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T2664.hs:31:9: error:
• Could not deduce: Dual a ~ Dual b
from the context: ((a :*: b) ~ Dual c, c ~ Dual (a :*: b))
bound by the type signature for:
newPChan :: forall c.
((a :*: b) ~ Dual c, c ~ Dual (a :*: b)) =>
IO (PChan (a :*: b), PChan c)
at T2664.hs:23:5-12
Expected: IO (PChan (a :*: b), PChan c)
Actual: IO (PChan (a :*: b), PChan (Dual b :+: Dual a))
NB: ‘Dual’ is a non-injective type family
• In a stmt of a 'do' block:
return
(O $ takeMVar v,
E (pchoose Right v newPChan) (pchoose Left v newPChan))
In the expression:
do v <- newEmptyMVar
return
(O $ takeMVar v,
E (pchoose Right v newPChan) (pchoose Left v newPChan))
In an equation for ‘newPChan’:
newPChan
= do v <- newEmptyMVar
return
(O $ takeMVar v,
E (pchoose Right v newPChan) (pchoose Left v newPChan))
• Relevant bindings include
v :: MVar (Either (PChan a) (PChan b)) (bound at T2664.hs:24:9)
newPChan :: IO (PChan (a :*: b), PChan c) (bound at T2664.hs:23:5)
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