Ryan, So, if I understand you correctly, my only option is to use an IORef instead of an STRef? What I'm trying to do is implement a mutable box type as part of a dynamically-typed language I'm implementing in Haskell (which is mainly an exercise to improve my Haskell programming; mission accomplished). It bothers me that I have to use an IORef for this, since I don't see what this has to do with I/O. Similarly, if I wanted to have a mutable array type, I couldn't use STArray; I'd have to use IOArray. Or, I suppose I could define a richer Value type that had extra constructors for stateful types. Mike Ryan Ingram wrote:

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 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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Why not:

data Value = forall a . Typeable a => V a

type STValue s = STRef Value

This should be: type STValue s = STRef s Value Antoine