[Haskell-beginners] Just how unsafe is unsafe

[Austin Seipp]

Excerpts from Andrew Wagner's message of Thu Feb 05 15:11:17 -0600 2009:
> My question is, to what extent is this true?

You can completely destroy the soundness of the type system:

    import Data.IORef
    import System.IO.Unsafe
    import Control.Monad

    cast :: a -> b
    cast x = f where
     f = unsafePerformIO $ do
         writeIORef r [x]
         b <- liftM head $ readIORef r
         return b

Austin