Avoid unnecessary writes on --force (see #555)

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## Changes in 0.36.0
  - Add `--canonical`
+ - Avoid unnecessary writes on `--force` (see #555)
  - When an existing `.cabal` does not align fields then do not align fields in
    the generated `.cabal` file.
  - Fix a bug related to git conflict markers in existing `.cabal` files: When a

src/Hpack.hs

@@ -267,7 +267,7 @@ writeCabalFile :: DecodeOptions -> Bool -> FilePath -> NewCabalFile -> IO ()
 writeCabalFile options toStdout name cabalFile = do
   write . unlines $ renderCabalFile (decodeOptionsTarget options) cabalFile
   where
-    write = if toStdout then Utf8.putStr else Utf8.writeFile name
+    write = if toStdout then Utf8.putStr else Utf8.ensureFile name
 
 makeCabalFile :: OutputStrategy -> GenerateHashStrategy -> Maybe ExistingCabalFile -> [String] -> Version -> Package -> NewCabalFile
 makeCabalFile outputStrategy generateHashStrategy mExistingCabalFile cabalVersion v pkg = cabalFile

src/Hpack/Utf8.hs

@@ -1,14 +1,18 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 module Hpack.Utf8 (
   encodeUtf8
 , readFile
-, writeFile
+, ensureFile
 , putStr
 , hPutStr
 , hPutStrLn
 ) where
 
 import           Prelude hiding (readFile, writeFile, putStr)
 
+import           Control.Monad
+import           Control.Exception (try, IOException)
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as Encoding
 import           Data.Text.Encoding.Error (lenientDecode)
@@ -48,8 +52,13 @@ decodeNewlines = go
 readFile :: FilePath -> IO String
 readFile = fmap decodeText . B.readFile
 
-writeFile :: FilePath -> String -> IO ()
-writeFile name xs = withFile name WriteMode (`hPutStr` xs)
+ensureFile :: FilePath -> String -> IO ()
+ensureFile name new = do
+  try (readFile name) >>= \ case
+    Left (_ :: IOException) -> do
+      withFile name WriteMode (`hPutStr` new)
+    Right old -> unless (old == new) $ do
+      withFile name WriteMode (`hPutStr` new)
 
 putStr :: String -> IO ()
 putStr = hPutStr stdout

test/Hpack/Utf8Spec.hs

@@ -18,7 +18,7 @@ spec = do
           B.writeFile name "foo\r\nbar"
           Utf8.readFile name `shouldReturn` "foo\nbar"
 
-  describe "writeFile" $ do
+  describe "ensureFile" $ do
     it "uses system specific newline encoding" $ do
       inTempDirectory $ do
         let
@@ -28,5 +28,5 @@ spec = do
         writeFile name c
         systemSpecific <- B.readFile name
 
-        Utf8.writeFile name c
+        Utf8.ensureFile name c
         B.readFile name `shouldReturn` systemSpecific