{-# OPTIONS -fglasgow-exts #-}


-- Simple IO Regions. Tests
-- Attempt to break the safety guarantees by using encapsulated polymorphic
-- types
-- $Id: IORegions98Test1.hs,v 1.1 2006/01/24 00:42:08 oleg Exp oleg $

module IORegions98Test1 where

import IORegions98E
import Monad
import Data.Typeable


-- We define the following just as an attempt to break the safety
-- guarantee. Users should NOT add the following two instances.
-- However, even if they do, the safety guarantees still hold
instance Typeable1 Q
instance Typeable2 IOM


data W1 a = W1 (forall marks. IOM marks a)
data W2   = W2 (forall marks. Q marks)
data W3 a = W3 (forall marks. Q marks -> IOM marks a)

instance Typeable1 W1
-- Hugs likes the commented-out instance
-- instance Typeable a => Typeable (W3 a)
instance Typeable1 W3
instance Typeable W2

-- test1 = withFile "/etc/motd" (\q -> return $ W2 q)
-- test2 = withFile "/etc/motd" (\q -> return $ W1 (qGetChar q))
test3 = withFile "/etc/motd" (\q -> return $ W3 (qGetChar))
-- test4 = withFile "/etc/motd" (\q -> return $ W3 (\q1 -> qGetChar q))

{- Of course the guarantees break if we add -fallow-incoherent-instances 
 and define

  instance Typeable a

But in that case, the total function coerce :: a -> b
becomes expressible, which breaks all guarantees in Haskell whatsoever.
See the message on the Haskell list on Jan 26, 2006.

-}