[Haskell-cafe] What unsafeInterleaveIO is unsafe

[Henning Thielemann]

On Sun, 15 Mar 2009, Ryan Ingram wrote:

> unsafeInterleaveIO allows embedding side effects into a pure
> computation.  This means you can potentially observe if some pure
> value has been evaluated or not;  the result of your code could change
> depending how lazy/strict it is, which is very hard to predict!
>
> For example:
>
>> -- given
>> f :: Integer -> Integer
>>
>> main = do
>>     r <- newIORef 0
>>     v <- unsafeInterleaveIO $ do
>>         writeIORef r 1
>>         return 1
>>     x <- case f v of
>>         0 -> return 0
>>         n -> return (n - 1)
>>     y <- readIORef r
>>     print y
>>
>> -- a couple of examples:
>> f x = 0 -- program prints "0"
>> -- f x = x -- program prints "1"

Interesting example. I have implemented the lazyio package. It asserts 
that unsafely interleaved IO actions are performed in order. This should 
prevent such problems, does it? However, they are only avoided within the 
LazyIO monad. Running the LazyIO monad in IO allows the same corruptions 
as shown above.

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lazyio