Handle errors in Main a bit more cleanly than with 'error', shutting down...

Handle errors in Main a bit more cleanly than with 'error', shutting down cleanly with the error message and no stack trace

app/Main.hs

@@ -9,21 +9,22 @@ import qualified Data.Text.IO as Text (getContents, readFile, writeFile)
 import Text.InvisiXML (InvisiXML(..), parse)
 import Text.XML.Light.Serializer (encode)
 import System.FilePath ((<.>), dropExtension)
+import System.Exit (die)
 
-endPoints :: Command -> (IO Text, FilePath)
+endPoints :: Command -> IO (IO Text, FilePath)
 endPoints (Command {input = StdIn, outputPrefix}) =
-  (Text.getContents, fromMaybe noOutputPrefix outputPrefix)
+  (,) Text.getContents <$> maybe noOutputPrefix return outputPrefix
   where
-    noOutputPrefix = error "output prefix (-o) is necessary when running on stdin"
+    noOutputPrefix = die "output prefix (-o) is necessary when running on stdin"
 endPoints (Command {input = FileInput f, outputPrefix}) =
-  (Text.readFile f, fromMaybe (dropExtension f) outputPrefix)
+  return (Text.readFile f, fromMaybe (dropExtension f) outputPrefix)
 
 run :: Command -> IO ()
-run command =
-  source >>= runExceptT . parse >>= either (fail . show) create
+run command = do
+  (source, prefix) <- endPoints command
+  source >>= runExceptT . parse >>= either (fail . show) (create prefix)
   where
-    (source, prefix) = endPoints command
-    create (InvisiXML {text, structure}) = do
+    create prefix (InvisiXML {text, structure}) = do
       Text.writeFile (prefix <.> "txt") text
       writeFile (prefix <.> "ixml") $ encode structure