Re: [Haskell-cafe] unsafePerformIO: are we safe?

You could do this: r :: IORef [a] r = unsafePerformIO $ newIORef [] cast :: a -> b cast a = unsafePerformIO $ do writeIORef r [a] x <- readIORef r return $ head x On 9/26/07, Dan Piponi wrote:

On 9/26/07, Lennart Augustsson wrote:

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Things can go arbitrarily wrong if you misuse unsafePerformIO, you can even subvert the type system.

So...if I was in a subversive kind of mood (speaking hypothetically), what would I have to do? -- Dan

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On 9/26/07, Jorge Marques Pelizzoni < jpeliz@icmc.usp.br> wrote:

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Hi, all!

This is a newbie question: I sort of understand what unsafePerformIO

does

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but I don't quite get its consequences. In short: how safe can one be in face of it? I mean, conceptually, it allows any Haskell function to have side effects just as in any imperative language, doesn't it? Doesn't it blow up referential transparency for good? Is there anything intrinsic to it that still keeps Haskell "sound" no matter what unsafePerformIO users do (unlikely) or else what are the guidelines we should follow when using it?

Thanks in advance. Cheers,

Jorge.

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