unsafe-perform-io

I found some sample code, and changed it a little counter = unsafePerform $ newIORef 0 newNode _ = unsafePerformIO $ do i <- readIORef counter writeIORef counter (i+1) return i Which returns 1 then 2 then 3 then 3 etc each time it's run. But when I change it to newNode = unsafePerformIO $ do i <- readIORef counter writeIORef counter (i+1) return i then I get 0 every time I run it. Why is this happening, and what can I do to fix it? In your second version newNode is a simple value, not a function. So haskell evaluates it exactly once and then gives you the result of that evaluation whenever you

I'm creating an FFI module to a library in C which wants a 1-time, non-reentrant function to be called before anything else is. This call is idempotent, but stateful, so I could just call it in every Haskell call. But it's slow and due to non-reentrancy it could cause conflicts. So is this the right time to use unsafePerformIO? I could wrap a Bool in an unsafe IORef or MVar to make these initialization calls idempotent by ignoring subsequent calls (calls where the global, hidden IORef state is False). If not, what is the right way to do this? Don Stewart I prefer the approach of initializing

I often find this pattern in Haskell code: options :: MVar OptionRecord options = unsafePerformIO $ newEmptyMVar ... doSomething :: Foo -> Bar doSomething = unsafePerformIO $ do opt <- readMVar options doSomething' where ... Basically, one has a record of options or something similar, that is initially set at the program's beginning. As the programmer is lazy, he doesn't want to carry the options record all over the program. He defines an MVar to keep it - defined by an ugly use of unsafePerformIO . The programmer ensures, that the state is set only once and before any operation has taken