Fwd: [Haskell-cafe] Auto-deriving Control.Exception.Exception

Antoine Latter aslatter at gmail.com
Sun Jan 4 15:13:50 EST 2009

Forwarding response to cafe

---------- Forwarded message ----------
From: Antoine Latter <aslatter at gmail.com>
Date: Sun, Jan 4, 2009 at 2:13 PM
Subject: Re: [Haskell-cafe] Auto-deriving Control.Exception.Exception
To: "eyal.lotem at gmail.com" <eyal.lotem at gmail.com>

On Sun, Jan 4, 2009 at 11:50 AM, eyal.lotem at gmail.com
<eyal.lotem at 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

At least, I'm pretty sure that's what happens.  The Haddock docs have
lost their links to the source code.


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