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
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