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

[Miguel Mitrofanov]

You're right.

The issue you've mentioned can be fixed easily - import Data.Generics 
instead of Generics and get rid of -package lang (I've copied them from 
the documentation without checking, seems like it's a bit outdated).

The real problem is that you can't use "Get a" in generics! And you have 
the same problem here, because "Get" constructor isn't exported either!

But we can make it work using a continuation trick:

{-# OPTIONS_GHC -fglasgow-exts -XGenerics #-}
module Test where
import Control.Monad
import Data.Binary
import Data.Generics
import Data.Int
import System.Random
class Binary' a where
   put' :: a -> Put
   get' :: (a -> Get StdGen) -> Get StdGen
   put' {| Unit |} Unit = return ()
   get' {| Unit |} f = f Unit
   put' {| a :+: b |} (Inl x) = putWord8 0 >> put' x
   put' {| a :+: b |} (Inr y) = putWord8 1 >> put' y
   get' {| a :+: b |} f =
       do w <- getWord8
          case w of
            0 -> get' $ \x -> f $ Inl x
            _ -> get' $ \y -> f $ Inr y
   put' {| a :*: b |} (x :*: y) = put' x >> put' y
   get' {| a :*: b |} f = get' $ \x -> get' $ \y -> f (x :*: y)
instance Binary' Int32 where
   put' = put
   get' f = get >>= f
instance Binary' StdGen
instance Binary StdGen where
   put = put'
   get = get' return

This time I've checked that it really compiles. Pretty much sure it works.