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POIs there a `replicateM` function for the internal monad in a monad transformer?
 

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  1. POIs there a `replicateM` function for the internal monad in a monad transformer?
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    <p>Suppose I have something like this:</p> <pre><code>data Environment = ... data MyState = ... data Report = ... updateState :: Environment -&gt; MyState -&gt; MyState updateState = ... report :: MyState -&gt; Report report = ... foo :: ReaderT Environment (State MyState) Report foo = do env &lt;- ask state &lt;- lift get let newState = updateState env state lift $ put newState return $ report newState </code></pre> <p>What's in my head are simulations of a time process in which I have parameters that will be stored in <code>Environment</code>, the dynamical state will be stored in <code>MyState</code> and info I wish to collect in each time step of the simulation will be stored in <code>Report</code>.</p> <p>Now, I wan't to run many steps of this simulation and get a list with the reports for each time step. </p> <p>I usually did this without a <code>ReaderT</code> and used to pass the parameters like this:</p> <pre><code> foo :: Enviroment -&gt; State MyState Report </code></pre> <p>Then I would just do:</p> <pre><code> manySteps :: Int -&gt; Enviroment -&gt; State MyState [Report] manySteps n env = replicate n $ (foo env) </code></pre> <p>I'm getting confused with the types of <code>lift</code> and <code>replicateM</code>. Is there a combination that would replicate the <code>State MyState</code> monad inside the transformer?</p> <p>In the future I'll replace <code>ReaderT Environment (State MyState) Report</code> for <code>ReaderT Environment (StateT MyState (Rand StdGen)) Report</code> so it's better to get things right before having this monster type :(. </p> <p><strong>edit:</strong> as a side question - is there a better strategy than using <code>ReaderT Environment (State MyState) Report</code> ? </p>
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    1. USRafael S. Calsaverini
    1. USRafael S. Calsaverini
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      1. USRalph
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    1. COThe strategy of using ReaderT and StateT looks reasonable. For real use I would suggest not using "ask,get,put" directly, define a short alias or helper with a descriptive name and use that instead. This allows you to edit the implementation of the Monad stack and only have to update the helpers.
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      1. USChris Kuklewicz
 

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