[Haskell-cafe] How to derive instance for type without exported constructor?

[Grigory Sarnitskiy]

Well, I've managed to produce a solution, quite ugly and unefficient. Still it works (and I really need it). StdGen serialization occurs only once during computation that lasts several hours, so the speed is not vital for me. Here is my solution:

module Main where

import System.Random
import Data.Binary
import Data.Int

data StdGen' = StdGen' Int32 Int32 deriving (Show)

gen2gen' :: StdGen -> StdGen'
gen2gen' gen = let
    [g1, g2] = words $ show $ gen
    g1' = read g1 :: Int32
    g2' = read g2 :: Int32
    in StdGen' g1' g2'

gen'2gen :: StdGen' -> StdGen
gen'2gen (StdGen' g1' g2') = let
    gen = read $ show g1' ++ ' ':(show g2') :: StdGen
    in gen

instance Data.Binary.Binary StdGen' where
    put (StdGen' aa ab) = do
            Data.Binary.put aa
            Data.Binary.put ab
    get = do
        aa <- get
        ab <- get
        return (StdGen' aa ab)

instance Data.Binary.Binary StdGen where
    put gen = put $ gen2gen' gen
    get = do
        gen' <- get
        return (gen'2gen gen')