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@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@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@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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