Have forceEff pretend to return a value again

src/Nix/Normal.hs

@@ -77,11 +77,7 @@ normalForm_ :: (Framed e m,
                MonadThunk t m (NValue t f m),
                MonadDataErrorContext t f m)
             => NValue t f m -> m ()
-normalForm_ = fmap void $ normalForm' $ \t k -> do
-    forceEff t (void . k)
-    -- This next return is safe, only because we never inspect this value, nor
-    -- is anything returned to the user due to 'fmap void' above.
-    return $ error "normalForm_: a value was expected"
+normalForm_ = void <$> normalForm' forceEff
 
 removeEffects :: (MonadThunk t m (NValue t f m), MonadDataContext f m)
               => NValue t f m -> NValueNF t f m

src/Nix/Thunk.hs

@@ -14,7 +14,7 @@ class Monad m => MonadThunk t m v | t -> m, t -> v where
     query :: t -> r -> (v -> r) -> r
     queryM :: t -> m r -> (v -> m r) -> m r
     force :: t -> (v -> m r) -> m r
-    forceEff :: t -> (v -> m ()) -> m ()
+    forceEff :: t -> (v -> m r) -> m r
     wrapValue :: v -> t
     getValue :: t -> Maybe v
 

src/Nix/Thunk/Basic.hs

@@ -105,12 +105,12 @@ forceThunk (Thunk n active ref) k = do
                     writeVar ref (Computed v)
                     k v
 
-forceEffects :: MonadVar m => NThunkF m v -> (v -> m ()) -> m ()
+forceEffects :: MonadVar m => NThunkF m v -> (v -> m r) -> m r
 forceEffects (Value v) k = k v
 forceEffects (Thunk _ active ref) k = do
     nowActive <- atomicModifyVar active (True,)
     if nowActive
-        then return ()
+        then return $ error "Loop detected"
         else do
             eres <- readVar ref
             case eres of