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

[Grigory Sarnitskiy]

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)
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