Add isLeft from Data.Either explicitly

...as it is only included in newer Versions of Data.Either
which are not available in all library versions Ganeti has
to work on.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

src/Ganeti/BasicTypes.hs

@@ -42,6 +42,7 @@ module Ganeti.BasicTypes
   , justOk
   , justBad
   , eitherToResult
+  , isLeft
   , annotateResult
   , annotateError
   , failError
@@ -274,6 +275,12 @@ eitherToResult :: Either a b -> GenericResult a b
 eitherToResult (Left  s) = Bad s
 eitherToResult (Right v) = Ok  v
 
+-- | Check if an either is Left. Equivalent to isLeft from Data.Either
+-- version 4.7.0.0 or higher.
+isLeft :: Either a b -> Bool
+isLeft (Left _) = True
+isLeft _        = False
+
 -- | Annotate an error with an ownership information, lifting it to a
 -- 'MonadError'. Since 'Result' is an instance of 'MonadError' itself,
 -- it's a generalization of type @String -> Result a -> Result a@.