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

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What's the modern way to apply a...

YueCompl

12 Apr 2021 12 Apr '21

10:13 a.m.

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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Erik Hesselink

12 Apr 12 Apr

10:50 a.m.

New subject: 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

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

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

YueCompl

12:27 p.m.

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

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 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#... 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... 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 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 mailto:jaro.reinders@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 mailto:vladislav@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 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_polymorphi... 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... https://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-func...

So, how can I do that nowadays?

Thanks, Compl

_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org mailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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YueCompl

12:58 p.m.

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

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 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 mailto:hesselink@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#... 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... 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 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 mailto:jaro.reinders@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 mailto:vladislav@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 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_polymorphi... 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... https://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-func...

So, how can I do that nowadays?

Thanks, Compl

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Erik Hesselink

2:50 p.m.

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

That is a lot, I'm not sure I understand that pattern synonym. Using `withTypeable` instead works for me: holdEvent :: Dynamic -> Dynamic holdEvent (Dynamic tr x) = case tr of App ft at -> case ft `eqTypeRep` typeRep @EventSink of Just HRefl -> withTypeable at $ toDyn (hcHoldEvent x) Nothing -> error "to handle" _ -> error "to handle" Cheers, Erik On Mon, 12 Apr 2021 at 18:58, YueCompl wrote:

...

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 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 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#... [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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YueCompl

13 Apr 13 Apr

10:07 a.m.

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

After struggled this far, I decide that I can neither trivially understand `pattern TypeRep`, nor the `withTypeable` at core. But this is what really amazing with Haskell, GHC and the community here - I can get my job done even without full understanding of what's going on under the hood, so long as the compiler says it's okay! The warning has gone due to unknown reason after I refactored the code a bit, surprisingly but well, I feel safe and comfort to use it now. Thanks to Erik, Vlad and Jaro again for your help. u/Iceland_jack made a ticket to [add pattern TypeRep to Type.Reflection](https://gitlab.haskell.org/ghc/ghc/-/issues/19691) and appears it's very welcomed. Though I don't expect it get shipped very soon or even could be back ported to GHC 8.8, so I end up with this shim: (there `PolyKinds` appears some unusual to be put into my `.cabal` due to its syntax change can break some of my existing code) ```hs {-# LANGUAGE PolyKinds #-} module Dyn.Shim ( pattern TypeRep, dynPerformIO, dynPerformSTM, dynContSTM, ) where import Control.Concurrent.STM (STM) import Data.Dynamic (Dynamic (..), Typeable) import Type.Reflection ( TypeRep, eqTypeRep, typeRep, withTypeable, pattern App, type (:~~:) (HRefl), ) import Prelude data TypeableInstance a where TypeableInstance :: Typeable a => TypeableInstance a typeableInstance :: TypeRep a -> TypeableInstance a typeableInstance tr = withTypeable tr TypeableInstance {- ORMOLU_DISABLE -} -- | Shim for the proposed one at: -- https://gitlab.haskell.org/ghc/ghc/-/issues/19691 pattern TypeRep :: forall k (a :: k). () => Typeable a => TypeRep a pattern TypeRep <- (typeableInstance -> TypeableInstance) where TypeRep = typeRep {- ORMOLU_ENABLE -} -- | Perform a polymorphic IO action which is wrapped in a 'Dynamic' -- -- The specified 'naAlt' action will be performed instead, if the wrapped -- computation is not applicable, i.e. not really an IO action. dynPerformIO :: IO Dynamic -> Dynamic -> IO Dynamic dynPerformIO naAlt (Dynamic trAct monotypedAct) = case trAct of App io TypeRep -> case io `eqTypeRep` typeRep @IO of Just HRefl -> Dynamic TypeRep <$> monotypedAct Nothing -> naAlt -- not an IO action _ -> naAlt -- not even a poly-type -- | Perform a polymorphic STM action which is wrapped in a 'Dynamic' -- -- The specified 'naAlt' action will be performed instead, if the wrapped -- computation is not applicable, i.e. not really an STM action. dynPerformSTM :: STM Dynamic -> Dynamic -> STM Dynamic dynPerformSTM naAlt (Dynamic trAct monotypedAct) = case trAct of App io TypeRep -> case io `eqTypeRep` typeRep @STM of Just HRefl -> Dynamic TypeRep <$> monotypedAct Nothing -> naAlt -- not an STM action _ -> naAlt -- not even a poly-type -- | Perform a polymorphic STM action which is wrapped in a 'Dynamic' -- -- The specified 'naAlt' action will be performed instead, if the wrapped -- computation is not applicable, i.e. not really an STM action. dynContSTM :: STM () -> Dynamic -> (Dynamic -> STM ()) -> STM () dynContSTM naAlt (Dynamic trAct monotypedAct) !exit = case trAct of App io TypeRep -> case io `eqTypeRep` typeRep @STM of Just HRefl -> exit . Dynamic TypeRep =<< monotypedAct Nothing -> naAlt -- not an STM action _ -> naAlt -- not even a poly-type ``` And my test case being a little more complex than the very first example, might be easier for others to grasp the usage, it runs like this: ```console λ> import PoC.DynPoly λ> testDynHold First got Nothing Then got Just 3 λ> ``` With the code: ```hs module PoC.DynPoly where import Control.Monad (void) import Data.Dynamic (Dynamic (..), fromDynamic, toDyn) import Data.IORef (modifyIORef', newIORef, readIORef, writeIORef) import Dyn.Shim import Type.Reflection (eqTypeRep, typeRep, pattern App, type (:~~:) (HRefl)) import Prelude dynHoldEvent :: Dynamic -> Dynamic dynHoldEvent (Dynamic trEvs monotypedEvs) = case trEvs of App trEs TypeRep -> case trEs `eqTypeRep` typeRep @EventSink of Just HRefl -> Dynamic TypeRep (holdEvent monotypedEvs) Nothing -> error "not an EventSink" -- to be handled properly _ -> error "even not a poly-type" -- to be handled properly where holdEvent :: forall a. EventSink a -> IO (TimeSeries a) holdEvent !evs = do !holder <- newIORef Nothing listenEvents evs $ writeIORef holder . Just return $ TimeSeries $ readIORef holder data EventSink a = EventSink { listenEvents :: (a -> IO ()) -> IO (), publishEvent :: a -> IO () } newtype TimeSeries a = TimeSeries {readTimeSeries :: IO (Maybe a)} newEventSink :: forall a. IO (EventSink a) newEventSink = do !listeners <- newIORef [] let listen listener = modifyIORef' listeners (listener :) publish a = readIORef listeners >>= void . mapM ($ a) return $ EventSink listen publish testDynHold :: IO () testDynHold = do (evs :: EventSink Int) <- newEventSink let !dynEvs = toDyn evs !dynHold = dynHoldEvent dynEvs !dynTs <- dynPerformIO (error "bug: dyn type mismatch?") dynHold case fromDynamic dynTs of Nothing -> error "bug: unexpected dyn result type" Just (ts :: TimeSeries Int) -> do v0 <- readTimeSeries ts putStrLn $ "First got " <> show v0 publishEvent evs 3 v1 <- readTimeSeries ts putStrLn $ "Then got " <> show v1 ``` Thanks with best regards, Compl

...

On 2021-04-13, at 02:50, Erik Hesselink wrote:

That is a lot, I'm not sure I understand that pattern synonym. Using `withTypeable` instead works for me:

holdEvent :: Dynamic -> Dynamic holdEvent (Dynamic tr x) = case tr of App ft at -> case ft `eqTypeRep` typeRep @EventSink of Just HRefl -> withTypeable at $ toDyn (hcHoldEvent x) Nothing -> error "to handle" _ -> error "to handle"

Cheers,

Erik

YueCompl

10:48 a.m.

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

A followup wish I have: ```hs case io `eqTypeRep` typeRep @IO of Just HRefl -> Dynamic TypeRep <$> monotypedAct Nothing -> naAlt -- not an IO action ``` The `Just HRefl` part as in above remains hard to understand for me, I had glanced it in doc of the 'Type.Reflection' module earlier, but had no chance to figure out the usage of `eqTypeRep` to be like this, at least on my own. The community is very helpful in this regards, in leading me to it. But may there can be better surface syntax / usage hints that more intuitive, i.e. costing less effort to reach the solution? I anticipate improvements but apparently lack expertise for progress, I tried `Just {}` and it won't compile already... I mean, things are already great as far, well, maybe the learning experience can be made even better. Best, Compl

...

On 2021-04-13, at 22:07, YueCompl via ghc-devs wrote:

After struggled this far, I decide that I can neither trivially understand `pattern TypeRep`, nor the `withTypeable` at core. But this is what really amazing with Haskell, GHC and the community here - I can get my job done even without full understanding of what's going on under the hood, so long as the compiler says it's okay! The warning has gone due to unknown reason after I refactored the code a bit, surprisingly but well, I feel safe and comfort to use it now.

Thanks to Erik, Vlad and Jaro again for your help.

u/Iceland_jack made a ticket to [add pattern TypeRep to Type.Reflection](https://gitlab.haskell.org/ghc/ghc/-/issues/19691 https://gitlab.haskell.org/ghc/ghc/-/issues/19691) and appears it's very welcomed. Though I don't expect it get shipped very soon or even could be back ported to GHC 8.8, so I end up with this shim:

(there `PolyKinds` appears some unusual to be put into my `.cabal` due to its syntax change can break some of my existing code)

```hs {-# LANGUAGE PolyKinds #-}

module Dyn.Shim ( pattern TypeRep, dynPerformIO, dynPerformSTM, dynContSTM, ) where

import Control.Concurrent.STM (STM) import Data.Dynamic (Dynamic (..), Typeable) import Type.Reflection ( TypeRep, eqTypeRep, typeRep, withTypeable, pattern App, type (:~~:) (HRefl), ) import Prelude

data TypeableInstance a where TypeableInstance :: Typeable a => TypeableInstance a

typeableInstance :: TypeRep a -> TypeableInstance a typeableInstance tr = withTypeable tr TypeableInstance

{- ORMOLU_DISABLE -}

-- | Shim for the proposed one at: -- https://gitlab.haskell.org/ghc/ghc/-/issues/19691 https://gitlab.haskell.org/ghc/ghc/-/issues/19691 pattern TypeRep :: forall k (a :: k). () => Typeable a => TypeRep a pattern TypeRep <- (typeableInstance -> TypeableInstance) where TypeRep = typeRep

{- ORMOLU_ENABLE -}

-- | Perform a polymorphic IO action which is wrapped in a 'Dynamic' -- -- The specified 'naAlt' action will be performed instead, if the wrapped -- computation is not applicable, i.e. not really an IO action. dynPerformIO :: IO Dynamic -> Dynamic -> IO Dynamic dynPerformIO naAlt (Dynamic trAct monotypedAct) = case trAct of App io TypeRep -> case io `eqTypeRep` typeRep @IO of Just HRefl -> Dynamic TypeRep <$> monotypedAct Nothing -> naAlt -- not an IO action _ -> naAlt -- not even a poly-type

-- | Perform a polymorphic STM action which is wrapped in a 'Dynamic' -- -- The specified 'naAlt' action will be performed instead, if the wrapped -- computation is not applicable, i.e. not really an STM action. dynPerformSTM :: STM Dynamic -> Dynamic -> STM Dynamic dynPerformSTM naAlt (Dynamic trAct monotypedAct) = case trAct of App io TypeRep -> case io `eqTypeRep` typeRep @STM of Just HRefl -> Dynamic TypeRep <$> monotypedAct Nothing -> naAlt -- not an STM action _ -> naAlt -- not even a poly-type

-- | Perform a polymorphic STM action which is wrapped in a 'Dynamic' -- -- The specified 'naAlt' action will be performed instead, if the wrapped -- computation is not applicable, i.e. not really an STM action. dynContSTM :: STM () -> Dynamic -> (Dynamic -> STM ()) -> STM () dynContSTM naAlt (Dynamic trAct monotypedAct) !exit = case trAct of App io TypeRep -> case io `eqTypeRep` typeRep @STM of Just HRefl -> exit . Dynamic TypeRep =<< monotypedAct Nothing -> naAlt -- not an STM action _ -> naAlt -- not even a poly-type

```

And my test case being a little more complex than the very first example, might be easier for others to grasp the usage, it runs like this:

```console λ> import PoC.DynPoly λ> testDynHold First got Nothing Then got Just 3 λ> ```

With the code:

```hs module PoC.DynPoly where

import Control.Monad (void) import Data.Dynamic (Dynamic (..), fromDynamic, toDyn) import Data.IORef (modifyIORef', newIORef, readIORef, writeIORef) import Dyn.Shim import Type.Reflection (eqTypeRep, typeRep, pattern App, type (:~~:) (HRefl)) import Prelude

dynHoldEvent :: Dynamic -> Dynamic dynHoldEvent (Dynamic trEvs monotypedEvs) = case trEvs of App trEs TypeRep -> case trEs `eqTypeRep` typeRep @EventSink of Just HRefl -> Dynamic TypeRep (holdEvent monotypedEvs) Nothing -> error "not an EventSink" -- to be handled properly _ -> error "even not a poly-type" -- to be handled properly where holdEvent :: forall a. EventSink a -> IO (TimeSeries a) holdEvent !evs = do !holder <- newIORef Nothing listenEvents evs $ writeIORef holder . Just return $ TimeSeries $ readIORef holder

data EventSink a = EventSink { listenEvents :: (a -> IO ()) -> IO (), publishEvent :: a -> IO () }

newtype TimeSeries a = TimeSeries {readTimeSeries :: IO (Maybe a)}

newEventSink :: forall a. IO (EventSink a) newEventSink = do !listeners <- newIORef [] let listen listener = modifyIORef' listeners (listener :) publish a = readIORef listeners >>= void . mapM ($ a) return $ EventSink listen publish

testDynHold :: IO () testDynHold = do (evs :: EventSink Int) <- newEventSink let !dynEvs = toDyn evs !dynHold = dynHoldEvent dynEvs !dynTs <- dynPerformIO (error "bug: dyn type mismatch?") dynHold case fromDynamic dynTs of Nothing -> error "bug: unexpected dyn result type" Just (ts :: TimeSeries Int) -> do v0 <- readTimeSeries ts putStrLn $ "First got " <> show v0 publishEvent evs 3 v1 <- readTimeSeries ts putStrLn $ "Then got " <> show v1 ```

Thanks with best regards, Compl

...
On 2021-04-13, at 02:50, Erik Hesselink mailto:hesselink@gmail.com> wrote:

That is a lot, I'm not sure I understand that pattern synonym. Using `withTypeable` instead works for me:

holdEvent :: Dynamic -> Dynamic holdEvent (Dynamic tr x) = case tr of App ft at -> case ft `eqTypeRep` typeRep @EventSink of Just HRefl -> withTypeable at $ toDyn (hcHoldEvent x) Nothing -> error "to handle" _ -> error "to handle"

Cheers,

Erik

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