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

Mon Apr 12 16:27:01 UTC 2021

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 <hesselink at 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 at haskell.org <mailto:ghc-devs at 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.reinders at gmail.com <mailto:jaro.reinders at 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 <vladislav at serokell.io <mailto:vladislav at 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 at haskell.org <mailto: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/ <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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>> 
> 
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