Oh, forgot to mention that there is a warning I also don't understand by far:
```log
src/PoC/DynPoly.hs:40:3: warning: [-Woverlapping-patterns]
Pattern match has inaccessible right hand side
In a case alternative: Dynamic (App eventSink TypeRep) evs' -> ...
|
40 | Dynamic (App eventSink TypeRep) evs' ->
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
```
I need to work out some extra stuff to test the solution in real case,
meanwhile this warning seems worrying ...
> On 2021-04-13, at 00:27, YueCompl via ghc-devs <ghc-devs@haskell.org> wrote:
>
> Thanks Erik,
>
> With the help from Iceland_jack <https://www.reddit.com/user/Iceland_jack>
> via /r/haskell <https://www.reddit.com/r/haskell> , I end up with a working
> solution like this:
>
> ```hs
> data TypeableInstance a where
> -- data TypeableInstance :: forall k. k -> Type where
> TypeableInstance :: Typeable a => TypeableInstance a
>
> typeableInstance :: forall (k :: Type) (a :: k). TypeRep a ->
> TypeableInstance a
> typeableInstance typeRep' = withTypeable typeRep' TypeableInstance
>
> pattern TypeRep :: forall k (a :: k). () => Typeable a => TypeRep a
> pattern TypeRep <-
> (typeableInstance -> TypeableInstance)
> where
> TypeRep = typeRep
>
> holdEvent :: Dynamic -> Dynamic
> holdEvent !devs = case devs of
> Dynamic (App eventSink TypeRep) evs' ->
> case eqTypeRep (typeRep @EventSink) eventSink of
> Just HRefl -> Dynamic TypeRep (hcHoldEvent evs')
> Nothing -> error "not an EventSink" -- to be handled properly
> _ -> error "even not a poly-type" -- to be handled properly
> 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
>
> ```
>
> I'm still wrapping my head around it, for how the `pattern TypeRep` works in
> this case.
>
> Or you think there exists a solution without using such a pattern?
>
> My function (hcHoldEvent) is polymorphic so not eligible to be wrapped as a
> Dynamic in the first place, or there also some way to specialize it at
> runtime? That'll be another interesting tool.
>
> Thanks with regards,
> Compl
>
>> On 2021-04-12, at 22:50, Erik Hesselink <hessel...@gmail.com
>> <mailto:hessel...@gmail.com>> wrote:
>>
>> Your function is not `forall a. a -> f a`, as in your initial example, but
>> requires its argument to be an `EventSink`. The value you unwrap from the
>> `Dynamic` is any existential type, not necessarily an `EventSink`. You'll
>> have to compare the TypeReps (with something like `eqTypeRep`[1], or wrap
>> your function in a `Dynamic` and use `dynApply` [2], which does the
>> comparison for you.
>>
>> Cheers,
>>
>> Erik
>>
>> [1]
>> https://hackage.haskell.org/package/base-4.15.0.0/docs/Type-Reflection.html#v:eqTypeRep
>>
>> <https://hackage.haskell.org/package/base-4.15.0.0/docs/Type-Reflection.html#v:eqTypeRep>
>> [2]
>> https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-Dynamic.html#v:dynApply
>>
>> <https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-Dynamic.html#v:dynApply>
>> On Mon, 12 Apr 2021 at 16:15, YueCompl via ghc-devs <ghc-devs@haskell.org
>> <mailto:ghc-devs@haskell.org>> wrote:
>> 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
>>> <mailto: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
>>> <mailto: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
>>>> <mailto: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/
>>>>
>>>> <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
>>>>
>>>> <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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>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>> <http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs>
>>>
>>
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