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

Albert Y. C. Lai trebla at vex.net
Thu Apr 15 18:27:18 UTC 2021

```On 2021-04-15 8:54 a.m., YueCompl via Haskell-Cafe wrote:
> -- * Things not working
> managedArrayAsSeries::SomeManagedArray->IODynamic
> managedArrayAsSeries (SomeManagedArrayma)=do
> vec <-do
> SomeArraycap fp <-arrayAtTheMoment ma
> return \$VS.unsafeFromForeignPtr0 fp cap
> letlen =return \$VS.length vec
> rs i =return \$vec VS.!i
> return \$toDyn \$Serieslen 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
<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.

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