On Mar 11, 2011, at 7:37 PM, Luke Palmer wrote:
> On Fri, Mar 11, 2011 at 8:18 PM, Joshua Ball <[email protected]> wrote:
>> Suppose I want the following functions:
>>
>> newRef :: a -> RefMonad (Ref a)
>> readRef :: Ref a -> RefMonad a
>> writeRef :: Ref a -> a -> RefMonad ()
>
> I would be delighted to see a pure, unsafe*-free implementation of
> your interface. I have never seen one, and I don't really expect it
> to exist. Likewise I would love to see a proof that it doesn't.
This message from Koen Claessen, dating back to 2001, discusses precisely this
issue:
http://www.mail-archive.com/[email protected]/msg09207.html
Quoting Koen:
"I conjecture this functionality cannot be implemented in Haskell 98, nor in
any of the known safe extensions of Haskell."
I think the folk consensus in the community is that Koen was right. While a
proof along the lines of "doing so would violate parametricity" seems
plausible, I can't recall anybody attempting a rigorous proof so far.
-Levent.
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