Hi!
I'm afraid I didn't understand your questions well enough to answer them.
My question is, why does this type check: instance Neuron TestNeuron where data LiveNeuron TestNeuron = LiveTestNeuron NeuronId mkLiveNeuron nid = LiveTestNeuron nid getNeuronId (LiveTestNeuron nid) = nid live _ _ = return () attach nerve = ((liftM mkLiveNeuron) . forkIO $ bracket (grow :: IO FooNeuron) dissolve (live nerve)) :: IO (LiveNeuron TestNeuron) FooNeuron is obviously different to TestNeuron (but both are instances of Neuron class). Type signature is: attach nerve = ((liftM mkLiveNeuron) . forkIO $ bracket (grow :: IO n) dissolve (live nerve)) :: IO (LiveNeuron n) This seems different to: attach nerve = ((liftM mkLiveNeuron) . forkIO $ bracket (grow :: forall n. IO n) dissolve (live nerve)) :: IO (LiveNeuron n) which does not type check. Maybe I am missing some basic understanding.
But it'd be worth reading http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/other-type-extension...
I had. Thanks. Mitar