make error slightly nicer

main/ips.hs

@@ -2,8 +2,9 @@ module Main (main) where
 
 import IPS (Bytes, makeBytes)
 import qualified IPS
-import System.Environment
-import System.IO
+import System.Environment (getArgs)
+import System.Exit (exitFailure)
+import System.IO (hPutBuf, withFile, IOMode(WriteMode))
 import qualified Data.ByteString as ByteString
 import qualified Data.Vector.Storable as Vector
 
@@ -15,7 +16,9 @@ main = do
       buf <- readBytes inf
       ips <- either error id <$> IPS.parseFile ipsf
       writeBytes outf $ IPS.apply ips buf
-    _ -> error "usage: $0 <in> <ips> <out>"
+    _ -> do
+      putStrLn "usage: ips <in> <ips> <out>"
+      exitFailure
 
 readBytes :: FilePath -> IO Bytes
 readBytes f = makeBytes <$> ByteString.readFile f