-- * 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/existential_quantification.html#restrictions

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.