System.IO.Unsafe

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System.IO.Unsafe |
Portability | portable
Stability | provisional
Maintainer | libraries@haskell.org
Description
"Unsafe" IO operations.
Synopsis
unsafePerformIO :: IO a -> a
unsafeInterleaveIO :: IO a -> IO a
Unsafe `IO` operations
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. However, it is less well known that `unsafePerformIO` is not type safe. For example:
         test :: IORef [a]
         test = unsafePerformIO $ newIORef []

         main = do
         	      writeIORef test [42]
         	      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!
unsafeInterleaveIO :: IO a -> IO a
`unsafeInterleaveIO` allows `IO` computation to be deferred lazily. When passed a value of type `IO a`, the `IO` will only be performed when the value of the `a` is demanded. This is used to implement lazy file reading, see `hGetContents`.
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