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 <mvanie...@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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