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
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
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- * 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