I think it's possible to do this *today* using unsafeCoerce#.

I was able to come up with this basic example below. In practice one would at the very least want to abstract away the gnarly stuff inside a
library. But since it sounds like you want to be the one to write a library that shouldn't be a problem.

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE UnliftedDatatypes #-}
module Main where
import GHC.Exts
import GHC.IO
import Unsafe.Coerce
import Data.Kind
data SA = SA (SmallMutableArray# RealWorld Any)
mkArray :: Int -> a -> IO (SA)
mkArray (I# n) initial = IO $ \s ->
    case unsafeCoerce# (newSmallArray# n initial s) of
        (# s', arr #) -> (# s', SA arr #)
readLifted :: SA -> Int -> IO a
readLifted (SA arr) (I# i) = IO (\s ->
    unsafeCoerce# (readSmallArray# arr i s)
    )
data UWrap (a :: UnliftedType) = UWrap a
-- UWrap is just here because we can't return unlifted types in IO
-- If you don't need your result in IO you can eliminate this indirection.
readUnlifted :: forall a. SA -> Int -> IO (UWrap a)
readUnlifted (SA arr) (I# i) = IO (\s ->
    case unsafeCoerce# (readSmallArray# arr i s) of
        (# s', a :: a #) -> (# s', UWrap a #)
    )
writeLifted :: a -> Int -> SA -> IO ()
writeLifted x (I# i) (SA arr) = IO $ \s ->
    case writeSmallArray# (unsafeCoerce# arr) i x s of
        s -> (# s, () #)
writeUnlifted :: (a :: UnliftedType) -> Int -> SA -> IO ()
writeUnlifted x (I# i) (SA arr) = IO $ \s ->
    case writeSmallArray# arr i (unsafeCoerce# x) s of
        s -> (# s, () #)
type UB :: UnliftedType
data UB = UT | UF
showU :: UWrap UB -> String
showU (UWrap UT) = "UT"
showU (UWrap UF) = "UF"
main :: IO ()
main = do
    arr <- mkArray 4 ()
    writeLifted True 0 arr
    writeLifted False 1 arr
    writeUnlifted UT 2 arr
    writeUnlifted UT 3 arr
    (readLifted arr 0 :: IO Bool) >>= print
    (readLifted arr 1 :: IO Bool) >>= print
    (readUnlifted arr 2 :: IO (UWrap UB)) >>= (putStrLn . showU)
    (readUnlifted arr 3 :: IO (UWrap UB)) >>= (putStrLn . showU)
    return ()

Cheers

Andreas

Am 02/08/2022 um 17:32 schrieb J. Reinders:

      

        
Could you use `StablePtr` for the keys?


      

      
That might be an option, but I have no idea how performant stable pointers are and manual management is obviously not ideal.



      

        
How does the cost of computing object hashes and comparing colliding
objects compare with the potential cache miss cost of using boxed
integers or a separate array?  Would such an "optimisation" be worth
the effort?


      

      
Literature on hash tables suggests that cache misses were a very important factor in running time (in 2001): https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.25.4189

I don’t know whether it has become less or more important now, but I have been told there haven’t been that many advances in memory latency.
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