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

22 Sep 2011


      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  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/Unsafe...

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

Edward Kmett