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

[Miguel Mitrofanov]

Well, normally - you can't (unless there is some equivalent to the 
constructor exported).

But there is a trick. You can use generic classes:

{-# OPTIONS_GHC -fglasgow-exts -XGenerics -package lang #-}
import Generics
class Binary' a where
   put' :: a -> Put
   get' :: Get a
   put' {| Unit |} Unit = return ()
   get' {| Unit |} = return Unit
   put' {| a :+: b |} (Inl x) = putWord8 0 >> put' x
   put' {| a :+: b |} (Inr y) = putWord8 1 >> put' y
   get' {| a :+: b |} =
     do w <- getWord8
        case w of
          0 -> liftM Left get'
          _ -> liftM Right get'
   put' {| a :*: b |} (x :*: y) = put' x >> put' y
   get' {| a :*: b |} =
     do x <- get'
        y <- get'
        return $ x :*: y
instance Binary' Int32 where
   put' = put
   get' = get
instance Binary' StdGen
instance Binary StdGen where
   put = put'
   get = get'

Last time I've checked it worked fine. A friend of mine have used it to 
create "instance Eq Chan", if I remember correctly.

Grigory Sarnitskiy wrote:
> In System.Random StdGen is defined as
> 
> data StdGen = StdGen Int32 Int32
> 
> but its constructor StdGen is not exported. How to make StdGen to be an instance of Binary? The following won't work:
> 
> 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)
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>