From ce38867d46ee5dccb4916f0da32c9e52c197f26f Mon Sep 17 00:00:00 2001
From: Mike Pilgrem <mpilgrem@users.noreply.github.com>
Date: Sat, 9 Nov 2024 22:35:11 +0000
Subject: [PATCH] Re CLC issue 292 Avoid using partial init and last

---
 lib/Data/Format.hs              | 18 ++++++++++++++----
 test/unix/Test/Format/Format.hs | 33 +++++++++++++++++++++++----------
 2 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/lib/Data/Format.hs b/lib/Data/Format.hs
index 8adb892b..cc2ead64 100644
--- a/lib/Data/Format.hs
+++ b/lib/Data/Format.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP  #-}
 {-# LANGUAGE Safe #-}
 
 module Data.Format (
@@ -26,6 +27,9 @@ module Data.Format (
 
 import Control.Monad.Fail
 import Data.Char
+#if MIN_VERSION_base(4,19,0)
+import Data.List (unsnoc)
+#endif
 import Data.Void
 import Text.ParserCombinators.ReadP
 import Prelude hiding (fail)
@@ -227,11 +231,17 @@ zeroPad Nothing s = s
 zeroPad (Just i) s = replicate (i - length s) '0' ++ s
 
 trimTrailing :: String -> String
-trimTrailing "" = ""
 trimTrailing "." = ""
-trimTrailing s
-    | last s == '0' = trimTrailing $ init s
-trimTrailing s = s
+trimTrailing s = case unsnoc s of
+    Nothing -> ""
+    Just (initial, '0') -> trimTrailing initial
+    _ -> s
+
+#if !MIN_VERSION_base(4,19,0)
+unsnoc :: [a] -> Maybe ([a], a)
+unsnoc = foldr (\x -> Just . maybe ([], x) (\(~(a, b)) -> (x : a, b))) Nothing
+{-# INLINABLE unsnoc #-}
+#endif
 
 showNumber :: Show t => SignOption -> Maybe Int -> t -> Maybe String
 showNumber signOpt mdigitcount t =
diff --git a/test/unix/Test/Format/Format.hs b/test/unix/Test/Format/Format.hs
index 9532f648..772d18cb 100644
--- a/test/unix/Test/Format/Format.hs
+++ b/test/unix/Test/Format/Format.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# OPTIONS -fno-warn-orphans #-}
 
 module Test.Format.Format (
@@ -6,6 +7,9 @@ module Test.Format.Format (
 
 import Data.Char
 import Data.Fixed as F
+#if MIN_VERSION_base(4,19,0)
+import Data.List (unsnoc)
+#endif
 import Data.Time
 import Data.Time.Clock.POSIX
 import Foreign
@@ -151,16 +155,25 @@ unixWorkarounds fmt s
 unixWorkarounds _ s = s
 
 compareFormat :: (String -> String) -> String -> TimeZone -> UTCTime -> Result
-compareFormat _modUnix fmt zone _time
-    | last fmt == 'Z' && timeZoneName zone == "" = rejected
-compareFormat modUnix fmt zone time =
-    let
-        ctime = utcToZonedTime zone time
-        haskellText = formatTime locale fmt ctime
-        unixText = unixFormatTime fmt zone time
-        expectedText = unixWorkarounds fmt (modUnix unixText)
-    in
-        assertEqualQC (show time ++ " with " ++ show zone) expectedText haskellText
+compareFormat modUnix fmt zone time = case unsnoc fmt of
+    Nothing ->
+        error "compareFormat: The impossible happened! Format string is \"\"."
+    Just (_, lastChar)
+      | lastChar == 'Z' && timeZoneName zone == "" -> rejected
+      | otherwise ->
+            let
+                ctime = utcToZonedTime zone time
+                haskellText = formatTime locale fmt ctime
+                unixText = unixFormatTime fmt zone time
+                expectedText = unixWorkarounds fmt (modUnix unixText)
+            in
+                assertEqualQC (show time ++ " with " ++ show zone) expectedText haskellText
+
+#if !MIN_VERSION_base(4,19,0)
+unsnoc :: [a] -> Maybe ([a], a)
+unsnoc = foldr (\x -> Just . maybe ([], x) (\(~(a, b)) -> (x : a, b))) Nothing
+{-# INLINABLE unsnoc #-}
+#endif
 
 -- as found in http://www.opengroup.org/onlinepubs/007908799/xsh/strftime.html
 -- plus FgGklz