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    <p>The way <code>pipes</code> does it is to type-class composition using the <code>Channel</code> type class:</p> <pre class="lang-hs prettyprint-override"><code>class Channel p where {-| 'idT' acts like a \'T\'ransparent proxy, passing all requests further upstream, and passing all responses further downstream. -} idT :: (Monad m) =&gt; a' -&gt; p a' a a' a m r {-| Compose two proxies, satisfying all requests from downstream with responses from upstream. -} (&gt;-&gt;) :: (Monad m) =&gt; (b' -&gt; p a' a b' b m r) -&gt; (c' -&gt; p b' b c' c m r) -&gt; (c' -&gt; p a' a c' c m r) p1 &gt;-&gt; p2 = p2 &lt;-&lt; p1 </code></pre> <p>... and derived a lifted composition over <code>EitherT</code> from the base composition. This is a special case of the the principle of proxy transformers, introduced in <code>pipes-2.4</code>, that allows lifting composition over arbitrary extensions.</p> <p>This lifting requires defining an <code>EitherT</code> specialized to the shape of the <code>Proxy</code> type in <code>Control.Proxy.Trans.Either</code>:</p> <pre class="lang-hs prettyprint-override"><code>newtype EitherP e p a' a b' b (m :: * -&gt; *) r = EitherP { runEitherP :: p a' a b' b m (Either e r) } </code></pre> <p>This specialization to the <code>Proxy</code> shape is necessary in order to be able to define a well-typed instance of the <code>Channel</code> class. Scala might be more flexible in this regard than Haskell.</p> <p>Then I just redefine the <code>Monad</code> instance (and other instances) along with all the ordinary <code>EitherT</code> operations for this specialized type:</p> <pre class="lang-hs prettyprint-override"><code>throw :: (Monad (p a' a b' b m)) =&gt; e -&gt; EitherP e p a' a b' b m r throw = EitherP . return . Left catch :: (Monad (p a' a b' b m)) =&gt; EitherP e p a' a b' b m r -- ^ Original computation -&gt; (e -&gt; EitherP f p a' a b' b m r) -- ^ Handler -&gt; EitherP f p a' a b' b m r -- ^ Handled computation catch m f = EitherP $ do e &lt;- runEitherP m runEitherP $ case e of Left l -&gt; f l Right r -&gt; right r </code></pre> <p>With this in hand I can then define the following lifted composition instance:</p> <pre class="lang-hs prettyprint-override"><code>-- Given that 'p' is composable, so is 'EitherP e p' instance (Channel p) =&gt; Channel (EitherP e p) where idT = EitherP . idT p1 &gt;-&gt; p2 = (EitherP .) $ runEitherP . p1 &gt;-&gt; runEitherP . p2 </code></pre> <p>To understand what is going on there, just follow the types:</p> <pre class="lang-hs prettyprint-override"><code>p1 :: b' -&gt; EitherP e p a' a b' b m r p2 :: c' -&gt; EitherP e p b' b c' c m r runEitherP . p1 :: b' -&gt; p a' a b' b m (Either e r) runEitherP . p2 :: c' -&gt; p b' b c' c m (Either e r) -- Use the base composition for 'p' runEitherP . p1 &gt;-&gt; runEitherP . p2 :: c' -&gt; p a' a c' c m (Either e r) -- Rewrap in EitherP (EitherP . ) $ runEitherP . p1 &gt;-&gt; runEitherP . p2 :: c' -&gt; EitherP e p a' a c' c m r </code></pre> <p>This lets you throw and catch errors within a particular stage without interrupting other stages. Here's an example I've copied and pasted from my <code>pipes-2.4</code> announcement post:</p> <pre class="lang-hs prettyprint-override"><code>import Control.Monad (forever) import Control.Monad.Trans (lift) import Control.Proxy import Control.Proxy.Trans.Either as E import Safe (readMay) promptInts :: () -&gt; EitherP String Proxy C () () Int IO r promptInts () = recover $ forever $ do str &lt;- lift getLine case readMay str of Nothing -&gt; E.throw "Could not parse an integer" Just n -&gt; liftP $ respond n recover p = p `E.catch` (\str -&gt; lift (putStrLn str) &gt;&gt; recover p) main = runProxy $ runEitherK $ mapP printD &lt;-&lt; promptInts </code></pre> <p>Here's the result:</p> <pre class="lang-hs prettyprint-override"><code>&gt;&gt;&gt; main 1&lt;Enter&gt; 1 Test&lt;Enter&gt; Could not parse an integer Apple&lt;Enter&gt; Could not parse an integer 5&lt;Enter&gt; 5 </code></pre> <p>The answer to the iteratee approach is similar. You must take your existing way of composing iteratees and lift it over <code>EitherT</code>. Whether or not you use type-classes or just define a new composition operator is up to you.</p> <p>Some other useful links:</p> <ul> <li><a href="http://www.haskellforall.com/2012/10/pipes-24-proxy-transformers-extra.html" rel="nofollow noreferrer"><code>pipes-2.4</code> announcement post</a></li> <li><a href="http://hackage.haskell.org/packages/archive/pipes/2.5.0/doc/html/Control-Proxy-Class.html" rel="nofollow noreferrer"><code>Control.Proxy.Class</code></a>, <a href="http://hackage.haskell.org/packages/archive/pipes/2.5.0/doc/html/Control-Proxy-Trans.html" rel="nofollow noreferrer"><code>Control.Proxy.Trans</code></a>, and <a href="http://hackage.haskell.org/packages/archive/pipes/2.5.0/doc/html/Control-Proxy-Trans-Either.html" rel="nofollow noreferrer"><code>Control.Proxy.Trans.Either</code></a></li> <li>A <a href="https://stackoverflow.com/questions/11402892/what-approach-to-error-handling-to-use-with-pipes-core/11417819#11417819">very similar Stack Overflow question</a> on the same subject (except for `pipes)</li> </ul>
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