Exception Handling

[Volker Stolz]

On Tue, Nov 28, 2000 at 11:15:14AM +0000, Smelly Pooh wrote:
> I'm writing an application at the moment (in Haskell duh) and the issue of
> exception handling has come up.  I don't want to use Maybe (because that would
> destructure the code somewhat) or any kind of Monad and I've settled on the
> idea of SPJs Exception extension.  The idea is to create a data type with a
> different constructor per exception as in
> 
> data MyException  =  Ex1 | Ex2 | Ex3
> 
> then use throwDyn/catchDyn to throw and catch it.  Unfortunately this requires
> MyException to be an instance of class Typeable, and the documentation on that
> is quite sparse.  Any advice or pointers would be greatly appreciated

\begin{code}
data DMsg = DOne | DTwo Int | DThree String | DQuit

instance Typeable DMsg where
  typeOf _ = mkAppTy msgTc []
msgTc :: TyCon
msgTc = mkTyCon "Msg"

-- To Dynamic:
let dyn = toDyn msg

-- and back:
let msg = fromDynamic dyn
case msg of
  Just _ -> ...
  Nothing -> error (..)
\end{code}
-- 
\usepackage[latin1]{inputenc}!
Volker Stolz * stolz@i2.informatik.rwth-aachen.de * PGP + S/MIME