RPC (Or: How do I disambiguate function application based on TypeRep values?) - haskell

Question

I'm building some infrastructure for doing remote procedure calls in Haskell, and for reasons that are too long to explain here, I cannot reuse existing libraries.
So here's the setup: I have a type class for serializing and deserializing data:
class Serializable a where
encode :: a -> B.ByteString
decode :: B.ByteString -> Maybe a
maxSize :: a -> Int
where B is Data.ByteString.
I can use this to implement serialization of integers, booleans, lists of serializables, tuples of serializables ect.
Now I want to send some arguments across a network to a server, which then performs a computation based on these arguments, and sends back a result. So I create an existential type representing things that can be serialized:
data SerializableExt = forall t . Serializable t => SerializableExt t
because I want to send something of type [SerializableExt].
So, of course, I need to create an instance Serializable SerializableExt. This is where the problem starts:
In order to implement decode :: B.ByteString -> Maybe SerializableExt I need to know the concrete type that the existential type SerializableExt wraps.
So I implement encode :: SerializableExt -> B.ByteString as serializing the concrete type along with the value:
encode (SerializableExt x) = encode (typeOf x, x)
using typeOf from Data-Typeable. The problem is now the implementation of decode :: B.ByteString -> Maybe SerializableExt:
decode bs =
let (tyenc, xenc) = splitPair bs -- Not really important. It just splits bs into the two components
in case (decode tyenc :: Maybe TypeRep) of
Just ty -> SerializableExt <$> _ -- Somehow invoke decode xenc, where the choice of which decode to execute depends on the value of ty.
_ -> Nothing
But I can't see how to fill in the hole here. Because of Haskell's separation of the value level and the type level I can't use the value of ty to disambiguate the invocation of decode xenc, right?
Is there a way to solve this issue, and actually put something in the hole which will do what I want? Or can you come up with another design?
EDIT: One way of doing it would be the following:
decode bs =
let (tyenc, xenc) = splitPair bs
in SerializableExt <$>
case (decode tyenc :: Maybe TypeRep) of
Just ty
| ty == typeRep (Proxy :: Proxy Int) -> decode xenc :: Maybe Int
| ty = typeRep (Proxy :: Proxy ()) -> decode xenc :: Maybe ()
| ...
_ -> Nothing
but this is bad for several reasons:
It's tedious to extend.
It cannot handle pairs (or generally: tuples) generically; every
combination of types needs to be handled.
It's not very Haskelly

Answer 1

Data.Dynamic lets us put arbitrary Haskell values into a single container, and get them out again in a type-safe way. That's a good start towards inter-process communication; I'll come back to serialization below.
We can write a program that takes a list of Dynamic values, checks for the number & types it needs, and returns a result in the same way.
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ScopedTypeVariables #-}
-- | Experiments with type-safe serialization.
module Main where
import Data.Proxy
import Data.Dynamic
import Data.Foldable
import Data.Type.Equality
import Type.Reflection
foo :: Int -> String -> String
foo i s = concat (replicate i s)
actor :: [Dynamic] -> Either String [Dynamic]
actor (di : ds : _) = case (fromDynamic di, fromDynamic ds) of
(Just i, Just s) -> Right [toDyn (foo i s)]
_ -> Left "Wrong types of arguments"
actor _ = Left "Not enough arguments"
caller :: Either String [Dynamic]
caller = actor [ toDyn (3::Int), toDyn "bar" ]
main :: IO ()
main = case caller of
Left err -> putStrLn err
Right dyns -> for_ dyns (\d -> case fromDynamic d of
Just s -> putStrLn s
Nothing -> print d)
We can use a TypeRep to guide selection of a class instance. (For ease of testing my code, I used String.)
class Serial a where
encode :: a -> String
decode :: String -> Maybe a
decodeAs :: Serial a => TypeRep a -> String -> Maybe a
decodeAs _ s = decode s
Finally, we'd like to serialize the TypeRep, and when decoding, check that the encoded type matches the type that we're decoding at.
instance Serial SomeTypeRep
encodeDyn :: (Typeable a, Serial a) => a -> (String, String)
encodeDyn a = (encode (SomeTypeRep (typeOf a)), encode a)
decodeDynamic :: forall a. (Typeable a, Serial a) => String -> String -> Maybe a
decodeDynamic tyStr aStr = case decode tyStr of
Nothing -> Nothing
Just (SomeTypeRep ty) ->
case eqTypeRep ty (typeRep :: TypeRep a) of
Nothing -> Nothing
Just HRefl -> decodeAs ty aStr