Deserializing an existential data type

Question

I need to write a Serialize instance for the following data type:

data AnyNode = forall n . (Typeable n, Serialize n) => AnyNode n

Serializing this is no problem, but I can't implement deserialization, since the compiler has no way to resolve the specific instance of Serialize n, since the n is isolated from the outer scope.

There's been a related discussion in 2006. I am now wondering whether any sort of solution or a workaround has arrived today.

Answer 1

You just tag the type when you serialize, and use a dictionary to untag the type when you deserialize. Here's some pseudocode omitting error checking etc:

serialAnyNode (AnyNode x) = serialize (typeOf n, serialize x)

deserialAnyNode s = case deserialize s of
          (typ,bs) -> case typ of
                         "String" -> AnyNode (deserialize bs :: String)
                         "Int" -> AnyNode (deserialize bs :: Int)
                         ....

Note that you can only deserialize a closed universe of types with your function. With some extra work, you can also deserialize derived types like tuples, maybes and eithers.

But if I were to declare an entirely new type "Gotcha" deriving Typeable and Serialize, deserialAnyNode of course couldn't deal with it without extension.