Plugin function definition

[alanz]

At the moment the plugin callback function for a particular command is defined as

type Dispatcher = forall m. (MonadIO m,GHC.GhcMonad m,HasIdeState m) 
                         => IdeRequest -> m IdeResponse

The monad used inside HIE is defined as

newtype IdeM a = IdeM { unIdeM :: GM.GhcModT (GM.GmOutT (StateT IdeState IO)) a}
      deriving ( Functor
               , Applicative
               , Alternative
               , Monad
               , MonadPlus
               , MonadIO
               , GM.GmEnv
               , GM.GmOut
               , GM.MonadIO
               , ExceptionMonad
               )

data IdeState = IdeState
  {
    idePlugins :: Plugins
  } deriving (Show)

There is a specific haskell-ide-plugin-api package for the Dispatcher type, and IdeM is defined
in the main haskell-ide-egine package.

There are a number of options for this.

1. Leave things as they are. But then the GhcModT is not available to a plugin action.
2. Make Dispatcher have the signature IdeRequest -> IdeM IdeResponse
   This means the IdeM type has to be in haskell-ide-plugin-api, and the various instances
   required become orphans, or need to be in haskell-ide-plugin-api.
3. Include GhcModT in the constraints on the Dispatcher

Another consideration is that the haskell-ide-engine looks like it may be a
general IDE backend, usable for other languages e.g. PureScript, Idris, et al.
In this case having anything GHC related is unneccessary. In this case,
perhaps the type should be simply

type Dispatcher = forall m. (MonadIO m,HasIdeState m) 
                         => IdeRequest -> m IdeResponse

[alanz]

Note: Dispatcher is in the process of being renamed to CommandFunc via #51

[alanz]

Closed via #130