Re: [Haskell-cafe] What's the modern way to apply a polymorphic function to a Dynamic value in GHC 8.8 and onwards?

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#... [2] https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-Dynamic.html#v:d... On Mon, 12 Apr 2021 at 16:15, YueCompl via ghc-devs 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 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 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 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_polymorphi...

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-func...

So, how can I do that nowadays?

Thanks, Compl

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