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

[YueCompl]

Dear Cafe,

I believe there should exist some trick that I haven't learned, to have this compile:

module PoC.Existential where

import Data.Dynamic
import qualified Data.Vector.Storable as VS
import Foreign
import Prelude

-- * Necessary artifacts

data Series a = Series
  { seriesLen :: IO Int,
    readSeries :: Int -> IO a
  }

data SomeArray = forall a.
  (Typeable a, VS.Storable a) =>
  SomeArray
  { arrayCap :: Int,
    arrayPtr :: ForeignPtr a
  }

class ManagedArray t where
  arrayAtTheMoment :: t -> IO SomeArray

data SomeManagedArray
  = forall t.
    (Typeable t, ManagedArray t) =>
    SomeManagedArray t


In following, I can confirm it works with the nested `do` block flattened (as shown later), but I really need it in the more complex real case, so please bear with me.


-- * 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


The error:

src/PoC/Existential.hs:36:5: error:
    • Couldn't match type ‘a’ with ‘a0’
      ‘a’ is a rigid type variable bound by
        a pattern with constructor:
          SomeArray :: forall a.
                       (Typeable a, Storable a) =>
                       Int -> ForeignPtr a -> SomeArray,
        in a pattern binding in
             a 'do' block
        at src/PoC/Existential.hs:35:5-20
      Expected type: IO (VS.Vector a0)
        Actual type: IO (VS.Vector a)
    • In a stmt of a 'do' block:
        return $ VS.unsafeFromForeignPtr0 fp cap
      In a stmt of a 'do' block:
        vec <- do SomeArray cap fp <- arrayAtTheMoment ma
                  return $ VS.unsafeFromForeignPtr0 fp cap
      In the expression:
        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
    • Relevant bindings include
        fp :: ForeignPtr a (bound at src/PoC/Existential.hs:35:19)
   |
36 |     return $ VS.unsafeFromForeignPtr0 fp cap
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


While these more trivial things work as expected:


-- * Things working

managedArrayAsSeries' :: SomeManagedArray -> IO Dynamic
managedArrayAsSeries' (SomeManagedArray ma) = do
  SomeArray cap fp <- arrayAtTheMoment ma
  let vec = VS.unsafeFromForeignPtr0 fp cap

  let len = return $ VS.length vec
      rs i = return $ vec VS.! i
  return $ toDyn $ Series len rs

arrayAsSeries :: SomeArray -> Dynamic
arrayAsSeries (SomeArray cap fp) =
  colAsSeries $ SomeColumn $ VS.unsafeFromForeignPtr0 fp cap

arrayAsSeries' :: SomeArray -> Dynamic
arrayAsSeries' (SomeArray cap fp) = do
  let vec = VS.unsafeFromForeignPtr0 fp cap
      len = return $ VS.length vec
      rs i = return $ vec VS.! i
  toDyn $ Series len rs

data SomeColumn
  = forall a.
    (Typeable a, VS.Storable a) =>
    SomeColumn (VS.Vector a)

colAsSeries :: SomeColumn -> Dynamic
colAsSeries (SomeColumn colVec) = toDyn $ Series len rs
  where
    len = return $ VS.length colVec
    rs i = return $ colVec VS.! i


Please teach me the trick!

Thanks with regards,
Compl

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