Allow access to handler effects in the local environment

effectful-core/src/Effectful/Dispatch/Dynamic.hs

@@ -46,14 +46,17 @@ module Effectful.Dispatch.Dynamic
   , localLiftUnlift
   , localLiftUnliftIO
 
-    -- *** Utils
+    -- *** Misc
+  , localSeqHandle
   , SharedSuffix
 
     -- * Re-exports
   , HasCallStack
   ) where
 
+import Control.Monad
 import Control.Monad.IO.Unlift
+import Data.Primitive.PrimArray
 import GHC.Stack (HasCallStack)
 import GHC.TypeLits
 
@@ -726,7 +729,32 @@ localLiftUnliftIO (LocalEnv les) strategy k = case strategy of
   ConcUnlift p l -> liftIO $ concUnliftIO les p l $ k unsafeEff_
 
 ----------------------------------------
--- Utils
+-- Misc
+
+localSeqHandle
+  :: (e :> es, SharedSuffix es handlerEs)
+  => LocalEnv localEs handlerEs
+  -> ((forall r. Eff (e : localEs) r -> Eff localEs r) -> Eff es a)
+  -> Eff es a
+localSeqHandle (LocalEnv les) k = unsafeEff $ \es -> do
+  eles <- copyRef es les
+  seqUnliftIO eles $ \unlift -> (`unEff` es) $ k $ unsafeEff_ . unlift
+
+copyRef
+  :: forall e es localEs. e :> es
+  => Env es
+  -> Env localEs
+  -> IO (Env (e : localEs))
+copyRef (Env hoffset hrefs hstorage) (Env offset refs0 storage) = do
+  when (hstorage /= storage) $ do
+    error "storages do not match"
+  let size = sizeofPrimArray refs0 - offset
+      i = 2 * reifyIndex @e @es
+  mrefs <- newPrimArray (size + 2)
+  copyPrimArray mrefs 0 hrefs (hoffset + i) 2
+  copyPrimArray mrefs 2 refs0 offset size
+  refs <- unsafeFreezePrimArray mrefs
+  pure $ Env 0 refs storage
 
 -- | Require that both effect stacks share an opaque suffix.
 --