maybe package:dhall

maybe :: Decoder a -> Decoder (Maybe a)
dhall Dhall.Marshal.Decode
Decode a Maybe. 
>>> input (maybe natural) "Some 1"
Just 1
mapMaybe :: Ord k => (a -> Maybe b) -> Map k a -> Map k b
dhall Dhall.Map
Transform all values in a Map using the supplied function, deleting the key if the function returns Nothing 
>>> mapMaybe Data.Maybe.listToMaybe (fromList [("C",[1]),("B",[]),("A",[3])])
fromList [("C",1),("A",3)]