Foreign.GreenCard

Documentation

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!

type MbString = Maybe String

marshall_bool_ :: Bool -> IO Int

unmarshall_bool_ :: Int -> IO Bool

marshall_string_ :: [Char] -> IO CString

unmarshall_string_ :: CString -> IO String

marshall_stringLen_ :: [Char] -> IO CStringLen

unmarshall_stringLen_ :: CString -> Int -> IO String

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.

Instances

Enum Float

Eq Float

Floating Float

Fractional Float

Num Float

Ord Float

Real Float

RealFloat Float

RealFrac Float

Show Float

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.

Instances

Enum Double

Eq Double

Floating Double

Fractional Double

Num Double

Ord Double

Real Double

RealFloat Double

RealFrac Double

Show Double

data Char :: *

The character type Char is an enumeration whose values represent Unicode (or equivalently ISO/IEC 10646) characters (see http://www.unicode.org/ 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).

Instances

Eq Char

Ord Char