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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe