[Haskell-cafe] Where is SNMap for stable names?

[Edward Kmett]

Then "don't do that." =) I should have mentioned, parametric keys are a no
no and can do bad things. ;)

-Edward

On Thu, Sep 22, 2011 at 2:33 PM, James Cook <mokus at deepbondi.net> wrote:

> I may be wrong, but I think the original SNMap was a map from 'StableName's
> to the specific values they were derived from, which also (IIRC) had some
> weak referencing aspect as well.  Using them as keys for arbitrary elements
> of the phantom type is actually not type-safe, because equality of
> 'StableName's does not imply equality of types.  Here's a simple
> demonstration (also at [1] with slightly more commentary) which defines a
> working equivalent of 'return unsafeCoerce'  without directly using any
> "unsafe" function:
>
> > import Prelude hiding (lookup)
> >
> > import Data.Functor.Identity
> > import System.Mem.StableName
> > import System.Mem.StableName.Map
> >
> > unsafeCoerceIO :: IO (a -> b)
> > unsafeCoerceIO = do
> >     sn1 <- makeStableName undefined
> >     sn2 <- makeStableName undefined
> >
> >     return $ \x ->
> >         let m = singleton sn1 (Identity x)
> >             Just (Identity y) = lookup sn2 m
> >          in y
> >
> > main :: IO ()
> > main = do
> >     unsafeCoerce <- unsafeCoerceIO
> >     unsafeCoerce () "what did you do to my argument stack?!"
>
> -- James
>
> [1]
> https://github.com/mokus0/junkbox/blob/master/Haskell/TypeExperiments/UnsafeStableMap.hs
>
>
>
>
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