[Haskell-cafe] Proving stuff about IORefs

[Derek Elkins]

On Sun, Oct 17, 2010 at 6:49 AM, Miguel Mitrofanov
<miguelimo38 at yandex.ru> wrote:
>
> On 17 Oct 2010, at 05:21, Ben Franksen wrote:
>
>> I want to prove that
>>
>>  f r == do
>>    s1 <- readIORef r
>>    r' <- newIORef s1
>>    x <- f r'
>>    s3 <- readIORef r'
>>    writeIORef r s3
>>    return x
>
> That is not true. Consider the following function:
>
> g r1 r2 = writeIORef r1 0 >> writeIORef r2 1 >> readIORef r1
>
> This function behaves differently depending on whether r1 and r2 are the same IORef or not. Therefore, the property you want to prove doesn't hold for the partially-applied function
>
> f = g r1

I originally was thinking along these lines, and this is an important
case, but there is an even more trivial example.

Let f be return.