| Portability | portable |
|---|---|
| Stability | provisional |
| Maintainer | sven.panne@aedion.de |
| Safe Haskell | None |
Graphics.UI.GLUT.Raw.APIEntry
Description
This is a purely internal module for handling an OpenGL-like extension mechanism for GLUT.
- type Invoker a = FunPtr a -> a
- getAPIEntry :: String -> IO (FunPtr a)
- getAPIEntryInternal :: String -> IO (FunPtr a)
- data FunPtr a
- unsafePerformIO :: IO a -> a
Documentation
getAPIEntry :: String -> IO (FunPtr a)
Retrieve a GLUT API entry by name. Throws a userError when no entry with the given name was found.
getAPIEntryInternal :: String -> IO (FunPtr a)
data FunPtr a
A value of type is a pointer to a function callable
from foreign code. The type FunPtr aa 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,,Ptra,FunPtraor a renaming of any of these usingStablePtranewtype. - the return type is either a marshallable foreign type or has the form
whereIOttis a marshallable foreign type or().
A value of type may be a pointer to a foreign function,
either returned by another foreign function or imported with a
a static address import like
FunPtr a
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
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 functionfoothat callsunsafePerformIO. If the call is inlined, the I/O may be performed more than once. - Use the compiler flag
-fno-cseto 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 (liketestin the example below). - Make sure that the either you switch off let-floating (
-fno-full-laziness), or that the call tounsafePerformIOcannot float outside a lambda. For example, if you say:f x = unsafePerformIO (newIORef [])you may get only one reference cell shared between all calls tof. Better would bef 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!