Fix filter/filter rules for Text and lazy Text

Predicates should be applied in the right order.
haskell · Feb 14, 2024 · 9c7a352 · 9c7a352
1 parent 456783a
commit 9c7a352
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Showing 3 changed files with 10 additions and 2 deletions.
diff --git a/src/Data/Text.hs b/src/Data/Text.hs
@@ -1659,7 +1659,7 @@ 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
@@ -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
@@ -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
     ]