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

[YueCompl]

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_type_work_in_this_case/guw5y80/?context=3 <https://www.reddit.com/r/haskell/comments/mre9ha/trick_to_have_existential_type_work_in_this_case/guw5y80/?context=3>).

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 <trebla at vex.net> 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/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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