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copied!<p>@Savanni D'Gerinel: you are on the right track. Let's extract your error-catching code from <em>verifyFibFile</em> to make it more generic, and modify it slightly so that it works directly in ErrorT:</p> <pre><code>catchError' :: ErrorT e IO a -> (IOError -> ErrorT e IO a) -> ErrorT e IO a catchError' m f = ErrorT $ catchError (runErrorT m) (fmap runErrorT f) </code></pre> <p><em>verifyFibFile</em> can now be written as:</p> <pre><code>verifyFibFile' :: FilePath -> FibIOMonad () verifyFibFile' path = do sequenceStr <- catchError' (liftIO $ readFile path) (throwError . FileUnReadable) ErrorT . return $ verifySequence sequenceStr' (0, 0) </code></pre> <p>Notice what we have done in <em>catchError'</em>. We have stripped the ErrorT constructor from the <code>ErrorT e IO a</code> action, and also from the return value of the error-handling function, knowing than we can reconstruct them afterwards by wrapping the result of the control operation in ErrorT again.</p> <p>Turns out that this is a common pattern, and it can be done with monad transformers other than ErrorT. It can get tricky though (how to do this with ReaderT for example?). Luckily, the <a href="http://hackage.haskell.org/package/monad-control" rel="nofollow">monad-control</a> packgage already provides this functionality for many common transformers.</p> <p>The type signatures in monad-control can seem scary at first. Start by looking at just one function: <a href="http://hackage.haskell.org/packages/archive/monad-control/0.3.1.4/doc/html/Control-Monad-Trans-Control.html#v%3acontrol" rel="nofollow">control</a>. It has the type:</p> <pre><code>control :: MonadBaseControl b m => (RunInBase m b -> b (StM m a)) -> m a </code></pre> <p>Let's make it more specific by making <code>b</code> be <code>IO</code>:</p> <pre><code>control :: MonadBaseControl IO m => (RunInBase m IO -> IO (StM m a)) -> m a </code></pre> <p><em>m</em> is a monad stack built on top of <em>IO</em>. In your case, it would be <em>ErrorT IO</em>.</p> <p><code>RunInBase m IO</code> is a type alias for a magical function, that takes a value of type <code>m a</code> and returns a value of type <code>IO *something*</code>, <em>something</em> being some complex magic that encodes the state of the whole monad stack inside IO and lets you reconstruct the <code>m a</code> value afterwards, once you have "fooled" the control operation that only accepts IO values. <em>control</em> provides you with that function, and also handles the reconstruction for you.</p> <p>Applying this to your problem, we rewrite <em>verifyFibFile</em> once more as:</p> <pre><code>import Control.Monad.Trans.Control (control) import Control.Exception (catch) verifyFibFile'' :: FilePath -> FibIOMonad () verifyFibFile'' path = do sequenceStr <- control $ \run -> catch (run . liftIO $ readFile path) (run . throwError . FileUnreadable) ErrorT . return $ verifySequence sequenceStr' (0, 0) </code></pre> <p>Keep in mind that this only works when the proper instance of <code>MonadBaseControl b m</code> exists. </p> <p><a href="http://www.yesodweb.com/blog/2011/08/monad-control" rel="nofollow">Here</a> is a nice introduction to monad-control.</p>

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