Change fromObj error messages

Currently fromObj doesn't detail what we're trying to read, which can
lead to cryptic messages: "Cannot read Int". The patch changes this
function to annotate the error messages with the key/value we're trying
to convert, by using a new version of fromJResult.

Since the display of the key in tryFromObj is now redundant (it was
already redundant in the 'not found' case), we remove it.

The new version of fromJResult (annotateJResult) simply prepends a
description string to the actual error message.

diff --git a/Ganeti/HTools/Utils.hs b/Ganeti/HTools/Utils.hs
index e7f63b3..736fcd8 100644 (file)
--- a/Ganeti/HTools/Utils.hs
+++ b/Ganeti/HTools/Utils.hs
@@ -97,6 +97,10 @@ fromJResult :: Monad m => J.Result a -> m a
 fromJResult (J.Error x) = fail x
 fromJResult (J.Ok x) = return x
 
+annotateJResult :: Monad m => String -> J.Result a -> m a
+annotateJResult s (J.Error x) = fail (s ++ ": " ++ x)
+annotateJResult _ (J.Ok x) = return x
+
 -- | Tries to read a string from a JSON value.
 --
 -- In case the value was not a string, we fail the read (in the
@@ -116,7 +120,8 @@ fromObj :: (J.JSON a, Monad m) => String -> [(String, J.JSValue)] -> m a
 fromObj k o =
     case lookup k o of
       Nothing -> fail $ printf "key '%s' not found in %s" k (show o)
-      Just val -> fromJResult $ J.readJSON val
+      Just val -> annotateJResult (printf "key '%s', value '%s'" k (show val))
+                  (J.readJSON val)
 
 -- | Annotate a Result with an ownership information
 annotateResult :: String -> Result a -> Result a
@@ -127,7 +132,7 @@ annotateResult _ v = v
 -- than fromObj
 tryFromObj :: (J.JSON a) =>
               String -> [(String, J.JSValue)] -> String -> Result a
-tryFromObj t o k = annotateResult (t ++ " key '" ++ k ++ "'") (fromObj k o)
+tryFromObj t o k = annotateResult t (fromObj k o)
 
 -- | Small wrapper over readJSON.
 fromJVal :: (Monad m, J.JSON a) => J.JSValue -> m a