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 12-04-2021 13:34, YueCompl via Haskell-Cafe 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_polymorphi...

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-func...

So, how can I do that nowadays?

Thanks, Compl

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