What's the modern way to apply a polymorphic function to a Dynamic value in GHC 8.8 and onwards?

[YueCompl]

Thanks to Vlad and Jaro, your solution of `apD` compiles, I think it should work.

But unfortunately my real case is a little different / more complex, a MWE appears like this:

```hs
holdEvent :: Dynamic -> Dynamic
holdEvent (Dynamic t evs') =
  withTypeable t $ Dynamic typeRep (hcHoldEvent evs')
  where
    hcHoldEvent :: forall a. EventSink a -> IO (TimeSeries a)
    hcHoldEvent !evs = do
      !holder <- newIORef Nothing
      listenEvents evs $ writeIORef holder . Just
      return $ TimeSeries $ readIORef holder

data EventSink a = EventSink
  { listenEvents :: (a -> IO ()) -> IO (),
    closeStream :: IO ()
  }

instance Functor EventSink where
  fmap = undefined

newtype TimeSeries a = TimeSeries {readTimeSeries :: IO (Maybe a)}

instance Functor TimeSeries where
  fmap = undefined

```

Now I'm clueless how to use the `withTypeable` trick to apply my polymorphic `hcHoldEvent` to `Dynamic`, naively written as in above, the error is:

```log
src/PoC/DynPoly.hs:20:49: error:
    • Couldn't match expected type ‘EventSink a0’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
        a pattern with constructor:
          Dynamic :: forall a.
                     base-4.13.0.0:Data.Typeable.Internal.TypeRep a -> a -> Dynamic,
        in an equation for ‘holdEvent’
        at src/PoC/DynPoly.hs:19:12-25
    • In the first argument of ‘hcHoldEvent’, namely ‘evs'’
      In the second argument of ‘Dynamic’, namely ‘(hcHoldEvent evs')’
      In the second argument of ‘($)’, namely
        ‘Dynamic typeRep (hcHoldEvent evs')’
    • Relevant bindings include
        evs' :: a (bound at src/PoC/DynPoly.hs:19:22)
        t :: base-4.13.0.0:Data.Typeable.Internal.TypeRep a
          (bound at src/PoC/DynPoly.hs:19:20)
   |
20 |   withTypeable t $ Dynamic typeRep (hcHoldEvent evs')
   |                                                 ^^^^

```

Thanks with best regards,
Compl


> On 2021-04-12, at 22:04, Jaro Reinders <jaro.reinders at gmail.com> wrote:
> 
> I have no experience in this area, but this compiles:
> 
> ```
> {-# LANGUAGE RankNTypes, ScopedTypeVariables #-}
> import Type.Reflection
> import Data.Dynamic
> 
> appD :: forall f. Typeable f => (forall a. a -> f a) -> Dynamic -> Dynamic
> appD f (Dynamic rep (x :: a)) = withTypeable rep (toDyn (f x))
> ```
> 
> Cheers,
> 
> Jaro


> On 2021-04-12, at 21:06, Vladislav Zavialov <vladislav at serokell.io> wrote:
> 
> Would something like this work for you?
> 
>  import Type.Reflection
>  import Data.Dynamic
> 
>  apD :: Typeable f => (forall a. a -> f a) -> Dynamic -> Dynamic
>  apD f (Dynamic t a) = withTypeable t $ Dynamic typeRep (f a)
> 
> - Vlad
> 
>> On 12 Apr 2021, at 14:34, YueCompl via ghc-devs <ghc-devs at haskell.org> wrote:
>> 
>> Dear Cafe and GHC devs,
>> 
>> 
>> There used to be a "principled way with pattern match on the constructor":
>> 
>> ```hs
>> data Dynamic where
>> Dynamic :: Typeable a => a -> Dynamic
>> 
>> apD :: Typeable f => (forall a. a -> f a) -> Dynamic -> Dynamic
>> apD f (Dynamic a) = Dynamic $ f a
>> ```
>> Source: https://www.reddit.com/r/haskell/comments/2kdcca/q_how_to_apply_a_polymorphic_function_to_a/
>> 
>> 
>> But now with GHC 8.8 as in my case, `Dynamic` constructor has changed its signature to: 
>> 
>> ```hs
>> Dynamic :: forall a. TypeRep a -> a -> Dynamic
>> ```
>> 
>> Which renders the `apD` not working anymore. 
>> 
>> 
>> And it seems missing dependencies now for an older solution Edward KMETT provides:
>> 
>> ```hs
>> apD :: forall f. Typeable1 f => (forall a. a -> f a) -> Dynamic -> Dynamic
>> apD f a = dynApp df a
>> where t = dynTypeRep a
>>       df = reify (mkFunTy t (typeOf1 (undefined :: f ()) `mkAppTy` t)) $ 
>>                 \(_ :: Proxy s) -> toDyn (WithRep f :: WithRep s (() -> f ()))
>> ```
>> Source: https://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-function-to-a-dynamic-value
>> 
>> 
>> So, how can I do that nowadays?
>> 
>> Thanks,
>> Compl
>> 
>> _______________________________________________
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>> ghc-devs at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> 

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