From 3873bb473e5797a6f2dc9197cf4536113d584ffe Mon Sep 17 00:00:00 2001
From: Julian Ospald <hasufell@posteo.de>
Date: Sat, 2 Dec 2023 18:02:28 +0800
Subject: [PATCH] Make osstr :: QuasiQuoter valid as a pattern

Similar to https://github.com/haskell/filepath/pull/210
---
 System/OsString/Internal.hs | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/System/OsString/Internal.hs b/System/OsString/Internal.hs
index 18825f8..e0d5254 100644
--- a/System/OsString/Internal.hs
+++ b/System/OsString/Internal.hs
@@ -3,6 +3,8 @@
 {-# LANGUAGE UnliftedFFITypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TemplateHaskellQuotes #-}
+{-# LANGUAGE ViewPatterns #-}  -- needed to quote a view pattern
 
 module System.OsString.Internal where
 
@@ -122,6 +124,7 @@ fromBytes = fmap OsString . PF.fromBytes
 
 -- | QuasiQuote an 'OsString'. This accepts Unicode characters
 -- and encodes as UTF-8 on unix and UTF-16 on windows.
+-- If used as pattern, requires turning on the @ViewPatterns@ extension.
 osstr :: QuasiQuoter
 osstr =
   QuasiQuoter
@@ -129,23 +132,25 @@ osstr =
   { quoteExp = \s -> do
       osp <- either (fail . show) (pure . OsString) . PF.encodeWith (mkUTF16le ErrorOnCodingFailure) $ s
       lift osp
-  , quotePat  = \_ ->
-      fail "illegal QuasiQuote (allowed as expression only, used as a pattern)"
+  , quotePat = \s -> do
+      osp' <- either (fail . show) (pure . OsString) . PF.encodeWith (mkUTF16le ErrorOnCodingFailure) $ s
+      [p|((==) osp' -> True)|]
   , quoteType = \_ ->
-      fail "illegal QuasiQuote (allowed as expression only, used as a type)"
+      fail "illegal QuasiQuote (allowed as expression or pattern only, used as a type)"
   , quoteDec  = \_ ->
-      fail "illegal QuasiQuote (allowed as expression only, used as a declaration)"
+      fail "illegal QuasiQuote (allowed as expression or pattern only, used as a declaration)"
   }
 #else
   { quoteExp = \s -> do
       osp <- either (fail . show) (pure . OsString) . PF.encodeWith (mkUTF8 ErrorOnCodingFailure) $ s
       lift osp
-  , quotePat  = \_ ->
-      fail "illegal QuasiQuote (allowed as expression only, used as a pattern)"
+  , quotePat = \s -> do
+      osp' <- either (fail . show) (pure . OsString) . PF.encodeWith (mkUTF8 ErrorOnCodingFailure) $ s
+      [p|((==) osp' -> True)|]
   , quoteType = \_ ->
-      fail "illegal QuasiQuote (allowed as expression only, used as a type)"
+      fail "illegal QuasiQuote (allowed as expression or pattern only, used as a type)"
   , quoteDec  = \_ ->
-      fail "illegal QuasiQuote (allowed as expression only, used as a declaration)"
+      fail "illegal QuasiQuote (allowed as expression or pattern only, used as a declaration)"
   }
 #endif