add file-io to build-depends when building with OsPath flag

Partly converted code to use functions from it, though more remain
unconverted. Most of withFile and openFile now use it.

Utility/FileIO.hs

@@ -0,0 +1,103 @@
+{- File IO on OsPaths.
+ -
+ - Since Prelude exports many of these as well, this needs to be imported
+ - qualified.
+ -
+ - Copyright 2025 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+{-# OPTIONS_GHC -fno-warn-tabs #-}
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+
+module Utility.FileIO
+(
+	withFile,
+	openFile,
+	readFile,
+	readFile',
+	writeFile,
+	writeFile',
+	appendFile,
+	appendFile',
+) where
+
+#ifdef WITH_OSPATH
+
+#ifndef mingw32_HOST_OS
+import System.File.OsPath
+#else
+-- On Windows, System.File.OsPath does not handle UNC-style conversion itself,
+-- so that has to be done when calling it. See 
+-- https://github.com/haskell/file-io/issues/39
+import Utility.Path.Windows
+import Utility.OsPath
+import System.IO (IO, Handle, IOMode)
+import System.OsPath (OsPath)
+import qualified System.File.OsPath as O
+import qualified Data.ByteString as B
+import qualified Data.ByteString.Lazy as L
+import Control.Applicative
+
+withFile :: OsPath -> IOMode -> (Handle -> IO r) -> IO r 
+withFile f m a = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.withFile f' m a
+
+openFile :: OsPath -> IOMode -> IO Handle
+openFile f m = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.openFile f' m
+
+readFile :: OsPath -> IO L.ByteString
+readFile f = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.readFile f'
+
+readFile' :: OsPath -> IO B.ByteString
+readFile' f = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.readFile' f'
+
+writeFile :: OsPath -> L.ByteString -> IO ()
+writeFile f b = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.writeFile f' b
+
+writeFile' :: OsPath -> B.ByteString -> IO ()
+writeFile' f b = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.writeFile' f' b
+
+appendFile :: OsPath -> L.ByteString -> IO ()
+appendFile f b = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.appendFile f' b
+
+appendFile' :: OsPath -> B.ByteString -> IO ()
+appendFile' f b = do
+	f' <- toOsPath <$> convertToWindowsNativeNamespace (fromOsPath f)
+	O.appendFile' f' b
+
+#endif
+
+#else
+-- When not building with OsPath, export FilePath versions
+-- instead. However, functions still use ByteString for the
+-- file content in that case, unlike the Strings used by the Prelude.
+import Utility.OsPath
+import System.IO (withFile, openFile, IO)
+import Data.ByteString.Lazy (readFile, writeFile, appendFile)
+import qualified Data.ByteString as B
+
+readFile' :: OsPath -> IO B.ByteString
+readFile' = B.readFile
+
+writeFile' :: OsPath -> B.ByteString -> IO ()
+writeFile' = B.writeFile
+
+appendFile' :: OsPath -> B.ByteString -> IO ()
+appendFile' = B.appendFile
+#endif