Type Families and defaults

[Simon Peyton-Jones]

| > I'm afraid I didn't understand your questions well enough to answer them.
| 
| My question is, why does this type check:

It's hard for me to answer a question like that!  To explain why something type checks I'd have to show every constraint and how it is solved.  

I think you have something more specific in mind.  Maybe you think "this should not typecheck".  E.g.  here's an expression (f x), and f has type Int -> <blah, and x has type Boo, so the application should fail to typecheck.

You say that example 1 (which typechecks) is different to example 2 (which does not). Again, in itself that's not surprising.  They differ, so their behaviours may differ.

Do remember that (e :: IO n)
means (e :: forall n. IO n)
unless you have ScopedTypeVariables

Sorry not to be more helpful.  

Simon

| 
| 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-
| extensions.html#scoped-type-variables
| 
| I had. Thanks.
| 
| 
| Mitar