Re: [Haskell-cafe] Safe top-level IORefs

4 Mar 2007

      Neil Mitchell wrote:
...
Hi
On 3/4/07, Roberto Zunino  wrote:
...
I'm posting the code of a module, IORefs, allowing top-level IORefs to
be safely declared and used. Usafety reports are welcome. Tested in
GHC 6.6.
That looks cool, does it work on Hugs?
I've tested it right now.

Yes, it works with -98, but you have to provide a (correct!) instance of
Typeable X on your own.

Declaring a ref becomes a bit more cumbersome:

data X  -- the actual name of the IORef
instance Typeable X where typeOf _ = mkTyConApp (mkTyCon "Main.X") []
instance IORefDefault X Int where ioRefDefault _ = 42  -- initial value
x = ioRef (undefined :: X) -- a convenient name for the IORef

It would be better to use drift to write the instance for you, since
getting that wrong breaks Typeable, and ioRef roo.

Finally, IORefs.ioRef must be changed to use asTypeOf.

Zun.

================================================================
-- Hugs & GHC version
ioRef :: IORefDefault a b => a -> IORef b
ioRef x = unsafePerformIO $
   do
   rs <- readIORef refs
   case typeOf x `M.lookup` rs of
      Nothing -> do
                 ref <- newIORef $ ioRefDefault (undefined `asTypeOf` x)
                 writeIORef refs $ M.insert (typeOf x) (Ref ref) rs
                 return ref
      Just (Ref aRef) -> case cast aRef of
                         Nothing  -> error $ "ioRef: impossible!"
                         Just ref -> return ref