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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