This JSON-RPC library is fully-compatible with JSON-RPC 2.0 and partially-compatible with JSON-RPC 1.0. It provides an interface that combines a JSON-RPC client and server. It can set and keep track of request ids to parse responses. There is support for sending and receiving notifications. You may use any underlying transport. Basic TCP client and server provided.

The recommended interface to this library is provided as conduits that encode outgoing messages, and decode incoming messages. Incoming messages are delivered as an IncomingMsg data structure, while outgoing messages are sent in a Message data structure. The former packs responses and errors with their corresponding request, and has a separate constructor for decoding errors.

A JSON-RPC application using this interface is considered to be peer-to-peer, as it can send and receive all types of JSON-RPC message independent of whether it originated the connection.

Type classes ToRequest, ToNotif are for data that can be converted into JSON-RPC requests and notifications respectively. An instance of aeson's ToJSON class is also required to serialize these data structures. Make sure that they serialize as a structured JSON value (array or object) that can go into the params field of the JSON-RPC object. Type classes FromRequest, FromNotif and FromResult are for deserializing JSON-RPC messages.

Errors are deserialized to the ErrorObj data type. Only a string is supported as contents inside a JSON-RPC 1.0 error. JSON-RPC 2.0 errors also have a code, and possibly additional data as an aeson Value.

This server returns the current time.

{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson.Types
import Data.Conduit
import qualified Data.Conduit.List as CL
import Data.Conduit.Network
import Data.Time.Clock
import Data.Time.Format
import Network.JsonRpc
import System.Locale

data TimeReq = TimeReq
data TimeRes = TimeRes UTCTime

instance FromRequest TimeReq where
    paramsParser "time" = Just $ const $ return TimeReq 
    paramsParser _ = Nothing

instance ToJSON TimeRes where
    toJSON (TimeRes t) = toJSON $ formatTime defaultTimeLocale "%c" t

srv :: AppConduits () () TimeRes TimeReq () () IO -> IO ()
srv (src, snk) = src $= CL.mapM respond $$ snk

respond :: IncomingMsg () TimeReq () ()
        -> IO (Message () () TimeRes)
respond (IncomingMsg (MsgRequest (Request ver _ TimeReq i)) Nothing) = do    
    t <- getCurrentTime
    return $ MsgResponse (Response ver (TimeRes t) i)

respond (IncomingError e) = return $ MsgError e
respond (IncomingMsg (MsgError e) _) = return $ MsgError $ e
respond _ = undefined

main :: IO ()
main = tcpServer V2 (serverSettings 31337 "127.0.0.1") srv

Corresponding TCP client to get time from server.

{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson.Types hiding (Error)
import Data.Conduit
import qualified Data.Conduit.List as CL
import Data.Conduit.Network
import qualified Data.Text as T
import Data.Time.Clock
import Data.Time.Format
import Network.JsonRpc
import System.Locale

data TimeReq = TimeReq
data TimeRes = TimeRes UTCTime

instance ToRequest TimeReq where
    requestMethod TimeReq = "time"

instance ToJSON TimeReq where
    toJSON TimeReq = emptyArray

instance FromResponse TimeRes where
    parseResult "time" = withText "time" $ \t -> case f t of
        Nothing -> fail "Could not parse time"
        Just t' -> return $ TimeRes t'
      where
        f t = parseTime defaultTimeLocale "%c" (T.unpack t)

cli :: AppConduits TimeReq () () () () TimeRes IO
    -> IO UTCTime
cli (src, snk) = do
    CL.sourceList [MsgRequest $ buildRequest V2 TimeReq] $$ snk
    ts <- src $$ CL.consume
    case ts of
        [] -> error "No response received"
        [IncomingError (ErrorObj _ m _ _ _)] -> error $ "Unknown: " ++ m
        [IncomingMsg (MsgError (ErrorObj _ m _ _ _)) _] -> error m
        [IncomingMsg (MsgResponse (Response _ (TimeRes t) _)) _] -> return t
        _ -> undefined

main :: IO ()
main = tcpClient V2 True (clientSettings 31337 "127.0.0.1") cli >>= print