Add two utility functions

These both are work with/on the Result type, so we add them to
BasicTypes. The functions will be used as more generic versions of
some more specialised functions that are right now spread across the
modules.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

htools/Ganeti/BasicTypes.hs

 {-
 
-Copyright (C) 2009, 2010, 2011 Google Inc.
+Copyright (C) 2009, 2010, 2011, 2012 Google Inc.
 
 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
@@ -25,9 +25,13 @@ module Ganeti.BasicTypes
   , isBad
   , eitherToResult
   , annotateResult
+  , annotateIOError
+  , exitIfBad
   ) where
 
 import Control.Monad
+import System.IO (hPutStrLn, stderr)
+import System.Exit
 
 -- | This is similar to the JSON library Result type - /very/ similar,
 -- but we want to use it in multiple places, so we abstract it into a
@@ -71,3 +75,17 @@ eitherToResult (Right v) = Ok v
 annotateResult :: String -> Result a -> Result a
 annotateResult owner (Bad s) = Bad $ owner ++ ": " ++ s
 annotateResult _ v = v
+
+-- | Annotates and transforms IOErrors into a Result type. This can be
+-- used in the error handler argument to 'catch', for example.
+annotateIOError :: String -> IOError -> IO (Result a)
+annotateIOError description exc =
+  return . Bad $ description ++ ": " ++ show exc
+
+-- | Unwraps a 'Result', exiting the program if it is a 'Bad' value,
+-- otherwise returning the actual contained value.
+exitIfBad :: Result a -> IO a
+exitIfBad (Bad s) = do
+  hPutStrLn stderr $ "Failure: " ++ s
+  exitWith (ExitFailure 1)
+exitIfBad (Ok v) = return v