Implement hWithEncoding

This function allows one to not only safely set the best encoding on a
file descriptor, but also restore the previous one. The idea is that it
will be used on file descriptors that come from the outside and so we
don’t know how they were used and will be used after us.

CHANGELOG.md

@@ -14,6 +14,7 @@ Initial release.
 
 - `withUtf8StdHandles`
 - `hSetEncoding`
+- `hWithEncoding`
 - `openFile`
 - `withFile`
 - `readFile`

README.md

@@ -57,17 +57,16 @@ All these functions will make sure that the content will be treated as if it
 was encoded in UTF-8 (it is 2020, what else can it be encoded in?).
 
 If, for some reason, you really need to use `withFile`/`openFile` from `base`,
-just call `hSetEncoding h`, where `h` is your handle and `hSetEncoding` comes
+or you got your file handle from somewhere else, wrap the code that works
-from `System.IO.Utf8` for your convenience:
+with it in a call to `hWithEncoding` from `System.IO.Utf8`:
 
 ```haskell
 import qualified System.IO as IO
 import qualified System.IO.Utf8 as Utf8
 
-doSomethingWithAFile :: IO ()
+doSomethingWithAFile :: IO.Handle -> IO ()
-doSomethingWithAFile = IO.withFile "file.txt" IO.ReadMode $ \h -> do
+doSomethingWithAFile h = Utf8.hWithEncoding h $ do
-  Utf8.hSetEncoding h
+    {- ... work with the file ... -}
-  {- ... work with the file ... -}
 ```
 
 ### Step 4: Write files using UTF-8

lib/System/IO/Utf8.hs

@@ -24,6 +24,7 @@ module System.IO.Utf8
   ( withUtf8StdHandles
 
   , hSetEncoding
+  , hWithEncoding
 
   , openFile
   , withFile
@@ -66,13 +67,10 @@ hSetBestUtf8Enc h = liftIO $ IO.hGetEncoding h >>= \case
 -- After the action finishes, restores the original encodings.
 withUtf8StdHandles :: IO a -> IO a
 withUtf8StdHandles action =
-    withConfiguredHandle stdin $
+    hWithEncoding stdin $
-    withConfiguredHandle stdout $
+    hWithEncoding stdout $
-    withConfiguredHandle stderr $
+    hWithEncoding stderr $
       action
-  where
-    withConfiguredHandle :: IO.Handle -> IO a -> IO a
-    withConfiguredHandle h = bracket (hSetBestUtf8Enc h) ($ h) . const
 
 
 -- | Set handle encoding to the best possible.
@@ -93,6 +91,13 @@ withUtf8StdHandles action =
 hSetEncoding :: MonadIO m => IO.Handle -> m ()
 hSetEncoding = liftIO . void . hSetBestUtf8Enc
 
+-- | Temporarily set handle encoding to the best possible.
+--
+-- This is like 'hSetEncoding', but it will restore the encoding
+-- to the previous one when the action is done.
+hWithEncoding :: (MonadIO m, MonadMask m) => IO.Handle -> m r -> m r
+hWithEncoding h = bracket (hSetBestUtf8Enc h) ($ h) . const
+
 
 -- | Like @openFile@, but sets the file encoding to UTF-8, regardless
 -- of the current locale.