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

[Vladislav Zavialov]

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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