[Haskell-cafe] using Typeable with STRefs

[Ryan Ingram]

Having the state be an instance of Typeable breaks the purity
guarantees of runST; a reference could escape runST:

  let v = runST (V `liftM` newSTRef 0)
  in runST (readSTRef $ fromJust $ getValue v)

Keep in mind that the state actually used by runST is "RealWorld";
runST is just a pretty name for unsafePerformIO.  So the state types
are actually the same, and the cast would succeed.

  -- ryan

On Mon, Mar 16, 2009 at 5:48 PM, Michael Vanier <mvanier42 at gmail.com> wrote:
> Hi,
>
> I'm having a problem using Typeable with STRefs.  Basically, I want to store
> STRefs (among other things) in a universal type.  STRef is an instance of
> Typeable2, which means that STRef s a is Typeable if s and a are both
> Typeable.  The problem is that the state type s is opaque and I can see no
> way to make it Typeable (other than making it RealWorld, and I don't want to
> use IO for this).  If this is the case, then AFAICT there is no point in
> having STRefs be instances of Typeable2.  Am I missing something?
>
> Here's the code I'd like to write:
>
> import Data.Typeable
> import Data.STRef
> import Control.Monad.ST
>
> data Value = forall a . Typeable a => V a
>  deriving Typeable
>
> getValue :: Typeable a => Value -> Maybe a
> getValue (V v) = cast v
>
> -- I need the Typeable s constraint for the code to compile, but I'd rather
> leave it out.
> test :: Typeable s => ST s Integer
> test = do ref <- newSTRef (10 :: Integer)
>         let refVal = V ref
>         case getValue refVal of
>           Nothing -> error "BAD"
>           Just r -> readSTRef r
>
> -- This doesn't compile, because s is not Typeable.       test2 :: Integer
> test2 = runST test
>
> Thanks in advance,
>
> Mike
>
>
>
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