Sometimes you want to catch two different sorts of exception. You could do something like

    f = expr `catch` \ (ex :: ArithException) -> handleArith ex
             `catch` \ (ex :: IOException)    -> handleIO    ex

    However, there are a couple of problems with this approach. The first is that having two exception handlers is
    inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the
    first, e.g.  in the example above, if handleArith throws an IOException then the second exception handler will 
    catch it. 
    Instead, we provide a function catches, which would be used thus:

    f = expr `catches` [Handler (\ex :: ArithException -> handleArith ex),
                        Handler (\ex :: IOException    -> handleIO    ex)]