From f508e185900637da22d6fbf4f139cabd428070f4 Mon Sep 17 00:00:00 2001
From: Simon Jakobi <simon.jakobi@gmail.com>
Date: Tue, 21 Jul 2020 03:07:46 +0200
Subject: [PATCH] alterF: Skip deleting the key when it's already absent (#288)

Fixes #287.
---
 Data/HashMap/Internal.hs        | 2 +-
 Data/HashMap/Internal/Strict.hs | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Data/HashMap/Internal.hs b/Data/HashMap/Internal.hs
index 656dc14c..a572ab5f 100644
--- a/Data/HashMap/Internal.hs
+++ b/Data/HashMap/Internal.hs
@@ -1282,7 +1282,7 @@ alterF f = \ !k !m ->
     mv = lookup' h k m
   in (<$> f mv) $ \fres ->
     case fres of
-      Nothing -> delete' h k m
+      Nothing -> maybe m (const (delete' h k m)) mv
       Just v' -> insert' h k v' m
 
 -- We unconditionally rewrite alterF in RULES, but we expose an
diff --git a/Data/HashMap/Internal/Strict.hs b/Data/HashMap/Internal/Strict.hs
index 971ebd7e..b774cf41 100644
--- a/Data/HashMap/Internal/Strict.hs
+++ b/Data/HashMap/Internal/Strict.hs
@@ -314,7 +314,7 @@ alterF f = \ !k !m ->
       mv = lookup' h k m
   in (<$> f mv) $ \fres ->
     case fres of
-      Nothing -> delete' h k m
+      Nothing -> maybe m (const (delete' h k m)) mv
       Just !v' -> insert' h k v' m
 
 -- We rewrite this function unconditionally in RULES, but we expose