Miguel Mitrofanov wrote:
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'
Isn't it to define an isomorphic type and unsafeCoerce to it pretty much equivalent? At least the following simplest example works just fine: module Main where import Unsafe.Coerce class Test a where test :: a -> Int data Foo = Foo Int Int data Bar = Bar Int Int instance Test Bar where test (Bar a b) = a + b instance Test Foo where test foo = test (unsafeCoerce foo :: Bar) main :: IO () main = print $ test (Foo 123 345) Cheers, Kyra