greencard-3.0.4: GreenCard, a foreign function pre-processor for Haskell.

Safe HaskellNone




module Foreign

module Foreign.C

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. Furthermore, when using unsafePerformIO to cause side-effects, you should take the following precautions to ensure the side effects are performed as many times as you expect them to be. Note that these precautions are necessary for GHC, but may not be sufficient, and other compilers may require different precautions:

  • Use {-# NOINLINE foo #-} as a pragma on any function foo that calls unsafePerformIO. If the call is inlined, the I/O may be performed more than once.
  • Use the compiler flag -fno-cse to prevent common sub-expression elimination being performed on the module, which might combine two side effects that were meant to be separate. A good example is using multiple global variables (like test in the example below).
  • Make sure that the either you switch off let-floating (-fno-full-laziness), or that the call to unsafePerformIO cannot float outside a lambda. For example, if you say: f x = unsafePerformIO (newIORef []) you may get only one reference cell shared between all calls to f. Better would be f x = unsafePerformIO (newIORef [x]) because now it can't float outside the lambda.

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!

data Float

Single-precision floating point numbers. It is desirable that this type be at least equal in range and precision to the IEEE single-precision type.

data Double

Double-precision floating point numbers. It is desirable that this type be at least equal in range and precision to the IEEE double-precision type.

data Char

The character type Char is an enumeration whose values represent Unicode (or equivalently ISO/IEC 10646) characters (see for details). This set extends the ISO 8859-1 (Latin-1) character set (the first 256 characters), which is itself an extension of the ASCII character set (the first 128 characters). A character literal in Haskell has type Char.

To convert a Char to or from the corresponding Int value defined by Unicode, use toEnum and fromEnum from the Enum class respectively (or equivalently ord and chr).