code>State</code> monad is just a newtype around:</p> <pre><code>s -&gt; (a, s) </code></pre> <p>Well, the <code>IO</code> version might do a little bit of side effects before returning the final values, which would look like:</p> <pre><code>s -&gt; IO (a, s) </code></pre> <p>That pattern is so common it has a name, specifically <code>StateT</code>:</p> <pre><code>newtype StateT s m a = StateT { runStateT :: s -&gt; m (a, s) } </code></pre> <p>The name has a <code>T</code> at the end because it is a monad <code>T</code>ransformer. We call <code>m</code> the "base monad" and <code>StateT s m</code> the "transformed" monad.</p> <p><code>StateT s m</code> is only a <code>Monad</code> if <code>m</code> is a <code>Monad</code>:</p> <pre><code>instance (Monad m) =&gt; Monad (StateT s m) where {- great exercise -} </code></pre> <p>However, in addition to that, all monad transformers implement the <code>MonadTrans</code> class, defined as follows:</p> <pre><code>class MonadTrans t where lift :: (Monad m) =&gt; m a -&gt; t m a instance MonadTrans (StateT s) where {- great exercise -} </code></pre> <p>If <code>t</code> is <code>StateT s</code>, then <code>lift</code>'s type specializes to:</p> <pre><code>lift :: m a -&gt; StateT s m a </code></pre> <p>In other words, it lets us "lift" an action in the base monad to become an action in the transformed monad.</p> <p>So, for your specific problem, you want the <code>StateT (IntMap k v) IO</code> monad, which extends <code>IO</code> with additional <code>State</code>. Then you can write your entire program in this monad:</p> <pre><code>main = flip runStateT (initialState :: IntMap k v) $ do m &lt;- get -- retrieve the map lift $ print m -- lift an IO action (k, v) &lt;- lift readLn put (insert k v m) </code></pre> <p>Notice that I still use <code>get</code> and <code>put</code>. That's because the <code>transformers</code> package implements all the concepts I described and it generalizes the signatures of <code>get</code> and <code>put</code> to be:</p> <pre><code>get :: (Monad m) =&gt; StateT s m s put :: (Monad m) =&gt; s -&gt; StateT s m () </code></pre> <p>That means they automatically work within <code>StateT</code>. <code>transformers</code> then just defines <code>State</code> as:</p> <pre><code>type State s = StateT s Identity </code></pre> <p>That means that you can use <code>get</code> and <code>put</code> for both <code>State</code> and <code>StateT</code>.</p> <p>To learn more about monad transformers, I highly recommend <a href="http://www.cs.virginia.edu/~wh5a/personal/Transformers.pdf" rel="noreferrer">Monad Transformers - Step by Step</a>.</p>
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    1. POStuck in the State Monad
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    1. USGabriel Gonzalez
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    1. POStuck in the State Monad
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    1. COAs always, an answer stuffed with wisdom. Thanks Gabriel, I'll have to process for a bit and see what the implications are for my application.
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    2. COThat was the perfect paper for me to read right now. I knew I was going to have to dig into Monad Transformers at some point, I just didn't realize how soon.
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    3. CO@MFlamer Yeah, Haskell is all about composing solutions from small building blocks. If monads are language building blocks (i.e. state, error handling, io), then monad transformers are composable building blocks (literally: assembling monad transformers is composition where monad transformers are monad morphisms between monad objects). When you build your own monad transformer stack, you are basically picking and choosing which language features you want to incorporate, unlike other languages which make the choice for you.
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