From e4e3f6085a519bb8f2acbe16ad55cbfc6b604882 Mon Sep 17 00:00:00 2001
From: Mike Pilgrem <mpilgrem@users.noreply.github.com>
Date: Sun, 6 Oct 2024 17:00:00 +0100
Subject: [PATCH] Avoid partial last in autogen Paths_<pkg>.hs

---
 Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs | 10 +++++++---
 changelog.d/pr-10432                                 | 10 ++++++++++
 templates/Paths_pkg.template.hs                      | 10 +++++++---
 3 files changed, 24 insertions(+), 6 deletions(-)
 create mode 100644 changelog.d/pr-10432

diff --git a/Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs b/Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs
index 25c924720ec..ad979c42951 100644
--- a/Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs
+++ b/Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs
@@ -69,7 +69,6 @@ render z_root = execWriter $ do
     return ()
   tell "\n"
   tell "import qualified Control.Exception as Exception\n"
-  tell "import qualified Data.List as List\n"
   tell "import Data.Version (Version(..))\n"
   tell "import System.Environment (getEnv)\n"
   tell "import Prelude\n"
@@ -306,9 +305,14 @@ render z_root = execWriter $ do
   tell "joinFileName \"\"  fname = fname\n"
   tell "joinFileName \".\" fname = fname\n"
   tell "joinFileName dir \"\"    = dir\n"
-  tell "joinFileName dir fname\n"
-  tell "  | isPathSeparator (List.last dir) = dir ++ fname\n"
+  tell "joinFileName dir@(c:cs) fname\n"
+  tell "  | isPathSeparator (lastChar c cs) = dir ++ fname\n"
   tell "  | otherwise                       = dir ++ pathSeparator : fname\n"
+  tell " where\n"
+  tell "  -- We do not use Data.List.NonEmpty.last, as that would limit the module to\n"
+  tell "  -- base >= 4.9.0.0 (GHC >= 8.0.1).\n"
+  tell "  lastChar x [] = x\n"
+  tell "  lastChar _ (x:xs) = lastChar x xs\n"
   tell "\n"
   tell "pathSeparator :: Char\n"
   if (zIsWindows z_root)
diff --git a/changelog.d/pr-10432 b/changelog.d/pr-10432
new file mode 100644
index 00000000000..1bb76adb0a1
--- /dev/null
+++ b/changelog.d/pr-10432
@@ -0,0 +1,10 @@
+synopsis: Avoid partial `Data.List.last` in autogenerated `Paths_<pkg>.hs`
+packages: Cabal
+prs: #10432
+significance:
+
+description: {
+
+- Autogenerated `Paths_<pkg>.hs` now avoids use of the partial function `Data.List.last`.
+
+}
diff --git a/templates/Paths_pkg.template.hs b/templates/Paths_pkg.template.hs
index 8e1e03d27e4..bea7d6813e3 100644
--- a/templates/Paths_pkg.template.hs
+++ b/templates/Paths_pkg.template.hs
@@ -26,7 +26,6 @@ import Foreign.C
 {% endif %}
 
 import qualified Control.Exception as Exception
-import qualified Data.List as List
 import Data.Version (Version(..))
 import System.Environment (getEnv)
 import Prelude
@@ -175,9 +174,14 @@ joinFileName :: String -> String -> FilePath
 joinFileName ""  fname = fname
 joinFileName "." fname = fname
 joinFileName dir ""    = dir
-joinFileName dir fname
-  | isPathSeparator (List.last dir) = dir ++ fname
+joinFileName dir@(c:cs) fname
+  | isPathSeparator (lastChar c cs) = dir ++ fname
   | otherwise                       = dir ++ pathSeparator : fname
+ where
+  -- We do not use Data.List.NonEmpty.last, as that would limit the module to
+  -- base >= 4.9.0.0 (GHC >= 8.0.1).
+  lastChar x [] = x
+  lastChar _ (x:xs) = lastChar x xs
 
 pathSeparator :: Char
 {% if isWindows %}