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.reind...@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 <vladis...@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
>>
>> _______________________________________________
>> ghc-devs mailing list
>> ghc-devs@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
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