haskell - To and From JSON for custom data types -

i have following classes:

type id = uuid  data mysubclass1 = mysubclass1     { svar1 :: utctime     , svar2 :: utctime     }  data mymainclass = mymainclass     { var1 :: id     , var2 :: mysubclass1     , var3 :: (int, string)     , var4 :: maybe string     } 

and here's json itself:

{   "var1" : ...,   "var2_1": ...,   "var2_2": ...,   "var3_1": ...,   "var3_2": ...,   "var4": ...,    "svar1": ...,   "svar2": ... } 

and need create instances of json them:

--ok instance fromjson mysubclass1   parsejson (object v) =      mysubclass1 <$> v .: "svar1"             <*> v .: "svar2"   --ok instance tojson mysubclass1   tojson (mysubclass1 svar1 svar2) =     object ["svar1" .= svar1       , "svar2" .= svar2     ] 

and here's have difficulty

instance fromjson mymainclass   parsejson (object v) =      mymainclass <$> v .: "var1"             <*> (mysubclass1 v .: "var2_1" v .: "var2_2") -- error            <*> (v .: "var3_1" v .: "var3_2") -- error            <*> v .:? "var4"   instance tojson mymainclass   tojson (mymainclass var1 var2 var3 var4) =     object ["var1" .= var1       , "var2" .= ---???       , -- what's next?     ] 

most of errors types don't match.