Handling errors in JSON to Haskell deserialization

My JSON deserializing code presented previously works, but when an error occurs, most likely because the JSON doesn't represent the objects you expect it to, we're a bit lost: we get a terse error message and our program aborts, because Data.Generics tend to call error when it encounters a problem, and our own calls from Maybe to Just may fail. So we need to build a way to report errors without crashing. Now, famously, there are several ways to report errors, and for a start I'll work with Either.

First, we create a error type that can help us represent both JSON parsing error and deserialization errors:

data JSOND13NError=JSONError ParseError
  | D13NError String String
  deriving Show

The two parameters in D13NError are the error message and the path in the JSON object where the error occured. They will be carried in the Either monad in a tuple of Strings

So parsing can either throw a JSONError or try to deserialize:

jsonStringToObj :: forall a. Data a => String -> Either JSOND13NError a
jsonStringToObj s= case parse JSON.json "JSON.parse" s of 
 Left err->Left (JSONError err)
 Right js->case (jsonToObj js "/") of
  Left (s,path) -> Left (D13NError s path)
  Right o-> Right o

The path starts at "/" for the deserialization

To work with Either we need a couple of helper function, one to translate the Maybe values we get from cast to Either:

toEither :: String -> Maybe a -> String -> Either (String,String) a
toEither s Nothing path=Left (s,path)
toEither _ (Just a) _=Right a

And then the monad declaration for Either(message,path):

instance Monad (Either (String,String)) where
 return a=Right a
 fail s=Left (s,"/")
 Right a >>= f2 =f2 a
 Left s >>= f2 =Left s 

With this, our main function becomes:

jsonToObj :: forall a. Data a => JSON.Value -> String -> Either (String,String) a
jsonToObj (JSON.String s) path=toEither "Not a String" (cast s) path
jsonToObj (JSON.Bool b) path=toEither "Not a Bool" (cast b) path
jsonToObj x path=do
 (values,cons)<-case x of
   JSON.Object m -> fieldJSONValues m myDataType path
   JSON.Number fl -> Right ([],if isPrefixOf "Prelude.Int" (dataTypeName myDataType)
    then mkIntConstr myDataType (round fl)
    else mkFloatConstr myDataType fl)
   JSON.Array [] -> Right ([],indexConstr myDataType 1)
   JSON.Array (x:xs) -> Right ([(x,path),((JSON.Array xs),path)],indexConstr myDataType 2)
 let StateT f=fromConstrM (StateT (\((x,path):xs) -> do
   r<-jsonToObj x path
   return (r,xs)
   )) cons
 r<- f values
 return (fst r)
 where
  getArg :: a' -> a'
  getArg = undefined
  getType :: a
  getType = undefined
  myDataType = dataTypeOf (getArg getType)

We use a StateT monad transformer to wrap our Either monad in the State. I'm not sure I understand 100% of how monad transformers actually work, but it works!

And the fieldJSONValues handles a few error conditions, along with building the path where we're at, by adding after the current path the name of each field in turn

fieldJSONValues :: (M.Map String JSON.Value) -> DataType -> String -> Either (String,String) ([(JSON.Value,String)],Constr)
fieldJSONValues m dt path | isAlgType dt=
 if idx>(maxConstrIndex dt)
  then Left ((printf "Constructor index %d too big" idx),path)
  else 
   if null fn 
    then Right ((map (\x->(fromJust $ M.lookup (show x) m',path++(show x)++"/")) (sort $ map (\x->((read x)::Int)) (M.keys m'))),c)
    else  
     let vals=map (\(x,y)->(fromJust x,y)) (filter (\(x,y)->isJust x) (map (\x->((M.lookup x m'),path++x++"/")) fn))
     in if (length vals) < (length fn)
      then Left ("Not enough fields",path)
      else Right (vals,c)
 where 
  idx=case M.lookup constrIndexField m of
   Just (JSON.Number f)->truncate f
   _ -> 1
  m'=M.delete constrIndexField m
  c=indexConstr dt idx 
  fn=constrFields c
fieldJSONValues m dt path | otherwise =Left ("Not an algebraic type",path)

Once I had the previous code working, using a different monad and adding error handling was easy. I suppose I could further abstract and try not to hard code the Either monad in it but use any type of Monad or MonadError, but this gives me what I want: no program crash and the ability to recover from errors!