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POUnwrapping a monad
 

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    <p>Given the below program, I am having issues dealing with monads.</p> <pre><code>module Main where import System.Environment import System.Directory import System.IO import Text.CSV -------------------------------------------------- exister :: String -&gt; IO Bool exister path = do fileexist &lt;- doesFileExist path direxist &lt;- doesDirectoryExist path return (fileexist || direxist ) -------------------------------------------------- slurp :: String -&gt; IO String slurp path = do withFile path ReadMode (\handle -&gt; do contents &lt;- hGetContents handle last contents `seq` return contents ) -------------------------------------------------- main :: IO () main = do [csv_filename] &lt;- getArgs putStrLn (show csv_filename) csv_raw &lt;- slurp csv_filename let csv_data = parseCSV csv_filename csv_raw printCSV csv_data -- unable to compile. </code></pre> <p>csv_data is an Either (parseerror) CSV type, and printCSV takes only CSV data.</p> <hr> <p>Here's the ediff between the working version and the broken version.</p> <pre><code>*************** *** 27,30 **** csv_raw &lt;- slurp csv_filename let csv_data = parseCSV csv_filename csv_raw ! printCSV csv_data -- unable to compile. \ No newline at end of file --- 27,35 ---- csv_raw &lt;- slurp csv_filename let csv_data = parseCSV csv_filename csv_raw ! case csv_data of ! Left error -&gt; putStrLn $ show error ! Right csv_data -&gt; putStrLn $ printCSV csv_data ! ! putStrLn "done" ! </code></pre> <p>reference: <a href="http://hackage.haskell.org/packages/archive/csv/0.1.2/doc/html/Text-CSV.html" rel="nofollow">http://hackage.haskell.org/packages/archive/csv/0.1.2/doc/html/Text-CSV.html</a></p>
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    1. USPaul Nathan
    1. USPaul Nathan
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    1. COThe question doesn't seem to have anything to do with monads. :)
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      1. USRotsor
    2. CO@Rotsor I was just going to post a comment reply, but it ballooned into an answer. The question is kinda sorta is about monads, but not really.
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      1. USDan Burton
    3. COPaul, isn't the problem the poor name assigned `printCSV :: CSV -> String`? It is a pure function, not a function to an IO type. If I write `putStrLn $ printCSV csv_data` for the last line, it compiles.
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