Forwarding response to cafe ---------- Forwarded message ---------- From: Antoine Latter Date: Sun, Jan 4, 2009 at 2:13 PM Subject: Re: [Haskell-cafe] Auto-deriving Control.Exception.Exception To: "eyal.lotem@gmail.com" On Sun, Jan 4, 2009 at 11:50 AM, eyal.lotem@gmail.com wrote:

If this is valid:

import qualified Control.Exception as Exc

genericFromException :: Typeable a => Exc.SomeException -> Maybe a genericFromException (Exc.SomeException e) = cast e instance Exc.Exception SomeType where fromException = genericFromException

then why not have an auto-deriving rule for Exception (fromException = cast . unSomeException)?

If you just say:

instance Exception YourType where

and leave the instance body empty, it will use the default definitions of 'fromException' and 'toException', which do the right thing by default. At least, I'm pretty sure that's what happens. The Haddock docs have lost their links to the source code. -Antoine