[Haskell-cafe] using Typeable with STRefs

Michael Vanier mvanier42 at gmail.com
Mon Mar 16 21:08:35 EDT 2009


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.


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 <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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