[Haskell-cafe] What's the modern way to apply a polymorphic function to a Dynamic value in GHC 8.8 and onwards?
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
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 ()))
So, how can I do that nowadays?
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