From a71aa7f16a1f52170437a5de3bf4923548c4f1a9 Mon Sep 17 00:00:00 2001
From: Brandon Chinn <brandonchinn178@gmail.com>
Date: Sun, 29 Dec 2024 14:08:14 -0800
Subject: [PATCH 1/4] Replace undefined with more descriptive error

---
 data/AlexTemplate.hs |  2 +-
 tests/Makefile       |  1 +
 tests/issue_262.x    | 26 ++++++++++++++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/issue_262.x

diff --git a/data/AlexTemplate.hs b/data/AlexTemplate.hs
index 8ca8c0a..ba7346a 100644
--- a/data/AlexTemplate.hs
+++ b/data/AlexTemplate.hs
@@ -94,7 +94,7 @@ data AlexReturn a
 
 -- alexScan :: AlexInput -> StartCode -> AlexReturn a
 alexScan input__ IBOX(sc)
-  = alexScanUser undefined input__ IBOX(sc)
+  = alexScanUser (error "alex rule requiring context was invoked by alexScan; use alexScanUser instead?") input__ IBOX(sc)
 
 alexScanUser user__ input__ IBOX(sc)
   = case alex_scan_tkn user__ input__ ILIT(0) input__ sc AlexNone of
diff --git a/tests/Makefile b/tests/Makefile
index 1eca78d..eede942 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -58,6 +58,7 @@ TESTS = \
         issue_119.x \
         issue_141.x \
         issue_197.x \
+        issue_262.x \
         monad_typeclass.x \
         monad_typeclass_bytestring.x \
         monadUserState_typeclass.x \
diff --git a/tests/issue_262.x b/tests/issue_262.x
new file mode 100644
index 0000000..4d5a217
--- /dev/null
+++ b/tests/issue_262.x
@@ -0,0 +1,26 @@
+{
+-- https://github.com/haskell/alex/pull/262
+-- https://gitlab.haskell.org/ghc/ghc/-/issues/25609
+--
+-- Error happens when using alexScan with a lexer that
+-- inspects the context.
+
+import Control.Exception
+import Data.List (isInfixOf)
+}
+
+%wrapper "basic"
+
+:-
+.* / { \state _ _ _ -> state == 'x' } { id }
+
+{
+main :: IO ()
+main = do
+  result <- try $ evaluate $ alexScan ('\n', [], "") 0 `seq` ()
+  case result of
+    Left (e :: SomeException)
+      | "use alexScanUser instead" `isInfixOf` show e
+      -> pure ()
+    _ -> error $ "Got unexpected result: " ++ show result
+}

From 837d2ba4b3da6bb4b7c639f1c3a868619502dca0 Mon Sep 17 00:00:00 2001
From: Brandon Chinn <brandonchinn178@gmail.com>
Date: Sat, 28 Dec 2024 19:03:06 -0800
Subject: [PATCH 2/4] Make sure alexScanUser is inlined

---
 data/AlexTemplate.hs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/data/AlexTemplate.hs b/data/AlexTemplate.hs
index ba7346a..80e6ac1 100644
--- a/data/AlexTemplate.hs
+++ b/data/AlexTemplate.hs
@@ -96,6 +96,11 @@ data AlexReturn a
 alexScan input__ IBOX(sc)
   = alexScanUser (error "alex rule requiring context was invoked by alexScan; use alexScanUser instead?") input__ IBOX(sc)
 
+-- If the generated alexScan/alexScanUser functions are called multiple times
+-- in the same file, alexScanUser gets broken out into a separate function and
+-- increases memory usage. Make sure GHC inlines this function and optimizes it.
+{-# INLINE alexScanUser #-}
+
 alexScanUser user__ input__ IBOX(sc)
   = case alex_scan_tkn user__ input__ ILIT(0) input__ sc AlexNone of
   (AlexNone, input__') ->

From 84a376f7ac58074da84b81729ccfcbf79aab270d Mon Sep 17 00:00:00 2001
From: Andreas Abel <andreas.abel@ifi.lmu.de>
Date: Mon, 30 Dec 2024 17:07:06 +0100
Subject: [PATCH 3/4] Add missing issue_262.x

---
 alex.cabal | 1 +
 1 file changed, 1 insertion(+)

diff --git a/alex.cabal b/alex.cabal
index b647a24..b723c77 100644
--- a/alex.cabal
+++ b/alex.cabal
@@ -90,6 +90,7 @@ extra-source-files:
         tests/issue_119.x
         tests/issue_141.x
         tests/issue_197.x
+        tests/issue_262.x
         tests/strict_text_typeclass.x
         tests/posn_typeclass_strict_text.x
         tests/tokens_monadUserState_strict_text.x

From 9f949c9bbe4c0b762da4dbc98b8836df4c7863d7 Mon Sep 17 00:00:00 2001
From: Andreas Abel <andreas.abel@ifi.lmu.de>
Date: Mon, 30 Dec 2024 17:31:51 +0100
Subject: [PATCH 4/4] Add LANGUAGE ScopedTypeVariables to issue_262.x

---
 tests/issue_262.x | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tests/issue_262.x b/tests/issue_262.x
index 4d5a217..a12af5f 100644
--- a/tests/issue_262.x
+++ b/tests/issue_262.x
@@ -5,6 +5,8 @@
 -- Error happens when using alexScan with a lexer that
 -- inspects the context.
 
+{-# LANGUAGE ScopedTypeVariables #-}
+
 import Control.Exception
 import Data.List (isInfixOf)
 }