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    <p>This is the <a href="http://www.haskell.org/haskellwiki/Scoped_type_variables" rel="nofollow noreferrer">Scoped Type Variables</a> GHC extension at work. Follow the link to learn more.</p> <p>Basically, you define an assertion about type that have to be met by the patter before it can match. So, yeah, it is akin to guards, but not completely so.</p> <p>How this particular example works? Dive into <a href="http://www.haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/src/Control-Exception-Base.html" rel="nofollow noreferrer">sources of "base" library</a> to find out that:</p> <pre><code>class (Show e) =&gt; Exception e where toException :: e -&gt; SomeException fromException :: SomeException -&gt; Maybe e data SomeException = forall e . Exception e =&gt; SomeException e instance Exception IOException where toException = IOException fromException (IOException e) = Just e fromException _ = Nothing instance Exception ArithException where toException = ArithException fromException (ArithException e) = Just e fromException _ = Nothing </code></pre> <p>We see that <code>IOException</code> and <code>ArithException</code> are different types implementing the typeclass <code>Exception</code>. We also see that <code>toException/fromException</code> is a wrapping/unwrapping mechanism that allows one to convert values of type <code>Exception</code> to/from values of types <code>IOException</code>, <code>ArithException</code>, etc.</p> <p>So, we could've written:</p> <pre><code>f = expr `catches` [Handler handleArith, Handler handleIO] handleArith :: ArithException -&gt; IO () handleArith ex = .... handleIO :: IOException -&gt; IO () handleIO ex = .... </code></pre> <p>Suppose that <code>IOException</code> happens. When <code>catchesHandler</code> processes first element of the handlers list, it calls <code>tryHandler</code>, which calls <code>fromException</code>. From the definition of <code>tryHandler</code> it follows that return type of the <code>fromException</code> should be the same as argument of <code>handleArith</code>. On the other hand, <code>e</code> is of type Exception, namely - (IOException ...). So, the types play out this way (this is not a valid haskell, but I hope that you get my point):</p> <pre><code>fromException :: (IOException ...) -&gt; Maybe ArithException </code></pre> <p>From the <code>instance Exception IOException ...</code> it follows immediately that the result is <code>Nothing</code>, so this handler is skipped. By the same reasoning the following handler would be called, because <code>fromException</code> would return <code>(Just (IOException ...))</code>.</p> <p>So, you've used type signatures of <code>handleArith</code> and <code>handleIO</code> to specify when each of them would be called, and <code>fromException/toException</code> made sure that it happened this way.</p> <p>If you want to, you could also constraint types of <code>handleIO</code> and <code>handleArith</code> inside the definition of <code>f</code>, using scoped type variables. Arguably, this could give you better readability.</p> <p>Finalizing, Scoped Type Variables are not a primary players here. They are just used for convenience. Main machinery for playing this kind of tricks is <code>fromException/toException</code> and friends. Scoped Type Variables just allow you to have syntax which more closely resemble guard patterns.</p>
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