diff --git a/src/Nix/Expr/Types.hs b/src/Nix/Expr/Types.hs
index c188c10cd..2eb1b6a6d 100644
--- a/src/Nix/Expr/Types.hs
+++ b/src/Nix/Expr/Types.hs
@@ -32,6 +32,7 @@ import           Data.Data
 import           Data.Fix
 import           Data.Functor.Classes
 import           Data.Hashable.Lifted
+import qualified Data.HashMap.Lazy             as MapL
 import qualified Data.List.NonEmpty            as NE
 import qualified Text.Show
 import           Data.Traversable
@@ -615,7 +616,19 @@ hashAt
 #else
 hashAt :: VarName -> Lens' (AttrSet v) (Maybe v)
 #endif
-hashAt = flip alterF
+hashAt = alterF
+ where
+  alterF
+    :: (Functor f)
+    => Text
+    -> (Maybe v -> f (Maybe v))
+    -> AttrSet v
+    -> f (AttrSet v)
+  alterF k f m =
+    maybe
+      (MapL.delete k m)
+      (\ v -> MapL.insert k v m)
+      <$> f (MapL.lookup k m)
 
 -- | Get the name out of the parameter (there might be none).
 paramName :: Params r -> Maybe VarName
diff --git a/src/Nix/Utils.hs b/src/Nix/Utils.hs
index 4c955455b..ccb62e7fa 100644
--- a/src/Nix/Utils.hs
+++ b/src/Nix/Utils.hs
@@ -172,19 +172,6 @@ uriAwareSplit txt =
         (e1 <> "://" <> suffix, PathEntryURI) : path
       | otherwise                                 -> (e1, PathEntryPath) : uriAwareSplit (Text.drop 1 e2)
 
-alterF
-  :: (Eq k, Hashable k, Functor f)
-  => (Maybe v -> f (Maybe v))
-  -> k
-  -> HashMap k v
-  -> f (HashMap k v)
-alterF f k m =
-  maybe
-    (M.delete k m)
-    (\ v -> M.insert k v m)
-    <$> f (M.lookup k m)
-
-
 -- | Analog for @bool@ or @maybe@, for list-like cons structures.
 list
   :: Foldable t