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

YueCompl compl.yue at icloud.com
Mon Apr 12 11:34:19 UTC 2021

Dear Cafe and GHC devs,

There used to be a "principled way with pattern match on the constructor":

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: 

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:

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?


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