[Haskell-cafe] Is unsafePerformIO safe here?

John Ky newhoggy at gmail.com
Sun Dec 7 20:52:13 EST 2008

Does that mean there is no place to store state while running the
interpreter and that I have to put the state elsewhere such as a file?  I
was hoping to avoid that as I'm only prototyping at this stage and don't
want to write a persistent layer just yet.



On Mon, Dec 8, 2008 at 11:51 AM, Thomas Davie <tom.davie at gmail.com> wrote:

> On 8 Dec 2008, at 01:28, John Ky wrote:
>  Hi,
>> Is the following safe?
>> moo :: TVar Int
>> moo = unsafePerformIO $ newTVarIO 1
>> I'm interested in writing a stateful application, and I wanted to start
>> with writing some IO functions that did stuff on some state and then test
>> them over long periods of time in GHCi.
>> I was worried I might be depending on some guarantees that aren't actually
>> there, like moo being discarded and recreated inbetween invocations of
>> different functions.
> Define safe... In this case though, I would guess it's not safe.  The
> compiler is free to call moo zero, one or many times depending on its
> evaluation strategy, and when it's demanded.  It's possible that your TVar
> will get created many times, and different values returned by the "constant"
> moo.
> That sounds pretty unsafe to me.
> Bob
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