exit with msg if no file given

src/Main.hs

@@ -1,8 +1,16 @@
 module Main where
 
+import           Control.Monad      (when)
 import           Gui                (runEditor)
 import           System.Environment (getArgs)
 
 main = do
   args <- getArgs
+  when (null args) $ error (msgWithUsage "Please specify a file")
   runEditor $ head args
+
+msgWithUsage :: String -> String
+msgWithUsage = \x -> x ++ "\n" ++ usage
+
+usage :: String
+usage = "Usage: hedit <file>"