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POHow do I combine IOError exceptions with locally relevant exceptions?
 

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  1. POHow do I combine IOError exceptions with locally relevant exceptions?
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    <p>I am building a Haskell application and trying to figure out how I am going to build the error handling mechanism. In the real application, I'm doing a bunch of work with Mongo. But, for this, I'm going to simplify by working with basic IO operations on a file.</p> <p>So, for this test application, I want to read in a file and verify that it contains a proper fibonnacci sequence, with each value separated by a space:</p> <pre><code>1 1 2 3 5 8 13 21 </code></pre> <p>Now, when reading the file, any number of things could actually be wrong, and I am going to call all of those <a href="http://www.haskell.org/haskellwiki/Error_vs._Exception" rel="nofollow">exceptions in the Haskell usage of the word</a>.</p> <pre><code>data FibException = FileUnreadable IOError | FormatError String String | InvalidValue Integer | Unknown String instance Error FibException where noMsg = Unknown "No error message" strMsg = Unknown </code></pre> <p>Writing a pure function that verifies the sequence and throws an error in the case that the sequence is invalid is easy (though I could probably do better):</p> <pre><code>verifySequence :: String -&gt; (Integer, Integer) -&gt; Either FibException () verifySequence "" (prev1, prev2) = return () verifySequence s (prev1, prev2) = let readInt = reads :: ReadS Integer res = readInt s in case res of [] -&gt; throwError $ FormatError s (val, rest):[] -&gt; case (prev1, prev2, val) of (0, 0, 1) -&gt; verifySequence rest (0, 1) (p1, p2, val') -&gt; (if p1 + p2 /= val' then throwError $ InvalidValue val' else verifySequence rest (p2, val)) _ -&gt; throwError $ InvalidValue val </code></pre> <p>After that, I want the function that reads the file and verifies the sequence:</p> <pre><code>type FibIOMonad = ErrorT FibException IO verifyFibFile :: FilePath -&gt; FibIOMonad () verifyFibFile path = do sequenceStr &lt;- liftIO $ readFile path case (verifySequence sequenceStr (0, 0)) of Right res -&gt; return res Left err -&gt; throwError err </code></pre> <p>This function does exactly what I want if the file is in the invalid format (it returns <code>Left (FormatError "something")</code>) or if the file has a number out of sequence (<code>Left (InvalidValue 15)</code>). But it throws an error if the file specified does not exist.</p> <p>How do I catch the IO errors that readFile may produce so that I can transform them into the FileUnreadable error?</p> <p>As a side question, is this even the best way to do it? I see the advantage that the caller of <code>verifyFibFile</code> does not have to set up two different exception handling mechanisms and can instead catch just one exception type.</p>
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