T2664.hs:31:52: error:
    • Could not deduce: b ~ a arising from a use of ‘newPChan’
      from the context: ((a :*: b) ~ Dual c, c ~ Dual (a :*: b))
        bound by the type signature for:
                   newPChan :: ((a :*: b) ~ Dual c, c ~ Dual (a :*: b)) =>
                               IO (PChan (a :*: b), PChan c)
        at T2664.hs:23:5-12
      ‘b’ is a rigid type variable bound by
        the instance declaration at T2664.hs:22:10-52
      ‘a’ is a rigid type variable bound by
        the instance declaration at T2664.hs:22:10-52
    • In the third argument of ‘pchoose’, namely ‘newPChan’
      In the first argument of ‘E’, namely ‘(pchoose Right v newPChan)’
      In the expression:
        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)