Documentation

Retrieve a GLUT API entry by name. Throws a userError when no entry with the given name was found.

A value of type FunPtr a is a pointer to a function callable from foreign code. The type a will normally be a foreign type, a function type with zero or more arguments where

• the argument types are marshallable foreign types, i.e. Char, Int, Double, Float, Bool, Int8, Int16, Int32, Int64, Word8, Word16, Word32, Word64, Ptr a, FunPtr a, StablePtr a or a renaming of any of these using newtype.
• the return type is either a marshallable foreign type or has the form IO t where t is a marshallable foreign type or ().

A value of type FunPtr a may be a pointer to a foreign function, either returned by another foreign function or imported with a a static address import like

 foreign import ccall "stdlib.h &free"
   p_free :: FunPtr (Ptr a -> IO ())

or a pointer to a Haskell function created using a wrapper stub declared to produce a FunPtr of the correct type. For example:

 type Compare = Int -> Int -> Bool
 foreign import ccall "wrapper"
   mkCompare :: Compare -> IO (FunPtr Compare)

Calls to wrapper stubs like mkCompare allocate storage, which should be released with freeHaskellFunPtr when no longer required.

To convert FunPtr values to corresponding Haskell functions, one can define a dynamic stub for the specific foreign type, e.g.

 type IntFunction = CInt -> IO ()
 foreign import ccall "dynamic" 
   mkFun :: FunPtr IntFunction -> IntFunction

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!