Add --silent (close #45)

README.md

@@ -39,5 +39,5 @@ To run `hpack` automatically on modifications to `package.yaml` add the
 following to your `~/.vimrc`:
 
 ```vim
-autocmd BufWritePost package.yaml silent !hpack > /dev/null
+autocmd BufWritePost package.yaml silent !hpack --silent
 ```

driver/Main.hs

@@ -2,14 +2,16 @@ module Main (main) where
 
 import           Prelude ()
 import           Prelude.Compat
-import           Control.Monad.Compat
+
+import           Control.DeepSeq
 import           Control.Exception
-import           System.IO
-import           System.IO.Error
+import           Control.Monad.Compat
 import           Data.List.Compat
 import           Data.Version (showVersion)
-import           Control.DeepSeq
 import           System.Environment
+import           System.Exit
+import           System.IO
+import           System.IO.Error
 
 import           Paths_hpack (version)
 import           Hpack.Config
@@ -29,17 +31,29 @@ header = unlines [
 main :: IO ()
 main = do
   args <- getArgs
-  if "--version" `elem` args
-    then putStrLn programVersion
-    else do
-      (warnings, name, new) <- run
-      forM_ warnings $ \warning -> hPutStrLn stderr ("WARNING: " ++ warning)
-      old <- force . either (const Nothing) (Just . stripHeader) <$> tryJust (guard . isDoesNotExistError) (readFile name)
-      if (old == Just (lines new)) then do
-        putStrLn (name ++ " is up-to-date")
-      else do
-        (writeFile name $ header ++ new)
-        putStrLn ("generated " ++ name)
-      where
-        stripHeader :: String -> [String]
-        stripHeader = dropWhile null . dropWhile ("--" `isPrefixOf`) . lines
+  case args of
+    ["--version"] -> putStrLn programVersion
+    ["--silent"] -> hpack False
+    [] -> hpack True
+    _ -> do
+      hPutStrLn stderr "Usage: hpack [ --version | --silent ]"
+      exitFailure
+
+hpack :: Bool -> IO ()
+hpack verbose = do
+  (warnings, name, new) <- run
+  forM_ warnings $ \warning -> hPutStrLn stderr ("WARNING: " ++ warning)
+  old <- force . either (const Nothing) (Just . stripHeader) <$> tryJust (guard . isDoesNotExistError) (readFile name)
+  if (old == Just (lines new)) then do
+    output (name ++ " is up-to-date")
+  else do
+    (writeFile name $ header ++ new)
+    output ("generated " ++ name)
+  where
+    stripHeader :: String -> [String]
+    stripHeader = dropWhile null . dropWhile ("--" `isPrefixOf`) . lines
+
+    output :: String -> IO ()
+    output message
+      | verbose = putStrLn message
+      | otherwise = return ()

hpack.cabal

@@ -1,4 +1,4 @@
--- This file has been generated from package.yaml by hpack version 0.5.3.
+-- This file has been generated from package.yaml by hpack version 0.5.4.
 --
 -- see: https://github.com/sol/hpack