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{-# LANGUAGE EmptyDataDecls, TypeFamilies, TypeOperators, ScopedTypeVariables #-}
module Overflow where
import Control.Concurrent
data (:*:) a b
data (:+:) a b
data family PChan a
data instance PChan (a :+: b) = E (IO (PChan a)) (IO (PChan b))
newtype instance PChan (a :*: b) = O (IO (Either (PChan a) (PChan b)))
type family Dual a
type instance Dual (a :+: b) = Dual a :*: Dual b
type instance Dual (a :*: b) = Dual a :+: Dual b
class Connect s where
newPChan :: (s ~ Dual c, c ~ Dual s) => IO (PChan s, PChan c)
pchoose :: (t -> a) -> MVar a -> IO (t,b) -> IO b
pchoose = undefined
instance (Connect a, Connect b) => Connect (a :*: b) where
newPChan = do
v <- newEmptyMVar
-- This version is in T2664a
-- correct implementation:
-- return (O $ takeMVar v, E (pchoose Left v newPChan) (pchoose Right v newPChan))
-- type error leads to stack overflow (even without UndecidableInstances!)
return (O $ takeMVar v, E (pchoose Right v newPChan) (pchoose Left v newPChan))
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