Re: [Haskell-cafe] Trick to have existential type work in this case?

19 Apr 2021

      What a concise explanation and practical solutions! Thanks!

With the hint from https://www.reddit.com/user/bss03/ https://www.reddit.com/user/bss03/ I've figured out the [1st solution](https://www.reddit.com/r/haskell/comments/mre9ha/trick_to_have_existential_t... https://www.reddit.com/r/haskell/comments/mre9ha/trick_to_have_existential_t...).

And your 2nd solution really updated my knowledge about CPS, I used to assume some "continuation" always has to be passed around in CPS, now I know it can be used for nesting of scopes, and to have it naturally "return" back to outer scope.

Thanks with best regards!
...
On 2021-04-16, at 02:27, Albert Y. C. Lai  wrote:
On 2021-04-15 8:54 a.m., YueCompl via Haskell-Cafe wrote:
...
-- * Things not working
managedArrayAsSeries :: SomeManagedArray -> IO Dynamic
managedArrayAsSeries (SomeManagedArray ma) = do
  vec <- do
    SomeArray cap fp <- arrayAtTheMoment ma
    return $ VS.unsafeFromForeignPtr0 fp cap
let len = return $ VS.length vec
      rs i = return $ vec VS.! i
  return $ toDyn $ Series len rs
That means you have this code fragment:
arrayAtTheMoment ma
...
...
=
\(SomeArray cap fp) -> return $ VS.unsafeFromForeignPtr0 fp cap
That means you have this function:
\(SomeArray cap fp) -> return $ VS.unsafeFromForeignPtr0 fp cap
Now you are violating the 1st restriction at
https://downloads.haskell.org/ghc/latest/docs/html/users_guide/exts/existent... https://downloads.haskell.org/ghc/latest/docs/html/users_guide/exts/existent...
There are two solutions.
1st solution: One more existential type.
data Vec = forall a. (Typeable a, VS.Storable a) => Vec (VS.Vector a)
Vec vec <- do
    SomeArray cap fp <- arrayAtTheMoment ma
    return (Vec (VS.unsafeFromForeignPtr0 fp cap))
2nd solution: CPS transform.
{-# language RankNTypes #-}
{-# language BlockArguments #-}
withSomeArray :: SomeArray
              -> (forall a. (Typeable a, VS.Storable a) => Int -> ForeignPtr a -> r)
              -> r
withSomeArray (SomeArray i p) f = f i p
sa <- arrayAtTheMoment ma
withSomeArray sa \cap fp -> do
  let vec = VS.unsafeFromForeignPtr0 fp cap
      -- or if you prefer: vec <- return (VS.unsafeFromForeignPtr0 fp cap)
      len = return (VS.length vec)
      rs i = return (vec VS.! i)
      etc.
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