How to derive instances of Data.Messagepack 1.0.0

问题

The previous version of Data.Messagepack, 0.7.2.5 supports deriving instances via Template Haskell. The current version (1.0.0), however, doesn't.

I was hence wondering if there is an alternative way to automatically derive MessagePack 1.0.0 instances, possibly using XDeriveGeneric?

回答1:

As a stop-gap measure, have a look at the msgpack-aeson directory of the message-pack github repo:

https://github.com/msgpack/msgpack-haskell/tree/master/msgpack-aeson

You could go from your data values <-> aeson <-> message-pack. Not necessarily efficient, but convenient since you can auto derive ToJSON and FromJSON with DeriveGeneric.

Example code:

{-# LANGUAGE DeriveGeneric, OverloadedStrings #-}

import Data.MessagePack.Aeson
import qualified Data.MessagePack as MP
import GHC.Generics
import Data.Aeson

data Foo = Foo { _a :: Int, _b :: String }
  deriving (Generic)

instance ToJSON Foo
instance FromJSON Foo

toMsgPack :: Foo  -> Maybe MP.Object
toMsgPack =   decode . encode

test = toMsgPack (Foo 3 "asd")

回答2:

You could write your own GMessagePack class and get instances by deriving Generic. I tried doing so to answer this question, but I can't recommend it. msgpack has no support for sums, and the one sum type supported by the Haskell msgpack library, Maybe, has a very poor encoding.

instance MessagePack a => MessagePack (Maybe a) where
  toObject = \case
    Just a  -> toObject a
    Nothing -> ObjectNil

  fromObject = \case
    ObjectNil -> Just Nothing
    obj -> fromObject obj

The encoding for Maybes can't tell the difference between Nothing :: Maybe (Maybe a) and Just Nothing :: Maybe (Maybe a), both will be encoded as ObjectNil and decoded as Nothing. If we were to impose on MessagePack instances the obvious law fromObject . toObject == pure, this instance for MessagePack would violate it.

来源：https://stackoverflow.com/questions/31892857/how-to-derive-instances-of-data-messagepack-1-0-0

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haskell

msgpack