[DP-67] remotable TH fails on type synonyms

[Imported from JIRA. Reported by Niklas Hambuechen @mail@nh2.me) as DP-67 on 2013-02-12 18:12:08]
Assume I write a function

f :: (ProcessId, ProcessId) -> Process ()

remotable ['f] works fine.

Then I make

type WorkStealingSlaveProcess a = (ProcessId, ProcessId) -> Process a
slave :: WorkStealingSlaveProcess ()

Now remotable ['f] fails, it says (my f is called slave here):

SimpleLocalnet.hs:1:1: Splicing declarations
    remotable ['slave]
  ======>
    SimpleLocalnet.hs:37:1-18
    slave__static :: Static (WorkStealingSlaveProcess ())
    slave__static = Control.Distributed.Static.staticLabel "Main.slave"
    __remoteTable :: RemoteTable -> RemoteTable
    __remoteTable
      = Control.Distributed.Static.registerStatic
          "Main.slave" (Data.Rank1Dynamic.toDynamic slave)
SimpleLocalnet.hs:56:22-37: Splicing expression
    mkClosure 'slave
  ======>
    ((closure
        (slave__static
         `Control.Distributed.Static.staticCompose`
           (staticDecode slave__sdict)))
     . Data.Binary.encode)

SimpleLocalnet.hs:56:22:
    Not in scope: `slave__sdict'
    In the result of the splice:
      $(mkClosure 'slave)
    To see what the splice expanded to, use -ddump-splices
    In the expression: $(mkClosure 'slave)
    In an equation for `slaveProcess':
        slaveProcess = $(mkClosure 'slave)

It simply doesn't generate the f__sdict function, which is because my type doesn't contain an arrow = see the following code from distributed-process (http://hackage.haskell.org/packages/archive/distributed-process/0.2.1.4/doc/html/src/Control-Distributed-Process-Internal-Closure-TH.html#remotable):

generateDefs :: Name -> Q ([Dec], [Q Exp])
generateDefs n = do
    proc <- [t| Process |]
    mType <- getType n
    case mType of
      Just (origName, typ) -> do
        let (typVars, typ') = case typ of ForallT vars [] mono -> (vars, mono)
                                          _                    -> ([], typ)

        -- The main "static" entry                                  
        (static, register) <- makeStatic origName typVars typ' 

        -- If n :: T1 -> T2, static serializable dictionary for T1 
        -- TODO: we should check if arg is an instance of Serializable, but we cannot
        -- http://hackage.haskell.org/trac/ghc/ticket/7066
        (sdict, registerSDict) <- case (typVars, typ') of
          ([], ArrowT `AppT` arg `AppT` _res) -> 
            makeDict (sdictName origName) arg
          _ -> 
            return ([], [])

We are in the second case because there is no ArrowT inside - it is hidden inside my type declaration.

Probably the solution is to "unfold" type aliases first.

If not, there should at least be a warning about this, instead of just returning [].

Start in Control.Distributed.Process.Internal.Closure.TH.html at generateDefs, especially the ArrowT pattern that decides whether to generate the serializable dictionary. Reproduce the type-synonym case from the issue with remotable and mkClosure, then verify that aliases are handled or that an explicit warning replaces the missing sdict failure.