How do I create dynamic exceptions

[Bayley, Alistair]

> From: Ross Paterson [mailto:ross@soi.city.ac.uk]
> 
> On Tue, Jul 22, 2003 at 02:59:12PM +0100, Bayley, Alistair wrote:
> > > handler :: Dynamic -> IO ()
> > > handler e = do
> > >   case (fromDynamic e) of
> > >    Nothing -> putStrLn ("dynamic cast failed")
> > >    Just (MkExc n s) -> putStrLn ("exception: " ++ (show 
> n) ++ " " ++ s)
> > 
> > > main :: IO ()
> > > main = catchDyn temp handler
> 
> You're catching a Dynamic wrapped as a Dynamic (catchDyn already does
> fromDynamic, as indicated by its type).  Change the handler to
> 
> > handler :: MyException -> IO ()
> > handler (MkExc n s) = putStrLn ("exception: " ++ (show n) 
> ++ " " ++ s)
> 
> any other dynamic exception fails through to the top level.


Thanks. I was applying some advice from Hal Daume:


> so, what you want is probably something like:
>
> > if rc < 0
> >   then throwIO (DynException (toDyn ("Err", rc)))
> >   else ...
>
> and then when you catch, use fromDyn(amic) to get the value out if you can
> predict its type...


... but in this case what I wanted was (just one more layer of indirection):


> handler :: Exception -> IO ()
> handler e = do
>   case e of
>     DynException err -> case (fromDynamic err) of
>       Nothing -> putStrLn "dynamic cast failure"
>       Just (MkExc n s) -> putStrLn ("exception: " ++ (show n) ++ " " ++ s)
>     _ -> putStrLn "unhandled exception"


> main :: IO ()
> main = catch temp handler  -- use catch rather than catchDyn


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