Make GHC.Driver.Main.hscTcRnLookupRdrName to return NonEmpty

3 files changed, 11 insertions, 4 deletions

diff --git a/compiler/GHC/Driver/Main.hs b/compiler/GHC/Driver/Main.hs
index b6ff27621b..976cf12b55 100644
--- a/compiler/GHC/Driver/Main.hs
+++ b/compiler/GHC/Driver/Main.hs
@@ -266,6 +266,7 @@ import GHC.SysTools.BaseDir (findTopDir)
 
 import Data.Data hiding (Fixity, TyCon)
 import Data.List        ( nub, isPrefixOf, partition )
+import qualified Data.List.NonEmpty as NE
 import Control.Monad
 import Data.IORef
 import System.FilePath as FilePath
@@ -445,11 +446,15 @@ ioMsgMaybe' ioA = do
 -- -----------------------------------------------------------------------------
 -- | Lookup things in the compiler's environment
 
-hscTcRnLookupRdrName :: HscEnv -> LocatedN RdrName -> IO [Name]
+hscTcRnLookupRdrName :: HscEnv -> LocatedN RdrName -> IO (NonEmpty Name)
 hscTcRnLookupRdrName hsc_env0 rdr_name
   = runInteractiveHsc hsc_env0 $
     do { hsc_env <- getHscEnv
-       ; ioMsgMaybe $ hoistTcRnMessage $ tcRnLookupRdrName hsc_env rdr_name }
+       -- tcRnLookupRdrName can return empty list only together with TcRnUnknownMessage.
+       -- Once errors has been dealt with in hoistTcRnMessage, we can enforce
+       -- this invariant in types by converting to NonEmpty.
+       ; ioMsgMaybe $ fmap (fmap (>>= NE.nonEmpty)) $ hoistTcRnMessage $
+          tcRnLookupRdrName hsc_env rdr_name }
 
 hscTcRcLookupName :: HscEnv -> Name -> IO (Maybe TyThing)
 hscTcRcLookupName hsc_env0 name = runInteractiveHsc hsc_env0 $ do
diff --git a/compiler/GHC/Runtime/Debugger.hs b/compiler/GHC/Runtime/Debugger.hs
index a89227aada..7448f62234 100644
--- a/compiler/GHC/Runtime/Debugger.hs
+++ b/compiler/GHC/Runtime/Debugger.hs
@@ -49,6 +49,7 @@ import GHC.Types.TyThing
 import Control.Monad
 import Control.Monad.Catch as MC
 import Data.List ( (\\), partition )
+import qualified Data.List.NonEmpty as NE
 import Data.Maybe
 import Data.IORef
 
@@ -57,7 +58,7 @@ import Data.IORef
 -------------------------------------
 pprintClosureCommand :: GhcMonad m => Bool -> Bool -> String -> m ()
 pprintClosureCommand bindThings force str = do
-  tythings <- (catMaybes . concat) `liftM`
+  tythings <- (catMaybes . concatMap NE.toList) `liftM`
                  mapM (\w -> GHC.parseName w >>=
                                 mapM GHC.lookupName)
                       (words str)
diff --git a/compiler/GHC/Runtime/Eval.hs b/compiler/GHC/Runtime/Eval.hs
index 0b62544433..74eba30421 100644
--- a/compiler/GHC/Runtime/Eval.hs
+++ b/compiler/GHC/Runtime/Eval.hs
@@ -121,6 +121,7 @@ import Data.Either
 import Data.IntMap (IntMap)
 import qualified Data.IntMap as IntMap
 import Data.List (find,intercalate)
+import Data.List.NonEmpty (NonEmpty)
 import Control.Monad
 import Control.Monad.Catch as MC
 import Data.Array
@@ -903,7 +904,7 @@ getRdrNamesInScope = withSession $ \hsc_env -> do
 
 -- | Parses a string as an identifier, and returns the list of 'Name's that
 -- the identifier can refer to in the current interactive context.
-parseName :: GhcMonad m => String -> m [Name]
+parseName :: GhcMonad m => String -> m (NonEmpty Name)
 parseName str = withSession $ \hsc_env -> liftIO $
    do { lrdr_name <- hscParseIdentifier hsc_env str
       ; hscTcRnLookupRdrName hsc_env lrdr_name }