From 0a8a5276ab9e5fed4f9d83e415ef87e2f00adcf9 Mon Sep 17 00:00:00 2001
From: meooow25 <soumiksarkar.3120@gmail.com>
Date: Wed, 14 Feb 2024 19:15:25 +0530
Subject: [PATCH] Fix filter/filter rules for Text and lazy Text

Predicates should be applied in the right order.
---
 src/Data/Text.hs           | 2 +-
 src/Data/Text/Lazy.hs      | 2 +-
 tests/Tests/Regressions.hs | 8 ++++++++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/Data/Text.hs b/src/Data/Text.hs
index 7d2514aa..320219c8 100644
--- a/src/Data/Text.hs
+++ b/src/Data/Text.hs
@@ -1686,7 +1686,7 @@ filter p = filter_ text p
 
 {-# RULES
 "TEXT filter/filter -> filter" forall p q t.
-    filter p (filter q t) = filter (\c -> p c && q c) t
+    filter p (filter q t) = filter (\c -> q c && p c) t
 #-}
 
 -- | /O(n+m)/ Find the first instance of @needle@ (which must be
diff --git a/src/Data/Text/Lazy.hs b/src/Data/Text/Lazy.hs
index 9b977d67..cd3a0bda 100644
--- a/src/Data/Text/Lazy.hs
+++ b/src/Data/Text/Lazy.hs
@@ -1690,7 +1690,7 @@ filter p = foldrChunks (chunk . filter_ T.Text p) Empty
 
 {-# RULES
 "TEXT filter/filter -> filter" forall p q t.
-    filter p (filter q t) = filter (\c -> p c && q c) t
+    filter p (filter q t) = filter (\c -> q c && p c) t
 #-}
 
 -- | /O(n)/ The 'find' function takes a predicate and a 'Text', and
diff --git a/tests/Tests/Regressions.hs b/tests/Tests/Regressions.hs
index 0418f4dc..60298c0d 100644
--- a/tests/Tests/Regressions.hs
+++ b/tests/Tests/Regressions.hs
@@ -176,6 +176,13 @@ t529 = do
     (T.pack (chr 33 : replicate 31 (chr 0) ++ [chr 65533, chr 0]))
     (decode (B.pack (33 : replicate 31 0 ++ [128, 0])))
 
+-- See Github #559
+-- filter/filter fusion rules should apply predicates in the right order.
+t559 :: IO ()
+t559 = do
+  T.filter undefined (T.filter (const False) "a") @?= ""
+  LT.filter undefined (LT.filter (const False) "a") @?= ""
+
 tests :: F.TestTree
 tests = F.testGroup "Regressions"
     [ F.testCase "hGetContents_crash" hGetContents_crash
@@ -193,4 +200,5 @@ tests = F.testGroup "Regressions"
     , F.testCase "t525" t525
     , F.testCase "t528" t528
     , F.testCase "t529" t529
+    , F.testCase "t559" t559
     ]