Main.hs

{-# OPTIONS_GHC -fplugin=MyPlugin #-}
module Main where

MyModule.hs

{-# LANGUAGE TypeFamilies #-}
module MyModule where

type family MyFam a b where

MyPlugin.hs

module MyPlugin where

import GHC.Plugins hiding ((<>))
import GHC.Tc.Types
import GHC.Unit.Finder.Types
import GHC.Tc.Plugin
import MyModule

plugin = defaultPlugin { tcPlugin = const (Just (TcPlugin tcPluginInitm undefined undefined)) }

myPackage = "this"
myModuleName = "MyModule"

tcPluginInitm :: TcPluginM ()
tcPluginInitm = do
  myModule     <- findModule myPackage myModuleName
  lookupResult <- tcLookupGlobal =<< lookupOrig myModule ( mkTcOcc "MyFam" )
  pprPanic "Result was:" ( ppr lookupResult )

findModule :: String -> String -> TcPluginM Module
findModule pkg modName = do
  findResult <- findImportedModule ( mkModuleName modName ) ( Just $ fsLit pkg )
  case findResult of
    Found _ res     -> pure res
    FoundMultiple _ -> error $ "MyPlugin: found multiple modules named " <> modName <> "."
    _               -> error $ "MyPlugin: could not find any module named " <> modName <> "."