The problem is that the caller of "get" is allowed to say what type of Query they want with this instance, for example: ] get :: Get (Query Int) because Int is an instance of Binary, and you claim ] instance Binary a => Binary (Query a) In fact, since the type of each query is unique, the tagging in "put" doesn't help you. So there is no way to write a generic Binary instance for Query a. But don't give up! Here's a solution in literate haskell...
{-# LANGUAGE ExistentialQuantification, FlexibleInstances, GADTs #-} module Query where import Data.Binary import Control.Monad (liftM)
data Query a where Lookup :: String -> Query (Maybe Int) Fetch :: [String] -> Query [Int]
One obvious strategy is to use existential types:
data SomeQuery = forall a. SomeQuery (Query a)
You can then make SomeQuery an instance of Binary using very similar code to your implementation. Now, the code that calls "get" needs to be able to deal with any Query type it gets back inside the SomeQuery. Another possibility is to drop the tagging altogether and use a helper class:
class BinaryQuery a where putQ :: Query a -> Put getQ :: Get (Query a)
instance BinaryQuery a => Binary (Query a) where put = putQ get = getQ
instance BinaryQuery (Maybe Int) where putQ (Lookup x) = put x getQ = liftM Lookup get
instance BinaryQuery [Int] where putQ (Fetch xs) = put xs getQ = liftM Fetch get
You can also combine the strategies and use SomeQuery when the stored value needs a tag:
instance Binary SomeQuery where put (SomeQuery x@(Lookup _)) = putWord8 0 >> put x put (SomeQuery x@(Fetch _)) = putWord8 1 >> put x get = getWord8 >>= \tag -> case tag of 0 -> liftM SomeQuery (get :: Get (Query (Maybe Int))) 1 -> liftM SomeQuery (get :: Get (Query [Int]))
The pattern matching in "put" specializes the type of x, allowing the query-level put to find the correct implementation. Similarily, in "get" we choose the correct get based on the input type. -- ryan