# How to make Claessen's Refs Ord-able?

Josef Svenningsson josefs@cs.chalmers.se
Wed, 20 Mar 2002 16:00:09 +0100 (MET)

```Hi,

Have you tried the following:

{-# NOINLINE refWInt #-}

?
/Josef

On Tue, 19 Mar 2002, Mike Gunter wrote:

>
> I'd like to extend the Ref type for observable sharing of Koen Claessen's
> Ph.D. thesis:
>
> > import IOExts     ( IORef, newIORef, readIORef, writeIORef, unsafePerformIO )
> >
> > newtype Ref a = MkRef (IORef a)
> >
> > ref :: a -> Ref a
> > ref x = MkRef (unsafePerformIO (newIORef x))
> >
> > deref :: Ref a -> a
> > deref (MkRef r) = unsafePerformIO (readIORef r)
> >
> > (<==>) :: Ref a -> Ref a -> Bool
> > (MkRef ref1) <==> (MkRef ref2)    = ref1 == ref2
>
> to make it possible for the Refs to be the keys for efficient finite maps.
> I.e. I want something with at least an efficient Ord-like compare and maybe
> also a hash function.
>
> The closest I've got is:
>
> > type    UniqTy    = Integer
> > newtype RefWInt a = RefWInt (UniqTy, a)   deriving (Show)
> >
> > curUniqInt    :: IORef UniqTy
> > curUniqInt    = unsafePerformIO (newIORef 0)
> > newRef a  = do
> >   v <- readIORef curUniqInt
> >   writeIORef curUniqInt (v + 1)
> >   return (v, a)
> >
> > refWInt x = RefWInt (unsafePerformIO (newRef x))
>
> which works properly for this program alone:
>
> > tri_1 = (a, b, b1)
> >   where a  = refWInt "a"
> >     b  = refWInt "b"
> >     b1 = refWInt "b"
>
> yielding (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (2,"b")).
>
> But if I add another copy of the same definition:
>
> > tri_2 = (a, b, b1)
> >   where a  = refWInt "a"
> >     b  = refWInt "b"
> >     b1 = refWInt "b"
>
>
> and use it by uncommenting the second line here:
>
> > main =   print tri_1
> >  --   >> print tri_2
>
> I get:
>
>   (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (1,"b"))
>   (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (1,"b"))
>
> with ghc.  (I get the desired
>
>   (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (2,"b"))
>   (RefWInt (3,"a"),RefWInt (4,"b"),RefWInt (5,"b"))
>
> with ghci.)
>
>
> Can I write code to always get the desired behavior?  How?
>
>         thanks,
>         mike
> _______________________________________________