|A collection of data types, classes, and functions for interfacing with another programming language.|
|unsafePerformIO :: IO a -> a|
This is the "back door" into the IO monad, allowing IO computation to be performed at any time. For this to be safe, the IO computation should be free of side effects and independent of its environment.
If the I/O computation wrapped in unsafePerformIO performs side effects, then the relative order in which those side effects take place (relative to the main I/O trunk, or other calls to unsafePerformIO) is indeterminate. You have to be careful when writing and compiling modules that use unsafePerformIO:
It is less well known that unsafePerformIO is not type safe. For example:
test :: IORef [a] test = unsafePerformIO $ newIORef  main = do writeIORef test  bang \<- readIORef test print (bang :: [Char])
This program will core dump. This problem with polymorphic references is well known in the ML community, and does not arise with normal monadic use of references. There is no easy way to make it impossible once you use unsafePerformIO. Indeed, it is possible to write coerce :: a -> b with the help of unsafePerformIO. So be careful!
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